RH813 – The Flagship Project

After designing several amplifiers with pentode and beam tetrode output tubes, and trying multiple options in the Universal and TTA amplifiers, the time has come for a “exclusive tube type” project. This amplifier was designed for the 813 beam tetrode tube, and due to size, voltage, heater voltage, and socket constraints, it is not possible to make it more universal than it already is. GK71 output tubes could be used instead of 813, but it would imply both different socket wiring and cap, and different heater voltage.

This project is by all means a “flagship”, in terms of output power, complexity, and costs involved. While I have tried to make it as easily reproducible and simple as possible, the voltages are nevertheless higher than in most common DIY designs, and the output power requires very serious output transformers. In order to make the amplifier as smooth as possible in operation, several additional features are introduced – thus in the end anyone can operate it without particular technical awareness: just turn it on, and enjoy.

The complexity and costs involved in this project imply that without the help of several DIY enthusiasts who post at the Audio-Talk forum, in particular Paul Barker, and the help of Per Lundahl of Lundahl Transformers, I would not be able to present it. Thus I would like to express my gratitude to those generous people before exposing the basics of the project.

 

Design Goals

The first and foremost design goal with this amplifier is achieving high output power – 30W and more, while using only one output tube per channel (i.e. pure SE operation). The 813, being a beam tetrode tube, lends itself very conveniently for that task, particularly having in mind that beam tetrodes and pentodes are more efficient than triodes.

Most high power SE amplifiers are built with transmitting triodes, like the 211, 845, or GM70. Leaving aside the question whether all these amps really achieve above 30W power ratings, truth is that all are operated at very high voltages, the B+ being 1kV and above. All voltages involved in tube amplifiers are dangerous, but it seems that everything above 500V requires extra care both in components choice, and how the amplifier is approached and measured. At high voltages arcs jump easily, and people can get electrocuted. That said, my idea was to keep the amplifier operating at voltages as low as possible, while achieving the 30W and more output power set as a goal. Most common digital multi-meters can measure DC up to 1kV, thus I assume that keeping the power supply around 800V is reasonable enough choice.

High power SE amplifiers with transmitting tubes operated at high voltages imply another difficulty – the fact that the driver tubes are usually operated at much lower voltages. While this can be easily solved with an additional power supply, my design goal is to keep it reasonable and simple – one power supply should be enough for all stages of the amplifier.

 

Design Choices

Keeping the B+ around 800V greatly simplifies the requirements for the capacitors – I have chosen to use motor run “cans” rated 600V AC, which should be safe for at least 1kV (and probably more). Besides the fact that those caps impart a particular sonic signature to the amplifier, used in this application they can be expected to last very long – the DIY-er will probably never have to change them. Another option is serial connected caps, where caps rated 500V DC WKG in series would be more than adequate – and due to the nature of the tubes used, and design choices made, even 450V DC WKG caps in series would probably be safe enough. Of course, the standard 400V caps could be used in a series of 3, for a very safe total rating of 1200V. The capacities required are relatively low, thus keeping the power supply as cheap and feasible as possible.

Putting capacitors in series decreases the total capacity, but by adopting a power supply with 2 chokes ripple is easily kept at bay. The example power supply illustrates the ripple at the load being just about 5mV (much less than most SE amplifiers), while ripple at the first cap is less than 150V – easy task for any motor-run cap (which is why they last so long in tube amps, and should be applied to the input cap position). With a hybrid bridge, output voltage will be higher for the difference in voltage drop between a 5U4 and the solid state diode used for the lower part of the hybrid bridge. The two chokes used, LL1638 – have a DC resistance of just 36 ohms, which helps improving the performance of the power supply: if different chokes with higher DCR were to be used, the additional voltage drop across the chokes should be accounted for.

The 813 beam tetrode has rather high CCS (Continuous Commercial Service) ratings, with 2250V maximum anode voltage, 1100V maximum screen grid voltage, 180mA maximum anode current, 100W maximum anode dissipation and 22W maximum screen grid dissipation – all intended for class AB1 operation, and even higher for ICAS ratings… at about 750V across the tube and slightly above 100mA cathode (total) current, this tube is literally “ticking”. Ug2 is set just below Ua, thus no single parameter is at more than 70% of the CCS rating, which makes this a very safe operating point, and probably should allow the output tube the longest possible lifetime.

Screen grid voltage was chosen to be just below the anode potential. This choice is dictated by the peculiar curves of the 813. The datasheet shows a slight kink in the lower part of the curves (well, they are actually more like straight lines), which is the least pronounced at low screen grid voltages, while it increases with the increase in screen grid voltage. Thus we might imagine that 400V Ug2 would be a good compromise – but unfortunately such choice would limit the power output to maybe 15W, which is definitely not worth pursuing in a project of this magnitude. While at 700V Ug2 the kink is more pronounced, the spacing of the curves allows for approximately 35W power output at several load resistance settings.

The curves show that 35-36W output can be achieved at similar operating points with both 10k and 5k load resistance. After trying (approximately) 5k, 7k, and 10k load resistance, I have chosen 5k as the load resistance for this amplifier, on the basis of “best sound”. The increase in load resistance does not yield appreciable improvement in performance – actually, both output power/distortion ratio and sound presentation are favorable with an approximately 5k load.

 

Voltage Regulation Tubes and Driver Section

The driver section is very similar to what already applied to 2^nd generation RH amps – RH300B and the RH2A3 family of amplifiers (RH-TTA). The ECC81 acts as driver tube, while the cathode follower tube makes life easier for the driver providing high input impedance, while at the same time it drives the output tube with low output impedance. Although the amplifier operates in A1 class (driving the output tube in voltage only), low output impedance is beneficial with most direct heated tubes due to the gradual increase in control grid current with the increase of driving signal values. Besides that, the 813 requires a relatively low valued grid bleeder resistor – another good reason to apply a cathode follower to driving this tube.

Just like in the other amplifiers mentioned that share the driver topology – the cathode follower tube could be almost any tube that can meet the current draw, voltage across tube, and anode dissipation requirements. The cathode follower tube is direct coupled to the driver tube, thus it is the driver tube that sets the operating point for the cathode follower tube – the current draw and voltage across the tube will be almost identical for any suitable tube used. In this amplifier I have chosen to use the 6SN7, but the same task could be performed by ECC82 or ECC81 family members – or other similar tubes like the 6J5, 6N7… For this driver topology it is important to correctly set the potential of the heaters circuit by connecting the heaters to a voltage divider like shown in the schematics. The driver tube will have the cathode at a very low potential, while the cathode of the cathode follower tube is going to see a relatively high potential, dictated by the voltage at the anode of the driver – depending on the particular tube used and a few other details, this will range between 100V and 160V DC – while the maximum allowed cathode-heater potential difference is 100V DC.

The purpose of the voltage regulator tubes in the circuit is not strictly shunt regulation of the voltage feeding the driver and cathode follower tubes. Actually, regulating this voltage is not particularly necessary in a class A circuit where there is little variation in current draw between minimum and maximum output power. Thus the purpose of these tubes is to facilitate smooth operation and to present a fail-safe feature.

If a directly heated rectifier tube is used, at power up the B+ will rise quite fast to its maximum level. Since the output tubes are directly heated and compelled to draw a set current (CCS circuit below the cathode), they will start drawing current immediately, and B+ will not rise above normal operating levels. This means that until the driver tube starts drawing current, the anode will be at a very high potential, and will set the grid of the cathode follower tube very positive as well. As the cathode follower tube gradually heats and starts drawing current, the cathode will be at a very high potential and arcing may occur between the elements inside the tube.

While there are several other mechanisms that could avoid this condition (pre-heating the driver tubes, placing a diode between the anode of the driver and the cathode of the cathode follower – allowing the current to flow across the anode and cathode resistors in series until the cathode follower tube heats up and starts drawing current, placing the cathode at a higher potential than the anode of the driver tube, which in turn makes the diode “disappear” from the circuitry), the solution with the voltage regulator string is in my opinion more elegant and predictable, and it offers a fail-safe function as well.

With the rapid rise of the B+ at power-up, the voltage regulator tubes light and start drawing current, thereby the driver and cathode follower tubes are immediately transferred from an 800V environment into the 400V environment for which the circuit was designed. As the tubes heat and start drawing current, the voltage regulator tubes will draw less and less current until the desired equilibrium point is achieved. Using 0A2 and 0B2 VR tubes in various combinations allows a fine tuning of this operating environment, between 324V (3x0B2) and 450V (3x0A2). In this manner, the tubes in the driver circuitry are not stressed at power-up, and in operation “enjoy” some voltage regulation.

The fail-safe function of the VR tubes becomes apparent in case one or both output tubes do not start (for instance, heater problems) drawing current. The VR tubes will draw at least 30mA current (actually, in such extreme conditions they may draw more current, up to probably 60mA, which should not prove damaging for a short period of time) providing some current draw to the power supply, keeping the voltage from exceeding a given maximum (in this case, it is almost impossible for the power supply as designed to exceed 1000V DC, thus the recommendation for 1000V DC caps rating, i.e. 500V in series: in practice, the VR tubes would draw up to 60mA and the B+ would not exceed 950V). It goes without saying that the VR tubes would ignite – being cold cathode tubes, they do not need anything else besides a minimum voltage potential to start (provided they are in good operating condition, and these tubes last quite a long time).

 

Heating the Output Tubes

The 813 has rather high power requirements – 10V at 5A – which is basically 50W. Compare it to the 5V 1.2A or 6W required to heat a 300B, and most problems become apparent.

Let’s start with what I see as the most important problem when heating direct heated tubes in SE amps – hum. With 50Hz (60Hz) AC, if a hummer (humdinger) pot or similar arrangement is applied to cancel the basic harmonic, we are nevertheless left with the residual 2^nd (and some higher) harmonics, i.e. 100Hz (120Hz) hum. The level of hum that can be heard depends mostly on heater voltage (increases for the square value of the heater voltage, i.e. it is much higher for 6.3V tubes like the 6B4G than for 2.5V tubes like the 2A3), and the gain of the tube. Since the gain of the tube in RH amps is decreased by the feedback loop between output tube and driver tube, we can enjoy the luxury of AC heaters even with a 300B, probably even on high efficiency speakers. But hum gradually increases with the increase in heater voltage and tube gain, and with a 2E22 it becomes difficult to ignore it completely, at least in the vicinity of the speakers. The 813 requires 10V and has rather high gain, thus 100Hz hum is virtually impossible to avoid, even in an RH type amp, and with relatively inefficient speakers: while it may not be disturbing at the listening position, the presence of hum probably influences the sound adversely when it comes to fine details in the music.

One obvious solution is DC heating, which should be hum free. I personally dislike the effect DC heating has on the sound, a specific type of detail muffling and bass tone exaggeration, and am aware of the other problems inherent with DC heating, like the difference in heater potential from one end of the heater to the other, or the need to switch current direction (polarity) at least from time to time in order to keep the filaments in best operating condition. Of course, the diatribe between AC or DC for the heaters of directly heated tubes seems to be never ending – both sides can rely on some advantages, and since DC is by far more complicated, the supporters of the latter may be inclined to last longer in this dispute, either due to stakes held (for instance, offering solutions for sale), or due to investments performed (buyers of such solutions). But at the cost of being accused of engineering (as opposed to audiophile) mentality, I would like to avoid the AC vs. DC sound issue, and stick to what matters: feasibility.

5A of current draw means that at least 9A will be drawn if rectification is applied. Adding some voltage losses in the process, we are safe at estimating that at least 100W of power is required for each heater – which is literally the double of what we should need with AC. Just rectifying with brute force, like high valued caps, does not present the best solution, neither as current inrush nor as final result (ripple). Thus some form of regulation is needed to filter out the ripple without investing in heater chokes and/or high valued caps. The most basic form would be voltage regulation, which is as well more energy efficient than current regulation (lower voltage drop). If we assume using very low drop Schottky rectifier diodes, and very low drop regulators capable of 5A, like the LM1084, we might be able to do it with the least energy loss. Still, at 1.5V drop and 5A current draw, we are facing at least 7.5W of heat for each regulator, which in turn requires using a hefty heat-sink, or a heat-sink with fan (a good heat-sink for older Athlon processors will make short work of the heat dissipation).

In the end, we get very low (but still audible, at least I can always hear traces of it) hum or buzz (I like to call it solid-state regulator buzz), and DC on the heaters (which I consider sub-optimal in operational terms, regardless of the sound quality discourse). A lot of energy lost in heating, a lot of complication and effort – and that is why I mention the residual hum or buzz: after going at that long a distance, I expect to get “no hum whatsoever”. Of course, even more elaborate alternatives are possible (several high-quality DC heater solutions are known, advocated, and discussed all over various forums – which pre-made solutions I have not tried due to obvious reasons), and you can always choose the car battery way… but even the car battery adds something audible, some (incredible but true) hum or buzz persists even with battery operated filaments (chemical buzz?).

Another solution is high-frequency AC, better known as “electronic transformer for halogen bulbs”. You can use adequately sized (minimum-maximum power) electronic transformers straight out of the box, and drop the voltage with resistors (0.15 ohm per heater leg will do the trick), but this solution is actually almost identical to what we started with in the first place – mains AC. The output of the electronic transformer is high frequency AC (usually between 30 and 60 kHz), but it is “modulated” in 100Hz format. Once your humdinger circuitry cancels the basic harmonic, you are left with 200Hz hum, which is slightly less audible than the standard 100Hz, but to my ears it is also more annoying (it actually resembles the electronic buzz). In order to be used for this application, the electronic transformer needs to be modified.

There are two basic modifications to be performed: filtering the initial 100Hz ripple, and setting the correct output voltage. It is rather easy to filter the initial ripple by placing a 270 to 330uF/400V cap (for European 230-240V AC mains) between the positive and negative pole of the bridge that rectifies the mains voltage at the input of the electronic transformer circuitry – but this will in turn cause the output voltage to rise. The rise of the RMS output voltage is proportional to the increase in DC voltage after the bridge (due to the decrease in 100Hz ripple, the average and RMS voltage will be much higher), and with the cap values mentioned is approximately 1.49x the original RMS voltage of the unit. While this excess voltage can be burnt with resistors, due to the high current draw this discourse borders with impractical: we are talking about some 35.5W dissipated in resistors, per heater (per tube). A modification of the output transformer (ferrite core, usually toroid) is necessary, lowering the number of turns in the secondary winding. This is rather easy to do with most electronic transformers.

What is not so easy is getting measurements. In order to measure HF AC, you will need a True RMS DMM capable of measuring at least 50kHz – or a scope. The output of the electronic transformer is not a sine wave, but a square wave, and since this square wave is not perfect, the RMS value is lower than that of a perfect square wave (which would be equal to the peak to peak value). Thus either you have an adequate True RMS DMM, or a scope – or you do a lot of estimation and calculus in order to get the correct value at the output.

Due to the complexity of the measurement and calculation, this rather easy procedure requires a lot of additional explanation. Since I am quite taken by the results achievable with HF AC for heating direct heated tubes, I intend to post about it in the future with more details. For those willing to try it, and not particularly afraid of learning by trial and error, the whole procedure consists in adding a cap, and unwinding some turns from the secondary of the ferrite core output transformer. The reward for using HF AC will be hum-free sound (less hum than with DC) with all the dynamic qualities of AC on the heaters. The best of both worlds: This might sound like repetition of what I already stated about other solutions in tube audio, but that exemplifies very well what I am after – finding better ways to use what is literally in front of our eyes, but remains unseen or misunderstood.

 

The Output Transformers

As already explained, I have chosen approximately 5k as the optimum load for this amplifier based on a combination of sound and performance. While 10k could offer approximately the same output power with slightly different parameters, 5k sounds better – more natural and true to life.

The problem with output transformers for an SE amplifier this powerful is just that – output power. While most SE transformers might be safe at the voltages involved in this project (the B+ is well below 1kV, and even paper insulation should be safe up to 1.5kV if properly applied), and the total DC current in the primary is not that high as to present a particular problem in terms of size – most transformers on the market just cannot provide adequate performance in terms of bandwidth at high output power. The main or most evident difference between an SE amplifier with 300B tubes, and an amplifier with 813s is output power – we are talking about maybe 10W against 35W. Even if safe, an output transformer for the usual SE amps will not be able to perform correctly at high power levels. This fact, and the availability of 813 tubes, has been keeping me from developing the project for quite some time.

The first pair of output transformers used in this project, courtesy of Paul Barker, are huge and potted units, designed for use with 845 and similar tubes – weighing over 7kg each! I have used these transformers in the initial phase of the project, and they are definitely up to the task – lots of bass and no constraints on output power. Due to multiple taps for the primary (5k, 7k) and the secondary (4, 8, and 16 ohms) – those transformers are easy to use as 5k, 7k, 10k – and even 2.5k and 3.5k if needed.

The second pair of output transformers used in this project come from Sweden, courtesy of Per Lundahl. While looking like the usual Lundahl transformers – C core, dual bobbin – they are actually twice the size and weight of the LL1623 I have used in the RH2A3/RH-TTA project. Those transformers - LL1688 are intended for 845 and similar transmitting triodes, and can easily be set for 5.5k, 9.5k, or 19k primary – with 4, 8, and 16 ohm speaker options. Since this time I was after just one setting, there is no wiring harness or switches – just 5.5k primary into 8 ohms.

While the huge potted transformers performed very well indeed, with great bass extention and lots of volume – I was surprised how much of a difference could a different type of transformer make in the same circuit, and even one which due to feedback mechanisms has lower output impedance and is therefore less demanding than no-feedback types. The improvement in mids and highs, the general liquidity of the sound, and sound-staging was something I did expect – but the LL1688 is better even in the definition and perceived depth of the bass notes. Overall, a clear winner.

 

Spice Models and Performance

I must admit having started this project without a proper model for the 813 output tubes. After all, some basic driver details are already known, and all I need are the curves – to draw some loadlines. Well, it seems that this is the sub-optimal way of designing amplifiers. Having a loadline is not enough, since the operation of the amplifier is represented by the interaction of the output tubes and the driver section – besides graphic representations of output power and distortion, and the eventual feedback applied, the distortion cancellation effect is very difficult to estimate…

While my 813 model is far from perfect, it is better than most models that can be found for pentodes and tetrodes. The difficulty is getting the “kink” in the lower part of the curves. As for the driver and cathode follower tubes, I have improved all the spice models for triodes that I am currently using to Koren 8 parameter models. Curves from the ECC81 datasheet are shown superimposed with curves obtained from the respective spice model. At this point, what the simulation shows is definitely how tubes should behave…

 

The Sound and General Impressions

I am always reluctant when it comes to discussing the sound of my designs – I prefer to hear from others how pleased they are with the various RH amps they have built. While I tend to explain in great detail the technical solutions, their eventual elegance and feasibility – the perception of music reproduction is much more subjective.

This amplifier differs from my other designs in complexity and power. Power was a design goal, while the added complexity was necessary in order to keep it user and DIY-er friendly as the previous designs. This basically means that once built and checked, it can be used for hours, day by day, without thinking about it – or playing a little bit with the combinations of driver tubes and operating points made possible by the adoption of VR tubes.

But the sound is really worth mentioning. While the general signature is the same as in other RH amps, due to the nature of the feedback and distortion cancellation applied – this amplifier shows some additional characteristics besides what can be ascribed to the intrinsic quality of the output tubes, more as a consequence of the high power available.

Audiophile DIY-ers with high efficiency speakers are probably fully satisfied with the power a 300B, or even 2A3 amp can offer – anything between 5 and 10W is probably enough. But with “the usual” lower or medium efficiency speakers most people own – each and every W counts. What made me take the good sound of an EL84 in triode mode and achieve it in pentode mode with at least twice the power (the RH84 that has started it all), finds its final gratification in this amplifier. We are not talking here about 5W vs. 2W, or whether a 300B can give you 8W or maybe 13W… this is power above 30W, and it seems very easy and so logical – once it is done. But the journey that got me there was a long one.

With more than 30W at disposal, we are talking about “commercial” power, something easily achievable with PP amps in the world of shops and magazines. A simple PP amp with EL34 is capable of 30W per channel… but there is a huge difference in sound. Once power is not the bottleneck in music reproduction, we can get to enjoy what was offered or hinted by lower powered amps. A nicely defined bass note becomes not only nicely defined and precise, but a loud, fast, and almost brutal attack! A well-defined piano note that had texture or vibrancy now has power and attack; it resonates in air without being distorted, almost regardless of listening volume. All of it by far exceeds what can be had from commercial PP amps, and explains why there is so much hype about the sound of SE amplifiers.

Endowed by the same sonic signature found in other amplifiers of the RH series while apparently being free of limitations due to its high power, this is definitely the “flagship” RH amplifier – with all the inherent characteristics. Some might ask whether it has the finesse of a 2A3, and the question would be a logical and frequently found one – after all, since the “Ongaku” conquered the world of commercial High End by blitz, there have been so many attempts to scale down the concept. But it is not a fair fight: at low listening levels the RH813 is just as nice, liquid, and enjoyable – as the RH2A3, to name one example. But what it can do transcends the “sound finesse” issue: where it can go, the RH2A3 cannot follow. I guess the question is not absolute, and needs to be rephrased – how efficient are your speakers? Because, if you own very efficient speakers, you probably do not need to go the length necessary, both in financial or psychological terms (higher voltages involved, and higher power consumption/heat dissipation – if that is of any concern, not to mention the fact that bigger is usually more expensive), to involve yourself in an affair with such a behemoth amplifier. On the other hand, the only reason why would you content yourself with a lower power amplifier is either not being able to use it (the neighbors might complain), or not being able to overcome the mentioned financial or psychological issues involved.

 

An Adequate Box and Other Issues

Starting with the size of the output tubes, and the maximum dissipation (CCS) possible, as well as the heater requirements in terms of power necessary – it is obvious that an amplifier like this is not easy to accommodate in a box, or on a shelf.

The design is relatively efficient – as SE amps can be. By adopting a toroid power transformer a great reduction in size, heat, and radiated field is achieved. While the toroid transformer could be a 300VA unit (output tubes heaters not operated by this transformer), it is rather difficult to wind the 750V secondary on a 300VA core. Therefore a 500VA core is probably necessary – increasing both cost and size (but slightly) – but as an aside the power transformer will be colder in operation. As a matter of fact, the one used in my RH813 does not exceed 50⁰C – even after 6 hours of operation on a hot summer day.

The power supply chokes have very low DCR, thus they dissipate very little heat – and the same is true of output transformers. This would leave us with the system necessary for the heaters, but instead of 100W per tube – between transformers and heat-sinks for the regulators and transistors – there is just a rather small box containing the modified electronic transformer… and this box, besides being small and efficient, does not exceed 45⁰C after 6 hours of operation on a hot summer day. Ah, the beauty of power-efficient solutions.

Thus we are left with the various resistors in the tube circuit, and the tubes themselves, as the most important source of heat. While some of the ceramic resistors get quite hot in operation (70-80⁰C), they do not represent an important source of heat, and the same is true of the driver and voltage regulation tubes; this leaves us with the output tubes. At 75W dissipation, and 50W for the heater, the 813s get quite hot and radiate a lot of power: due to the various heat and power optimizations, they are the most important source of heat in this amplifier.

Putting the amplifier in a standard size and dimension box is very feasible – but the height of the output tubes, and the heat they radiate, means that the amplifier is not suitable to be placed on a shelf. The heat radiated from the output tubes goes a long way towards burning the shelf above… thus with an amplifier that is radiating mainly from the tubes, and is not suitable for shelf placement, should probably be built in a tall and narrow box. The box should be organized in two distinct levels, the bottom level containing the power transformer and the output transformers, while on top of this level lie the chokes and caps and other elements of the power supply. At the highest level, the opening of the box supports the audio circuit itself, where tubes protrude from a top cover which is supported high enough to enable air circulation. This is basically what I intend to build as a box for this amplifier – to be placed on the floor, beside the shelf with other amps and audio equipment. Although tall and narrow, the box will have a low center of gravity, keeping it stable.

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.