RH-TTA – Tube Tester Amplifier

The RH300B project has spawned a schematics variation for the 2A3 tube, and from there several similar variations derive, RH2A3/1619, RH6B4G/6L6 – as presented in the previous blog entry. Indeed, the 1619 is very much compatible with the 2A3, with its 2.5V heaters, and the overall compatibility of the octal socket pin disposition as found in the special octal version of the 2A3, manufactured originally for the Audio Innovations amplifiers during the 90s. On the other hand, the 6B4G (by which I intend NOS types) and the more common Russian 6C4C (6S4S) have an identical octal pinout (5S) that is basically compatible with the 7AC pinout of the 6L6... not to mention that the 8EP pinout of the EL34 is compatible as well if we consider grounding the third grid (g3) instead of connecting it to the cathode (as a matter of fact, I prefer grounding g3 i.e. the beam former – to connecting it with the cathode, since a gradual increase of voltage on g3 leads to the forming of a kink or wave similar to that of a tetrode).

Why should we not have the best of both worlds? All we need are additional heater secondary windings for 6.3V tubes: plural, since each direct heated tube should have its own, and indirect heated tubes will not mind having their own heater secondary. Now here we are facing two possibilities – either a single 6.3V 2.5A secondary where the voltage can be reduced across resistors, or separate 6.3V 1.5A secondary and 2.5V 2.5A secondary. The latter solution is what I have chosen, since adding resistors both clutters the interior of amplifiers, and generates unnecessary heat by burning down voltage (not to mention an increase in power consumption).

The Best of Both Worlds

The result is an amplifier that can use a very wide array of tubes – from 2A3 octal and 1619, to 6A5G, 6B4G and most of the pin compatible indirect heated tubes (even 6V6 and 6F6 can be used if the rectifier applied to the power supply has a higher voltage drop, like the 5Y3). The basic schematics is as published under RH6B4G/6L6 – but for a small detail: in order to avoid the switch that selects cathode type (well, let’s get rid of at least one switch), I have chosen to connect pin 8 (cathode in 7AC and 8EP pinouts) directly to the “virtual cathode point” that is created on pin 6 (this pin is unused in all the pin-outs mentioned). Incidentally, pin 8 is the mid-filament point to which the indirect heated cathode is connected on the 6A5G.

The connection of pin8 to pin 6 is the only relevant schematics difference between the RH6B4G/6L6 schematics, and the RH Tube Tester Amplifier schematics… so far.

This is basically an amplifier that can use a wide array of tubes capable of at least 15W dissipation… now that brings to mind a few other similar tubes with different bases. The 307A that I have already designed for (RH307A), and the 2E22 come to mind, as well as the 1624, and the evergreen 807 – all of those require the UX5 socket and an anode cap. While the compatibility of these tubes with the amplifier could be solved with adapters – UX5 to octal socket – they all require anode caps which would complicate the wiring of the adapters, and the anode caps are not even the same size… Thus I have chosen to install parallel UX5 sockets.

The UX5 Alternative

The parallel UX5 socket is wired by connecting the appropriate pins – 1 and 5 on the UX5 are the cathode (filament) connections and should be connected to pins 2 and 7 on the octal socket with twisted wires (AC heating!), pin 2 of the UX5 is the screen grid (g2) and should be connected with pin4 of the octal socket.

Now for another difference: the maximum g2 voltage of most tubes that can be used in this amp is actually at least 300V, and thus the 1N5370B (56V 5W) zener diode in the schematics gets exchanged for a 22V zener of the same series (1N5358B – 22V 5W), which will just allow for the voltage drop across the primary of the output transformer, keeping the anode voltage at a slightly higher potential than the screen grid voltage.

If you are planning to use 2E22 tubes in your RH-TTA (like I do), you should connect pin 2 of the UX5 socket to pin 4 of the octal socket via an adequately biased zener diode (i.e. the cathode represented by the line on the zener diode should be connected to pin 4 on the octal socket). Of course, the zener in this case should be the already mentioned 1N5370B – 56V5W zener – since the maximum g2 voltage for 2E22 is 250V (g2 to cathode).

The grid or pin 3 on the UX5 socket should be connected via a grid stopper resistor (anything between 200 and 500 ohm would do) to the same point to which the grid stopper resistor leading to the octal socket is connected (i.e. the connection between coupling cap and grid bleeder resistor). Connection by wire is a possibility as well, but I think that this solution will provide better protection of the circuit from oscillations.

Last but not least, pin 4 of the UX5 socket – this pin is either g3 in the pentodes (307A, 2E22) or NC for the 1624. Thus, depending on whether you are planning to use the 807 or not, you may connect it directly to pin 6 on the octal socket (virtual cathode point). If you are not planning to use the 807 in this amplifier (like myself, since I have got no 807 tubes), you can also connect this pin simply to ground, ensuring that g3 on 307A and 2E22 is always at 0V potential.

A perfectly safe solution for the anode connection is a 4mm “banana” plug like the one you might use for the loudspeakers – if it is well isolated. Make sure that the plastic isolation is good enough for at least 400V DC if you are installing the UX5 socket and the anode “banana” jack on a metal sheet: if you are using wood or some other isolating material (like Plexiglas) no particular care has to be taken to isolate this jack. Having a jack instead of a fixed wire allows the use of sets of cables with anode cap on one side (9mm for 307A, 1624, 807; 14mm for 2E22) and a 4mm plug on the other, so they can be removed from the amp when you are not using them, while the socket and plug that you are not using can be covered (protected from dust and fingers).

What About UX4 Tubes?

Indeed, the 2A3 was originally meant for the UX4 socket – and as such it was manufactured in a wide array of versions – double-plate, bi-plate, mono-plate. The 2A3 is currently produced on UX4 socket by several manufacturers, while the special edition octal 2A3 is probably not produced anymore – so what if your main point of interest is the classic UX4 2A3?

The first option would be the standard RH2A3 schematics, with UX4 sockets. Or, if you would like to use other similar tubes, like 6B4G, 6A3, or 6A5G – and you are not all that into odd direct heated pentodes and tetrodes – you could build the RH-TTA but with an UX4 socket instead of the UX5 socket. Indeed, all those UX5 tubes are NOS only, and while prices may still be low enough to be intriguing – you might not be interested in buying any if you do not own some already. Having an UX4 socket in parallel with the octal socket is still allowing for a wide range of relatively common tubes, most of which are still being produced, or available at quite affordable prices.

Finally, you can use UX4-to-octal adapters. Besides buying the adapters, you could build your own, just like I had to do. To build an adapter you will need an empty octal base (preferably new, but you can also remove them from dead or shorted octal tubes) and a suitable UX4 socket – by suitable, I mean the round body type that can be removed from its metal retainer. Since the base will most probably be plastic (phenolic or similar), plastic sockets might be preferable for this use – but I chose to use ceramic sockets since I had those available.

Adapters are rather easy to make – all you need to do is solder short insulated pieces of wire to the UX4 socket lugs (making shure that once you insert the socket in the octal base the lugs will not short with the pins of the base) and remove just enough insulation from the wire pieces as to fit the length of the octal pins: the pin will be filled with wire which should protrude slightly on the other side, while inside the base the wire remains insulated. Once the wires are in place, apply solder with rosin and paste, ensuring that it flows inside the pins and makes good contact with the wire. When you finish soldering and the check shows that no mistakes were made connecting, and the connections are sound, you can glue the socket to the base. While cyan-acetate glues might seem appropriate, they are rather conducive to messy work and do not guarantee good connection unless the fit is tight. Two-component epoxy, on the other hand, will hold perfectly regardless of material type (ceramics on phenolic plastic or similar) and will fill the empty space if the fit is not tight. Do not forget to cover the exposed parts with tape that you will remove later to prevent ugly spills of glue or epoxy.

Once you make your adapters (or buy them) – you are ready to use UX4 based tubes on the TTA! And this means some unexpected guests, like the 45…

Enter the RH45

The 45 is highly coveted as one of the best sounding tubes of all times. A predecessor of the 2A3, it shares socket type, pinout, and heaters voltage with that much wider known and nowadays more used DHT. It seems that, unlike the 50 – the 45 is not yet extinct, and that holds particularly true for SE amps applications. After all, you do not need a matched pair – similar tubes are good enough – while with the current setting arrangements in RH amplifiers the only thing you need to worry is finding some of these triodes in pristine condition.

The 45 family can be basically divided as older globe types and the later ST shaped types. Interestingly enough, while many call ST shaped tubes “Coke bottles”, and find them rather sexy looking – in this case form only follows function. While the globe shape allowed for larger (and easier to manufacture) inside structures, the ST or Shoulder Tube (Type) boasts the shoulder that helps better arrange and fix the internal structure. Globe shaped tubes of the same type probably sound different than ST shaped tubes due to the different structure and possibly being more or less prone to micro-phonics.

What is the maximum anode dissipation of the 45 - or, for that matter, the maximum anode dissipation the 2A3? This is rather difficult to find out. While it is mostly assumed that maximum anode dissipation of 2A3 types is 15W (some datasheets actually state that value as “design center values”, which means they may be exceeded), data on the 45 is even more difficult to find. Guessing from several sources, the “historical” 45 had probably 10-11W of maximum anode dissipation. Now, those were certainly not “absolute maximum ratings”. This is not a story about constructing better than necessary – rather about technology being not precise enough. Just like the old skyscrapers were over-engineered and more massive than it seems necessary in today’s terms, the clue is material tolerances. The standards for steel and concrete were less tight than nowadays, while they had to build skyscrapers that would not crumble with the change of wind direction, or stormy weather, or medium intensity earthquakes – tubes were built to do their task, for instance amplify music in a home radio receiver. 2W of output power during many hours (years) of use meant having to use a given size or thickness for the anodes… and so on. Whether the 45 is really a 10W anode dissipation tube, or more powerful than that – this really depends on what you expect to get from it, and for how long: that is the real tube power equation.

 

Starting from available UX4 sockets or adapters in an RH2A3 (or RH-TTA), there are several important issues for implementing the 45 (i.e. in the RH45 amplifier):

a)      current draw should be set at 36mA – either by adding a 15 ohm resistor in series with the current setting resistor of the RH2A3 (TTA), or using a 36 ohm resistor in an amplifier to be used exclusively with 45 tubes (the RH45 amplifier);

b)      lower current draw will cause a rise in the B+, thus it is advisable to use 5R4 rectifiers with higher voltage drop (5R4GY and 5R4WGY) or 5Y3 rectifiers – another alternative that comes to mind is an 80 type rectifier (to be swapped for 5Z3 in the same UX4 socket when other higher dissipation tubes are used drawing more current); this will ensure that the B+ stays below 340V;

c)       36mA current draw and 330-340V B+ means a bias voltage (cathode to ground potential) of 55V approximately, which leaves about 270V across the tube, for slightly below 10W anode dissipation – so far so good, but the LM317 will need to be replaced with a TL783 as the voltage across the rectifier will probably exceed the maximum voltage rating of the LM317 regulator.

The TL783 is a totally transparent replacement for the LM317: same pinout, almost identical reference voltage, same packages available – which means that it is very easy to implement. While some think of the TL783 as an LM317 with higher voltage MOS type pass element, the differences actually exceed the initial expectations. Nevertheless, for setting a precise current draw at higher voltages (good for up to 125V across the regulator) the TL783 is perfectly suited, and will add a measure of reliability to any RH amplifier, regardless of expected bias voltage. The only issue with the TL783 is its price (almost triple that of an LM317) and availability (not all resellers have it on stock – but if you live outside of Serbia you will source it rather easily).

If the intended version is RH45 only, the TL783 is not strictly necessary, since the 470 ohm resistor can be increased to about 1k ohm, increasing the voltage drop across it and keeping the LM317 safe. But increasing the resistor in an RH-TTA (or any of the RH2A3 versions) is not feasible since the voltage will be either too low for current regulation, or too high for LM317 implementation, depending on the current draw.

Last but not least, with the TL783 the DIYer does not need to worry too much about the switch that excludes the voltage dropping resistor from the cathode circuit – leaving it excluded for the 2A3 or 45 will not harm the TL783 (but would kill the LM317), particularly if the heat-sink used allows for 2-3W of dissipation. With the TL783 the main purpose of the voltage dropping resistor remains reducing the dissipation of the regulator, keeping it as cool as possible.

Output Power

The RH-TTA is a further development of the RH2A3 presented earlier, thus output power and distortions are as already shown. While 2A3 and 6B4G tubes will allow about 5W output power, more can be had from the 1619 and the other DH pentodes and DH beam tetrodes mentioned, due to the higher efficiency of pentodes and beam tetrodes in particular. The 307A is a 15W dissipation pentode, similar in output power to the 1619 – while the 1624 is a 1619 in ST shaped envelope with increased dissipation to 25W. The 2E22 is a 30W dissipation pentode, but output power remains basically the same as the other pentodes/beam tetrodes mentioned, since the output power is limited by the voltage across the tube and the fixed current draw. The 1624 and the 2E22 can be operated as well with GZ34/5AR4 rectifiers, increasing the available B+ for 35V approximately. The same is true of other indirect heated tubes that might be used in the RH-TTA and have higher than 15W anode dissipation ratings, like the EL34 or the modern 6L6 types: output power is limited by the available B+ and current, as well as the necessity to apply higher primary resistance to keep distortions at bay. Output power will generally range between 5 and 7.5W, which is more than enough for serious listening even on relatively inefficient speakers (88-90dB/W/m). The RH-TTA is not about an important increase of power using pentode and beam tetrode tubes – but about the possibility to choose in accordance with taste and availability.

The novelty in this case is output power with the 45 tube (RH45 amplifier) – almost 3W. Whether you like the simulation results or not, listening to the amp clearly shows that output power is slightly lower than with 2A3 tube types, but the result is unexpectedly loud and satisfying. Basically, it is almost the same power output possible with a classic “no feedback” 2A3 SE amp. I have not tried to push the 45 in the same operating conditions applied to the 2A3, which would result in 15W dissipation: while I am curious enough to try it (and see whether the anodes would develop red sports or, probably, not) – that would be totally unnecessary and irrelevant, just like I consider pushing the 307A to 25W dissipation an unnecessary pass-time: if you need more power, there are tubes fitting the same socket that might be used for that purpose, like the 2A3 instead of the 45, or the 2E22 instead of the 307A. The 45 is too rare to be squandered for power at all costs. That said, 3W is almost 50% more than your average 45 SE amplifier, without excessive stress for the tubes.

I noticed the RH300B output power being discussed on a forum – while some were questioning the feasibility, others were quick to point out the merits of their designs. All in all, between those who understand what it takes, and those that do not understand, there was no mention of the particular sound quality achieved by the various RH amplifiers. Design and engineering are not meant to be the purpose, rather the means to achieve sonic excellence – but  those who never built one, or listened to an RH amplifier cannot discuss the sonic merits, being confined to accepting or negating design accomplishments. Anyway, for those interested, here is a simulation of the RH45 without Rfb at approximately 1.2% distortion...

I guess this example is illustrative enough - while correct application of feedback is paramount in RH amplifiers, some merit should be given to distortion cancellation as well.

Output Transformers

Just like I suggested when writing about the RH2A3/1619, for the operation of the RH-TTA with a wide array of tubes you will need a flexible transformer. While you could get away with a usual 3k primary with 8 or 4 ohms secondary – if you use the 4 ohms secondary with your 8 ohm speakers you will get approximately 6k primary loading – such arrangement is valid only if you do not change your speakers. Assuming 4 ohm speakers, the 8 ohm output will become approximately 1.5k – which is not a value usable with the RH-TTA: if that is the case you would obviously need to change your output transformers as well.

The Lundahl LL1623 is extremely flexible, and besides the possibility to configure from 1.6k to 5.6k primary and a choice of 4, 8, and 16 ohm secondary configurations – you can easily arrange the “neighboring” values to be changed with switches, like I did. Operating 3 switches simultaneously (it is difficult to find a two way 12 contact switch, or a rotary switch that has adequate current capability) I can choose between 3k and 5.6k primary loading into 8 ohms (on both channels at the same time), and if in the future I change speakers (for instance, 4 ohm units) I would just have to reconfigure the wiring harness. I regard this characteristic as paramount, particularly in a DIY project – since transformers are virtually “forever”: they do not change their characteristics appreciably with time, and when built with modern isolation materials they should have a much extended lifespan. Therefore, if nothing else, you should be able to reuse your transformers in some new project one day, and that is where flexibility means good investment.

If the RH-TTA is to be realized as a more limited solution – like just using 2A3, 6A3, 6B4G, and 6A5G, for instance… any good 3k primary output transformer would do, including the one I mentioned already. If the choice is to use only the UX5 pentodes and beam tetrodes, any good 5k-6k primary output transformer would do, again – including the one I mentioned.

The RH45 (standalone version) would require a 5k-6k output transformer. Since the output power is rather limited at almost 3W, it could even be built with output transformers salvaged from EL84 consoles, the type of output transformers that DIYers often use to build RH84 amplifiers with success. But I guess that most would opt for higher quality alternatives, since the 45 is a highly coveted and rather rare tube.

Amplifiers, Tubes, Lifetime, and Boredom

On the other hand, expensive boutique tubes and rare NOS tubes are not something I would recommend. Good examples for this might be the (original) NOS WE 300B tubes, including the later manufactured WE300B tubes (no comments necessary here I believe) or the EML tubes. While the various Emission Labs tubes might be of exceptional manufacturing quality, viewed from a historical perspective I cannot see a rational reason to use such tubes at exorbitantly high prices (take a look at the price list and ask yourself are you buying historical rarities or tubes made yesterday “to exacting standards”: the prices are ridiculous since you probably cannot use them as a means to avoid taxation, unlike investments in real estate). To rephrase the previous tought, while they might last a long time, they will not last forever, and only time can tell whether they will last as long as the “original” WE300B tubes were reported to last in the theater amplifiers (decades, or tens of thousands of working hours). Even if they do last 20 years of everyday operation, I guess one might get tired of listening to the same amp and the same tubes for 20 years? (while WE was probably happy for not having to replace the tubes in the amps they rented and which represented a source of income and business venture). Well, life is not a permanent condition, too – while you might reuse your transformers in a new amp one day, with different output tubes, what good is a tube that might last 20 years and it costs more than several complements of tubes for several amplifiers that you might use in those 20 years? Besides that, high precision and attention to detail is today represented as a path to great sound, while just throwing a glance on the interior of some of the tubes renowned as great sounding (45s, particularly the globe versions) should make you wonder how is it possible to show such low attention to detail (was it, really) and still get lasting quality and great sound? Last but not least, even if a tube is capable of exceptional sound quality, it will not provide said sound quality without a good output transformer, good passive components, maybe good drivers… not to mention good schematics exploiting the quality of the components: a tube is just a component that we use for a given period of time in our lives.

I am aware that manufacturing tubes in small volume nowadays, and eventually starting from scratch, is not cheap or investment efficient, I frequently ask myself how is it possible for the Chinese factories to produce acceptable replicas of “extinct” tubes? While those tubes can hardly be considered exact replicas, they indeed tend to work just fine in their own right. I am not questioning the quality difference between boutique and Chinese production, although it is often perceived as higher than it actually is, but addressing the quality vs. price ratio, as an obvious function of the perceived value. Sometimes marketing is used to mix true and useful information with not-so-true statements backed by assumptions…

Anyway, this type of “boredom” with common places in life and all the repetitive experiences of our everyday routine (and the same tubes for 20 years) is something that I am addressing with the RH-TTA: you may as well build several amps, but finding place for them on your shelves might be a problem - or you might not have the time to build new amps as often as you might want to. A flexible amplifier will let you use and explore many tubes, changing nuances and enjoying your music without too much effort: not even having to remove cables to connect another output, or another amplifier. While the RH Universal was directed at those requiring power and simplicity, the RH-TTA is directed at those who are interested in exploring the possibilities while output power is an issue relegated to the background.

The Tube Tester Amplifier

This amplifier is capable of using indirect heated tubes without modification, but most tubes I have used or tried it with are direct heated types. At first I was thinking about “DH-Universal”, but the presence of a number of switches that modify the characteristics to the tube, and the paralleled sockets are reminiscent of a tube tester.

First of all, there are 3 switches necessary to choose between 3k and 5.6k primary resistance – those should not be operated while the amplifier is operating, of course, just like all the other switches: all those choices are not something that is made “on the run”, but a choice between conditions required to operate. Located between the output tubes are two additional switches: one is used to exclude the voltage dropping resistor from the cathode circuit (necessary when operating the pentodes and beam tetrodes, since their bias voltage is less than 20V and the resistor would preclude correct operation of the current draw regulator), having a “triode” and “pentode” setting. The other switch is used to choose current draw, 60mA or 36mA (for the 45), which is achieved by bypassing the additional 15 ohm resistor.

Using the switch to bypass a resistor means that if the switch loses contact, the worst case scenario is the most benign – tubes will be operated at 36mA, and the voltage dropping resistor will remain in the cathode circuit at all times. The operation of the amplifier will be compromised in terms of performance for the duration of the fault, but neither the tubes nor other components will be at risk.

On either side of the rectifier tube, near the cathode connections of the output tubes, two larger switches (25A contacts) are used to choose between 2.5V or 6.3V secondary. This obviously allows to use i.e. 2A3 or 6B4G tubes. The unused secondary is left “open”, thus no current is drawn and no additional heat dissipated. Indeed, trying to operate 2A3 tubes on 6.3V might damage them… but I guess the DIYer is going to be aware of that – on the other hand, 6B4G tubes will not light their filaments at 2.5V and there should be no sound output or current draw since the cathodes will not be able to emit electrons: this condition might damage the tubes as well, contrary to what many believe.

The last switch is located out of sight on the back side near the power transformer, since it is rather rarely used. The purpose of this switch is to allow 5.5V heaters operation with the correct voltage – it is an 8 contacts 25A switch which is basically bypassed from side to side with 0.39 ohm resistors. When the switch is closed, the resistors are excluded and 6.3V are directed to the heaters voltage switch. When the switch is open, the current flows through the resistors: at 1A current draw each resistor drops 0.39V, thus 5.52V are delivered to the heaters voltage switch, to be used for the 307A.

If built with UX4 sockets instead of UX5, or if UX4 to octal adapters are used, the 300B tube can be used as well – the 8 contacts switch has to be fitted with different resistor values (0.56 ohm) to provide 5V to the 300B heaters. It is questionable whether it makes sense to use the 300B tube in the RH-TTA, since the output power is going to be limited by the available voltage and current draw, in a similar way in which other more powerful tubes are limited. The 300B will yield just 5.5W at 1% distortion – but the DIYer who would build the RH-TTA is probably satisfied to get 5W from the 2A3 and might be interested in getting approximately the same power from 300B tubes – without building another amplifier.

While the 300B can actually yield a lot more output power, operating it at low anode dissipation will without doubt guarantee long tube life and trouble-free operation. Those for whom 5W might be enough would probably enjoy the 300B and its characteristic sound in this circuit…

Last But Not Least – The Sound

All this text, and no mention of how does the amplifier perform in the music reproduction scope… basically, the sound quality issue was covered in my post on the RH2A3/1619 amplifier, thus the points worth mentioning are both comparison between the sound of various tubes, and the implication of having a finalized well laid-out box against a breadboard amplifier.

The RH-TTA ended up being quite large by my standards – it almost dwarfs the RH300B. I was unable to fit all the necessary parts in the rather small box in which I have fitted the RH300B, but I used the extra place to ensure that there is no interaction between the various elements.





All transformers are hidden inside the box, and the toroid transformers are vertically mounted, further minimizing the effect of their field on the amplifier. With 2.5V output tube types no noise or hum is audible even with an ear on the woofer or midrange of my 88dB/W/m speakers – although the heating is AC. Enough said: no need for DC on the heaters in this amplifier, and that is probably one of the reasons it sounds as good as it does – but this is a different topic, one that may require further research and discussion.

The sound has improved in comparison to the previous impression – maybe the output transformers have gone through some break-in period, and probably the remaining components have also benefited from a break-in period. I am not a big fan of the breaking-in theory: either it works or it does not… but it is a matter of fact that improvements in time can be heard. Good quality stuff usually sounds good from the first note, although it might improve with the passage of time. The basic tone quality of the amplifier remains unchanged regardless of output tube used, but each tube brings its intrinsic sound quality to the mix, like a distinct flavor.

The overall winner in the triode class is the NOS 6B4G (double-plate, black anodes), showing a margin of midrange quality above the Shuguang special octal 2A3 (bi-plate, black anodes) version: the new generation of mono-plate current production 2A3 tubes probably sound slightly better and can be directly compared to the NOS 6B4G. By the way, I was using this same pair of 6B4G tubes for years in a classic no feedback SE design, and they have probably worked at least 2000 hours – but they still look and test like new… and their price when manufactured was nowhere near the asking price of the current production boutique tubes. As a rule of thumb, the 2A3 family tubes are slightly forward in the midrange, but the midrange nevertheless shows warmth of tone.

The 1619 is my favorite in the direct heated pentodes/beam-tetrodes class. While being a rather ugly metal tube (no heaters to warm up your sight), it has no cap and the envelope is grounded, thus very safe. The sound is very liquid, with an overall quality that goes a long way towards beating the particular qualities of the 307A or the warmth of the 2E22. I have not tried the 1624 in this circuit since I haven’t got any – and it would be interesting to assess whether it is better or worse than it’s lower power metal sibling. As a rule of thumb, the pentodes tend to leave the impression of better extension and smoother frequency response, without midrange forwardness – but without the midrange warmth shown by the DHTs. This is more than anything else a matter of taste – the choice between rich overall tone, or pronounced yet warm midrange (more or less).

Finally, the highly coveted 45: I am not mentioning it in the triode class since it is operated at lower current and has lower power dissipation. The 45 tube is in a class of its own. My expectations were indeed very high, and while this is not “sound like I have never heard before”, the 45 does, even in the newer ST shape (the only 45s I have are Sylvania ST shape, provided by a DIY friend) confirm the intrinsic qualities that it was coveted for.

The particular sound of the 45 is rather different than the 6B4G some consider as its descendant (in particular, the bi-plate version is often considered as two 45s in a single tube). The 45 shows none of the mellowness expected from DHTs – on the contrary, it is lightning fast and detailed. While bass is well defined, fast and quite strong (unexpectedly so), the mids and highs are extremely detailed, liquid, and fast at the same time. The closest approximation to this type of sound is actually the “fake mesh” globe shape 300B of current production. Just like this particular 300B might seem bass shy, the 45 seems to lack some bass volume – at least until the listener understands that it is more about a lack of oomph, while the vibrations are very much present. But the particular quality of the mids and highs is what fascinates the listener – making it difficult to change the 45 for another tube, just like many prefer the sound of the “fake mesh” 300B to other more solid types, due to the transparency and liquidity of mids and highs. The less pronounced bass, the power limitation (on rather inefficient speakers in a rather large room) – it all fades away against the quality of the mids and highs. There is neither the particular richness of tone like with the pentodes, nor the forward but warm midrange characteristic of the DHTs: the strength of the 45 seems to be the speed, and liquidity, combined with a forgiving distortion pattern reminiscent of dust showing in rays of sunshine. On the other hand, the 3W provided by the 45 in this amplifier can go quite loud, and the soundstage thrown is very large and airy.

Credits

Just like on previous occasions, I would like to thank all those who support my work – in particular Mr. Per Lundahl, and a group of DIY-ers based in England who post on the audio-talk forum. New designs and developments would remain just ideas and afterthoughts without the help of friends from all over the world who follow my work and my blog.

The RH300B Story

The RH300B project was the first envisaged in the 2^nd generation series, but the last to be developed. The development of this amplifier was long stalled by material reasons, mostly because I had no 300B tubes to build and breadboard it. Thanks to the generosity of a DIY friend who sent me two pairs of these DHT tubes, I was finally able to breadboard, troubleshoot, and finalize the project.

While an initial circuitry was published on some forums and sent to some DIY-ers back in 2005, when the time came to finalize the project some design choices were to be reconsidered. Thus the RH300B introduces a different driver stage in respect to all previous RH amplifiers, while simplification was introduced in the power supply section.

 

For the first time, I was hesitant and reluctant to publish the schematics of this amplifier. A lot of work, knowledge, and insight have gone into the design of the amplifier, and the various difficulties encountered on the way clearly show that these projects are not to be taken lightly. Good ideas and lots of knowledge are not enough; building an amp definitely requires some material means. RH amplifiers have met the criticism of all those who felt threatened by the publicity gained in particular by the RH84, and while many DIY-ers have built and enjoyed one of the several designs, the RH series has also spawned lots of copy-cats, and word is out that some are building RH amps for their local clients – mostly DIY-ers without the time at hand, or the experience required to build their own amps: while the clients should not be judged for feeling challenged by the prospect of building a tube amplifier, since it is among other issues dangerous as well (these voltages could kill you), the “hired builders” should at least give some credit to the original designer and avoid changing important parts just to make it look different at first site. There is a site (I will not paste shortcuts, but you can easily find it based on the description) showcasing a parallel SE amplifier with 807 tubes which is basically a copy of the original RH807 adopting 2^nd generation solutions taken from the RH88, while the only difference is fixed biasing with negative voltage instead of a cathode resistor under the output tubes; the site owner has reluctantly agreed to mention my work by adding a blurred statement how his amp is in fact inspired by work of a compatriot of his, whose triode connected 807 amp (sic!) was in turn inspired by one of my designs… is there any shame left in people? Thus this amplifier represents the last in the 2^nd generation series, published for all DIY-ers.

The Schematics

As it is evident from the schematics, the driver section now has a cathode follower directly coupled to the driver itself. While the driver is a rather standard choice for RH amplifiers – an ECC81 family tube, with anode-to-anode feedback connection to the output tube – the cathode follower portion of the driver is not. The cathode resistor of the driver tube is split in two, of which one resistor is bypassed with a capacitor – which is also a first in RH amps. This particular split is necessary in order to maintain a correct biasing of the driver tube while increasing the gain, in order to keep the input sensitivity below 2V RMS for full output (as declared at 1% distortion).

The cathode follower is not strictly necessary, as the driver tube is well capable of driving the output tube on its own. This particular detail appeared as a necessity while contemplating biasing methods for the output tube, since fixed bias requires the adoption of a grid resistor lower than 50k. With such a load, the driver was not capable to perform its task adequately, and the cathode follower represents the best way to solve the problem, since it does not invert the signal nor add any gain, while it is possible to couple it directly to the driver tube – avoiding additional capacitors in the signal path.

In the end, fixed bias was discarded, since the only apparent benefit would have been a lower B+ voltage. On the other hand, the negative bias power supply represents and additional cost, complication, and a source of potential trouble for the DIY-er. Furthermore, it would require checking the bias voltage of the tubes from time to time, as well as adjustments every time the output tubes are changed. All the amplifiers I have designed so far were easy to build and straight-forward in use, which to me represents an important feature – thus fixed bias (i.e. bias by negative voltage applied to the grid) was discarded as it brings more potential problems than eventual benefits.

Lowering the output impedance and increasing the current draw has enabled lowering the B+ voltage which mostly compensates for the only possible gain from fixed bias – as a result, DC RMS voltages are never higher than 470V (with 5U4 rectifiers) which helps keep costs down while the second cap in the power supply remains safely in the 500V WKG category (of course, with both channels operating, i.e. drawing current).

Finally, the cathode follower has remained in the schematics, since it obviously helps the driver tube perform its task, and keeps distortions at even lower levels. While the driver tube should be an ECC81 family member (12AT7, ECC801, 6201, CV4024, etc.), the cathode follower can be almost any small signal triode tube – almost, because once you choose the socket and the basing connections, there is little variation possible – 5687 is one example - and of course you have to keep in mind the operating conditions for this tube, which preclude some choices (most ECC88 tubes cannot be used, and even E88CC types are not advisable). But if we stick to the more common noval tubes, it could be an ECC82/12AU7, ECC81/12AT7, even an ECC83/12AX7 – while in the octal domain, a 6SN7 would do the job perfectly well (12SN7 would suit just as well, but you will have to provide 12.6V for the heaters, and connect the heaters of the driver tube in series mode instead of parallel). Whichever tube you choose, as the cathode follower section is directly coupled to the driver tube it will be biased to approximately the same conditions. What will vary is the bias voltage of the CF tube (grid to cathode differential) as well as the input impedance (approximately ranging from 2meg to 5meg) and the gain (in CF slightly lower than 1, depending on the mu of the tube used). It is thus not unimportant which tube is chosen or used – as it will have an impact on the sound of the amplifier.

The cathode follower section requires referencing the heaters supply for the driver tubes at approximately 20% B+ voltage. Most datasheets recommend not exceeding the heater-cathode differential of tubes by more of 100V DC, thus it would not be a good idea to have the cathode of the CF tube at approximately 160V DC without doing something about the potential of the heaters. The best option is to reference the heaters to a potential between the cathodes of the two driver tubes, and a 220k/47k voltage divider will perform this task perfectly well.

Of course, having two triodes per channel helps employ one double triode per channel in a mono-block amplifier configuration… there is no reason why you could not use the ECC81 as the CF tube as well, and as a matter of fact it provides the highest input impedance among all the proposed tubes.

The output tube, as you already know, is the 300B. NOS 300B tubes are probably extinct by now, or too expensive, and I cannot expect that DIY-ers – people who are not ready (willing, or able, choose your game) to spend 5 figures in USD or EUR on Hi-End amplifiers (Hi-Cost no doubt, the Hi-End sound is in the ear of the beholder) – would spend 4 figures on the output tubes alone. Thus the amplifier was designed, bread-boarded, and finalized keeping in mind the current production 300B tubes.

The biasing of the output tube is automatic as customary in RH amplifiers, set by a current source (or sink, if you like) made by a simple LM317 and current setting resistor. Regardless of the tube inserted in the socket, the current draw will always remain the same, and since this is a triode, there is no second grid to take into account – all current drawn is “anode current”. The current is set in such manner as to be near the upper limit for 300B tubes, while the voltage drop across the tube, deriving from the combination of current draw and B+ voltage, puts the anode dissipation between 30.5W and 33W, depending on the chosen rectifier tube (5R4 or 5U4 types). The anode dissipation is on the upper conservative level, which basically means that you will not unnecessarily shorten tube life while enjoying the best sound those tubes can provide. This relatively low anode dissipation is certainly adequate for the mesh anode types, either “mesh anode” or “plate with holes”. Those seeking additional thrills can always use a GZ34 (5AR4) rectifier for higher B+ voltages resulting in higher anode dissipation (about 36W) – this is feasible but not at all necessary.

As for heat and tube life, I have measured the temperature on the hottest point on the glass of the globe “mesh” 300B tubes after several hours of operation at approximately 33W dissipation, and it does not exceed 100⁰C. Real datasheets for current production 300B varieties do not present the same richness of data as the datasheets from the golden age of tubes, but I guess that an output tube in an SE amplifier, operating at temperatures lower than that of the rectifier tube in the same amplifier, where the rectifier tube is definitely running inside the safe envelope of operation, means that the output tube is not stressed and will not encounter a premature end – at least due to anode dissipation – regardless of what some tube dealers would like you to believe. On the other hand, filament defects (breakage) are definitely not caused by the ability of the anode to disperse heat, and should be addressed in a different manner.

Last but not least, the output tube cathode circuitry is virtually the same as the configuration already explained for the RH307A amplifier. Since the LM317 can only handle up to 35V input, the inserted resistor keeps the environment safe for the current regulator. Regardless of 300B type, or rectifier used, the voltage across the regulator cannot exceed 30V. Another solution would be the TL783, which is good for up to 125V input – but besides being relatively difficult to source, it would have to dissipate up to 7W, requiring a very serious heat-sink (temperatures inside the amplifier should be taken into account as well). The voltage dropping resistor should be powerful enough to withstand up to 5W dissipation for a long period of time (11W types and higher recommended).

The heating circuitry is AC, but 50Hz hum is absolutely low and completely inaudible on my speakers, due to the cathode circuitry solution and the general schematics of the amplifier. Even on more efficient speakers the AC heaters hum should not be prominent enough to annoy the listener. While the question which heaters power solution provides the best sound – AC, DC, unregulated, voltage regulated, or current regulated – it is my opinion the AC probably sounds best due to the fact that the whole filament is kept at the same potential level. But leaving this discussion aside, since each and every amplifier should be regarded as a whole, another great advantage of AC powered heaters is the simplicity of the solution. If the inevitable mains frequency hum is not an issue (in this amplifier it should definitely not be a problem), AC is the best, cheapest, and most practicable solution by far.

The Power Supply

As already mentioned, the power supply is simplified in respect to previous RH amplifiers. This is a normal cap input power supply, where the rectifier is combined with solid state diodes in a hybrid bridge. The hybrid bridge is more efficient than the standard full wave with central tap rectifier, and puts less stress on the rectifier tube. It is also by far less complicated to set and use than an all tube bridge rectifier, which would require more tubes and additional secondary windings. The sound (contribution to the sound of the amplifier) of the hybrid bridge is virtually identical to the sound of the same rectifier tube used in a classic central tap arrangement, providing a win-win combination. Rectifiers can be switched easily, either to tune the sound of the amplifier, or to increase (or decrease) anode dissipation. Since output tube biasing is automatic and always the same (set by CCS), there is no need to adjust anything when changing either output tubes or the rectifier tube.

The power supply is cap input, and this first cap should be a 600V poly or oil type – a good choice would be motor run caps rated at least 400V AC. The second cap should be a 500V type, and it can be a good quality electrolytic. There is no need to bypass any of the two caps with smaller caps. The choke is not critical as the only important element is DC current handling – a 200mA type is the minimum requirement. As for inductance, 5H or more is recommended for a good level of filtering.

The Transformers

The RH300B can be built with just one power transformer, or with several, depending on the size of the enclosure and the willingness of the DIY-er to order custom wound transformers. I have built mine employing a 150VA custom wound toroid for the high tension and rectifier heaters, and another smaller custom wound 30VA toroid for the heaters of the output tubes and the drivers. The two cannot be squeezed in a 150VA core, but could be wound on a 200VA core which would probably be a more expensive solution due to the large number of secondary windings. This even larger toroid would also require more space, both as sheer physical size, and probably due to the more complicated placing of the secondary windings output wiring. There is absolutely no need for a more powerful high tension secondary than provided by the schematics: while the DC current draw of the B+ is slightly above 210mA, the AC current draw as measured on the HT secondary is 270mA – less than foreseen by PSUD2 or calculated using the usual formulas. The temperature of the transformers in operation is up to 60⁰C and 40⁰C, respectively, in the box, after several hours of operation – thus it can be regarded as absolutely recommended.

While the schematic requires 5V secondary windings for the output tubes, it is advisable to adopt 6.3V secondary windings and drop the voltage to 5V across 0.56 ohm resistors on each leg. This will limit the current drawn by the cold filaments at power-up and definitely increase the life of the tubes, since filament failure through breakage at power-up is a common problem with many current production direct heated tubes.

A word about the output transformers – as shown in the schematics, those should be 2.5k primary impedance types. The secondary, of course, should be adequate to the nominal impedance of the speakers used. As for size, while size does matter, manufacturers tend to rate their amplifiers as output power, which is a relative point of view. You could build a 2A3 amp and a 300B amp using the same output transformer, as both tubes would work into 2.5k loads – but while a classic SE 2A3 amp will provide approximately 3W, the 300B will provide approximately 8.5W, thanks to its higher anode dissipation, and in particular due to the higher current draw.

Thus the most important part when choosing output transformers for the RH300B should be whether they are built to operate with 100mA DC current across the primary winding. If that request is met, all the remaining details (physical size, DC resistance of the primary, etc.) will work towards achieving a more or less extended and defined output, introducing additional distortion and loss in a more or less pronounced manner. While the RH300B thanks to the circuitry will have an excellent damping factor (DF), and will sound perfectly well on most output transformers that are fit for the current, primary impedance, and output power – since this is really a top-notch project, you should try not to spare on the output transformer. What is true of the RH84, re-built by many with high quality parts and output transformers to enjoy its qualities even more, relates even more to this amplifier, as the parts quality will be rewarded with increased sound quality.

End Results and Remarks

This long awaited amplifier, while being the first to be designed in the 2^nd generation series, was the last to be finalized and built. The RH amps started as an alternative to classic DHT SE amps, succeeding in offering better sound and at least similar output power, step by step, DHT by DHT. The RH Universal easily over-powers classic 300B amps while providing better bass and definition, while the RH307A offers approximately the same power of classic SE 300B amps from a less expensive tube (while providing better bass and definition), and adds the thrill of direct heating, the particular sound of direct heated tubes… What is to be expected of the RH300B, since this is the tube mostly addressed by previous RH amplifiers with pentode (and beam tetrode) output tubes?

All RH amplifiers sound quite similar – since they have been designed by the same person adopting the same scheme type, which is quite rigorous on the tubes, imparting a particular type of sound. The main difference, beside output power, is the output tube itself, as the intrinsic sound quality of the tube will define the overall sonic quality of the amplifier. Direct heated tubes definitely have some interesting edge when it comes to sound, and the 300B is probably the most coveted of all DHTs – due to its intrinsic sound qualities, and the relatively high power it can provide, which is necessary in our world of relatively inefficient speakers spawned by several decades of powerful transistor amplifiers rule on the commercial Hi-Fi market.

Well, this amplifier is virtually a nail in the coffin of classic SE DHT amps. Whatever the 300B tube used (since there is currently a relative multitude of current production 300B types, and of course the almost extinct NOS WE) would provide in a classic SE design, it will do better and provide more in the RH300B amplifier. This is not any more the case of comparing pentodes with the 300B – but using the same “weaponry”. As a matter of fact, this was an easy win.

The simulation clearly shows 12W output at 1% distortion, with almost 2V RMS input sensitivity. I do not have any equipment that I could use to provide measurements, so DIY-ers will have to believe my word – or the simulation presented. Nevertheless, what matters is the sound, and the relative loudness that the amplifier can achieve in a system. In practice, the RH300B goes just as loud as the RH Universal does – on my normal real-world 88dB/W/m speakers – quite loud by anyone’s standards. There is no hint of the usual syrupy sound of SE 300B amps, which is so good for taming wild high efficiency horn speakers… instead, the sound is extremely detailed and controlled, with great bass definition and depth. Such statements are not to be taken lightly, but those who have successfully built any of the previous RH amplifiers probably know that their expectations will be met and exceeded.

If you ask me, my only regret is the fact that the 300B, being a triode, does not have a second grid that I could use to my advantage, like a lever to increase performance even further… It is interesting how many attempts have been made to copy the 300B, including indirect heated prototypes, curve clones, that were made out of pentodes in which the second and third grid were missing, not wound on their supports during manufacturing. All this trouble to construct a good triode, when there is so much to be had from pentodes – only because the knowledge required for that task has been lost or misplaced. Humans sometimes find it easier to replace an object, or resource, than to learn how to use it properly.

What Next?

This amplifier represents in a way the end of the 2^nd generation series. The next, 3^rd generation of RH amps will strive to go further, including different and maybe unexpected output tubes, and the circuitry solutions necessary to make these tubes work at their best.

Designing these amps takes a lot of time and thought, as I try to see the eventual problems in advance and design including simpler and more effective solutions, never straying from the path of excellence: the sound comes first, since those amplifiers are designed to enable enjoyment when listening to music, not winning awards. Bread-boarding an amplifier requires resources which are neither cheap nor easy to source (particularly in my country). The RH300B exemplifies this more than any of my previous amplifiers, as it was not possible to bread-board and build the finalized amp without the output tubes, and the lack of other resources, like output transformers and chokes, hit me more than ever before: for instance, the output transformers had to be extracted from the RH307A (those are 2.5k into 4/8 ohm transformers, when connected to 8 ohm speakers they present a 5k load to the output tube – the same ones pictured in the RH84 PPE prototype), and I had to find a way to design a good sounding power supply using only one choke in a cap input configuration…

There are no publicity banners, nor a Pay-Pal button on this blog. My efforts have always been totally non-commercial. But without help from DIY-ers who respect my work, I am not going to be able to continue designing and sharing amplifiers. All those who would like to help with output transformers, chokes, tubes, sockets, caps… are welcome to contact me.

RH-307A v2 "Super"

The RH-307A has undergone further development, with the aim to improve output power and explore the possibilities of the circuit, as well as to improve reliability - achieving the same project quality other RH amps are known for.

The Driver Circuitry

I have initially considered the 15W dissipation rating of the 307A tube a limiting factor, and the 9.1W output foreseen in the datasheet as overly optimistic. Thus the circuit was developed around the 2^nd generation RH driver circuitry idea - using the feedback resistor as the only anode resistor. This circuitry inevitably leads to a limitation in power output: graphically represented, the sine wave of the driver tube is in opposite phase with the sine wave of the output tube – and the peaks of the two will inevitably collide at a given power output, unless the difference in potential between driver and output tubes is larger than the combined voltage swings. The latter is virtually impossible to achieve at common operating voltages, and represents a limitation to the output power – which does not necessarily represent an important factor if the output power cannot be much higher anyway due to other limitations, like the anode dissipation of the output tube.

The 307A has actually shown itself as an unexpectedly powerful output tube, which probably due to its direct heated nature largely surpasses the output power of other pentodes with similar maximum anode dissipation. Thus the next step was the introduction of a “classic” RH driver circuitry, albeit of the more “modern” style that can be found in the RH Universal v2. The resistor ratio (anode to Rfb) is slightly skewed, although the purpose of such values is not limited to confusing the critics or those who taught they knew everything that was to be known about the subject of anode to anode feedback… Anyhow, this type of circuitry does not limit the output power by colliding the two sine waves, thus power is only limited by the characteristics of the output tube.

Reliability

Some 307A (VT225) tubes will exhibit a tendency towards screen grid arcing at voltages near the maximum operating condition as shown in the datasheet. In practice g2 might arc (particularly at power-on) if operated near 300V, and this arcing actually happens towards the nearby g3, since the suppressor grid is connected to a low potential point (the cathode or virtual cathode point).

This does not occur with all 307A tubes, nor does it manifest itself regularly in affected tubes. Most documented cases where this tube was used in amplifiers are related to triode strapped operation, thus arcing has not happened as most tie both g2 and g3 to the anode. In pentode operation, tying the suppressor grid to the screen grid, or the anode – would cause the appearance of the “tetrode kink”, with all the negative consequences, and is therefore out of the question.

A very easy solution to this problem is operating the 307A in pentode mode with lower screen grid voltages – conditions that this “filamentary pentode” was actually intended for as a transmitting tube. I have thus made the choice to set approximately 200V as the g2 operating voltage – a value totally safe from arcing in all conditions on all of the 307A tubes I have tried.

With solid state devices in the circuit, like zener diodes and voltage/current regulators, the arcing which has otherwise not damaged the tubes or other parts of the amplifier, has succeeded in killing both the zener diode and LM317 on the interested channel… as a further reliability measure, and also after reconsidering carefully the datasheet, I have decided to connect the suppressor grid (g3) directly to ground, instead of connecting it to the “virtual cathode point” as it would have been customary and usual. When g3 is grounded, the arc from g2, if it ever happens, will miss the cathode and relevant circuitry (LM317). Also, it should be taken into account that the operation of the 307A can be controlled as well by the suppressor grid, and the potential at which it is set – another good reason to connect it directly to ground (i.e. 0V).

Additional Effects

Operating the screen grid at lower voltage means that the anode will draw less current with the same cathode to control grid voltage differential. If this amplifier had a cathode resistor, the value of the resistor would have to be adjusted. But since the current draw is controlled by a current setting device (LM317 with current setting resistor), the result will be a lower cathode to control grid voltage differential, about 15V instead of the almost 30V of the original version where g2 was operated at 300V.

Needless to say, loosing 15V less in the cathode circuitry means drastically cutting on the dissipation for the LM317, with all the theoretical and practical reliability improvements. This also means having 15V more across the tube (anode to cathode) which leads to an increase of anode dissipation since the current is fixed. While some have reported operating the 307A tubes at 22 or 25W dissipation, and 80mA current, I have chosen to stick to the values given in the datasheet – 15W maximum dissipation and 60mA maximum current (set to approximately 43mA anode current). Since the 307A/VT225 is not a tube in current production, and the stock is going to dwindle in years to come, although I think that life is too short to be squandered with sub-optimal solutions – there is no need to burn your (rare) tubes too quickly.

Since the tube is forced to conduct a set current, this means that the characteristics change – a lower input signal will lead to the same output power – the sensitivity of the amplifier increases almost twofold. The effect of this change is obviously audible – the amplifier is more dynamic sounding than the original version: the percussion attacks are more pronounced, it seems as if the amplifier has gained speed. If “syrupy” is how many would define classic 2A3 and particularly 300B SE amps, this is quite the opposite.

The change in screen grid operating point is achieved by increasing the value of the zener diodes. While it could be easily achieved using one 150V zener diode, two 75V 5W zeners are a far better choice. It goes without saying that the power of the zeners needs to be quite high, since their power rating is greatly derated with temperature (and tube amps tend to be warm or even hot). Still, a 150V 5W zener should do, but its dynamic resistance is much higher than the dynamic resistance of two 75V 5W zeners in series – this dynamic resistance is obviosly very important to achieve the particular sound in amplifiers where zeners are used to drop voltage and set the g2 operating point.

The zeners used to drop voltage to screen grids (instead of resistors) seem to be susceptible to performance degradation, and the result is a relatively fast decay in bass (low frequency) performance. This is a topic which, unfortunately, is totally un-documented elsewhere on the net! The results of zener performance degradation can be easily experienced experimentally by connecting a grid stopper resistor between g2 and zener diode – the bass will be filtered and bass levels lowered, as if some RC filter was introduced. Removing the resistor restores low frequency extension, and the same effect can be experienced when a degraded (but otherwise normal in operation) zener diode is changed for a new zener diode of the same type.

Of course, one way to avoid this issue would be supplying the screen grids from a regulated source, or even an additional power supply… but besides being large and complicated, this solution would also miss one important issue – the specific sound achieved when screen grids are connected to the B+ via zener diodes. Thus the “free lunch” solution would be using zener diodes of much higher power, possibly achieving the desired voltage drop by putting them in series, and, last but not least, keeping them as cool as possible by physically separating the zener diodes from sources of heat – for instance, not soldering the diodes directly to the g2 pin…

The series of two 75V 5W zener diodes, placed separately and not soldered directly to the sockets or to the anode resistors, definitely solves this issue with the least of cost and complication – while keeping the particular sound character.

The classical driver circuitry removing swing limitations allows to obtain full output power, but also more freedom in the choice of driver tubes. In this case, it meant getting back to the ECC81 family of tubes, and in particular to the 6201. It is more than obvious that an amplifier will work as foreseen by the simulation or mathematical calculations based on the schematics – but there is more to sound than simulation or mathematics. While circuit simulation with good models allows setting the best values for resistors and estimating frequency response, power output, and distortions – the quality and intrinsic characteristics of the tubes used will have an important influence on the sound, which cannot be simulated. Just like the 6AU6 pentodes are no match sonically for the 6201, or the E180CC, the E88CC is also not playing in the same league. While my choice of 12AU6 was relatively limited (although RCA black anode always means high quality in my dictionary), I had a lot of various ECC88 family member to play with… even the famed CCa does not come close to the sonic performance of the 6201 as a driver in RH amplifiers. Getting back to the ECC81 family, and in particular to the 6201 – is like the “return of the King”.

“Super”

Well, after solving the reliability issues, improving sensitivity and speed, and changing to a preferred driver, I became aware that the small output transformers are probably a limiting factor, since the amplifier is already capable of sound volumes way higher than the RH84 (the SE version). Thus the small output transformers capable of maybe 5W output have been swapped for larger units (E108 size lamination).

Larger than necessary output transformers are regarded as a trade-off, most are afraid of bandwidth losses (high frequency loss due to higher parasitic capacities). Not in this case… the output transformers used are 5k into 8 ohms, with primaries foreseen for 100mA DC current: if output transformers could be estimated by power, these would probably be rated around 25W. While this looks as a total overkill, the results are awesome with this amplifier: not limited by the small output transformers, the bandwidth extension is nothing short of astonishing, and output power is almost at RH-Universal v2 levels. This should come as no surprise, since the WE datasheet states 9.1W output power at 300V across the tube into a 4.5k load. With 350V across the tube, anode to anode feedback loop, approximately 43mA anode current draw… if the datasheet is of any relevance, no wonder there are about 9W of undistorted output into a 5k load. Thus the “Super” in the name of the amplifier – if built with adequately sized transformers, it will undoubtedly outperform classical 300B SE amplifiers both in sound and power output.