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.

RH84 PPE – "Parallel Pentode Edition"

The RH84 seems to be my most famous amplifier – and probably the favorite EL84 DIY amplifier. Recent builds of the RH84 revision 2 confirm the good sound that many DIY audiophiles have enjoyed so far. Thus, the RH84 remains a good vehicle to further new ideas and improvements.

Critics have mostly criticized the choice of the driver tube – ECC81 – claiming that the application of a pentode driver tube would yield more power and/or less distortion. Still, to most using a pentode as driver actually means using it in triode mode (g2 and g3 strapped to the anode) – and while this is a valid if meaningless approach, it should not be mistaken for a pentode used in pentode mode as driver, which is what the critics claimed as superior, but have never shown in practice, either as one of the numerous copycats, or as an RH inspired improvement.

The Pentode Edition in the name of the amplifier stands for “all pentode”. The driver chosen for this amplifier is the 6AU6 (actually, 12AU6 – since I have got no 6AU6s, and the difference is confined to the heater voltage). At 6.2mA current draw it’s operating point is just where most would put it (if they knew how)…

For my taste, or more precisely, speakers – the RH84 could be improved by having more power. 5W is barely enough for 88dB/W/m speakers, if you like listening to music at moderate and high levels: this is obviously not an issue for fortunate owners of the various large Klipsch, Lowther, Tannoy, Altec, or JBL speakers, with efficiencies higher than 94dB/W/m. While many of those who built the RH84 over the years own such speakers, those that have lower efficiency speakers have stuck to this amp due to its good sound… and eventually gone further in the search of more power. Unfortunately, the power output is a limitation of the output tube – with 12W dissipation, an indirectly heated pentode cannot yield more than 5-6W. Since the EL84 cannot be driven into class A2, no additional power can be had above it physical limits – and there is no driver than can change that – as it should be clear to everyone with enough common sense and some electronics knowledge.

The only way to have more power from the cheap and affordable EL84 is the parallel approach: two EL84 in parallel have a combined anode dissipation of 24W, and output power is comparable to, if not slightly exceeding, the EL34. The parallel approach has been criticized by purists due to the fact that identical tubes do not exist and even matching cannot solve all the issues…

OK, so you cannot have everything from life, thus improvisation and compromise are necessary – this is not a perfect world. With the right design choices and approach, the most important issues related to parallel output tube implementation can be overcome and minimized.

 

Let’s start with the output tubes: as shown in the schematics, the two devices share the anode connection to the primary of the output transformer, and the zener diode connection to the B+ (22V of drop, slightly more than the drop across the output transformer, keeping the g2 at a constantly lower potential than the anode). The lower half of the tubes is actually where the separation occurs – each tube is controlled by a separate current sink, matched to equal current. The two grid stopper resistors connect to the common grid resistor for both output tubes. If the tubes are perfectly matched, the bias voltage across the current sink will be identical – but eventual mismatches will be automatically addressed as an equalizing voltage across the grid resistor: in practice, with reasonably similar tubes (not matched) this voltage is in the 0.02V range, meaning a grid current of 6.06x10^-8 A (sorry if it’s more than the eclectic taste and knowledge of some allows). Even quite mismatched tubes (worn and almost new tube) will not lead to more than 0.4V, i.e. grid current of 1.2121x10^-6 A… thus it can be concluded that the two tubes tend to balance without problematic repercussions.

The current sink device chosen for this version of the RH84 is the lowly and cheap 7805. Besides the need to provide some ideas and guidance to the DIYer, this was also done to keep costs lower: the 7805 is 33% cheaper than the LM317 (it that is of any importance) and if 10 pcs from the same batch are acquired, it should be easy and feasible to match accurately for output voltage two pairs of devices. Because, the output voltage is the reference voltage that will define and limit the current draw of each current sink, and thus each EL84. Furthermore, since the output voltage is 5V, approximately half the bias of the EL84 in this design, the resistor will perform half of the dissipation (approximately 250mW) leaving the remaining 1/4W to the 7805 which can handle up to 600mW without heatsink. If that was not enough, the screw hole is connected to pin 2 or ground, and thus is at ground level – meaning that it can be simply bolted onto the metallic chassis of the amplifier (if it is cold enough and not heated by the transformers and tubes…). The current setting resistor can be a 1/2W unit, and I have used 0.6W metal film precision resistors for this position.

Each EL84 cathode is separately bypassed to ground by means of approximately 100uF valued capacitors – I have chosen to place in parallel two 47uF/22V ROE tantalum caps, but any cap type above the 22V rating would do.

The driver in the RH84 PPE obviously is a pentode, used as a pentode (in pentode connection). While small signal pentode tubes can always be used as triodes by strapping g2 and g3 (if not already connected to the cathode inside the tube) to the anode – I see very little reason to pursue that direction, since there are many types of triodes readily available with a wide range of gain and current draw characteristics, thus an adequate driver can always be found without resorting to triode strapping pentode tubes. On the other hand, using a small signal pentode in pentode mode can bring some advantages over triodes, mostly in the available gain “department”… of course, using a pentode as driver in the RH amps (or any other similarly conceived amplifier which does not operate in class A2, or with such output tubes that are not suitable for class A2 operation, provided the pentode used as driver could drive the grid of the output tube with the current necessary… to be precise) will improve neither power nor distortion: it’s used to show that it can be done, and how it should be done properly, with excellent sonic results.

Small signal pentodes are used in a slightly different manner than output pentodes – the g2 resistor in small signal pentodes has a very important function in setting both current draw and gain, and the screen grid should be adequately bypassed to ground, since the combination of g2 resistor and cap forms an RC filter: in this case, 10uF to 20uF caps should suffice. Another detail to keep in mind is the voltage value of this bypass cap – while in operation the g2 will be below anode level and roughly at ½B+, at power-up the grid will not draw any relevant current and the cap should have the same rating as all the other power supply caps!

The 6AU6 was chosen as driver tube due to the fact that it is not some exotic hard to find tube, it should be relatively cheap – and because I had some 12AU6, mostly CEI (gray anode) and RCA (black anode), at hand. The 7 pin sockets are similar in looks to the common noval sockets and are easily available. I am not aware whether there are any xAU6 tubes from current production, but the NOS stock of those tubes does not seem to be dwindling yet.

Another small signal pentode I would use for the task is the EF86 – if I am correct, those can be sourced from current production as well. They require standard noval sockets and are therefore even more easily applicable to the driver task. I guess prices are slightly higher… anyway, I did not use it because I have got none, but here’s a simulated schematics with resistor values for those who would like to try the EF86 in pentode mode as RH84 drivers.

What about the old faithful, ECC81? Of course, it can be used as driver for the parallel RH84 as well, and the results of the simulation are, of course, quite in line with the results of the 6AU6 and EF86 versions – attached is a resistor configuration similar to the RH Universal which is actually optimized for use with ECC81. As a design issue, the ECC81 is a double triode, thus requires only one socket for two units (and half the space in the amplifier, not to mention less heater wiring). Furthermore, there are no additional g2 resistors and bypass caps – less parts equates not only to less cost, but more importantly less complexity and potential problems.

When it comes to sound, I guess we are on very subjective ground. Ultimately, higher quality tubes, drivers in particular, will lead to better sound – that is beyond subjectivity. The PPE is designed as to sound just as good with a garden variety xAU6 as the RH84 sounds with a garden variety ECC81 – just like the EL84 in the RH84 amps sounds in pentode mode just as good as in triode mode... But the power part of this well-known sentence does not hold water here – while in pentode mode the RH84 has (much) more power than in triode mode, the pentode driver does not add to the power, nor improve on the distortion values. On the other hand, there are several great ECC81 family members out there (6201, ECC801, ECC801S, etc.) that are quite tough to beat (in my book, and in the books of many others). Frankly, the RCA black anode 12AU6 are hardly a match for the Philips SQ 6201 – for my taste. As I already stated, this is subjective opinion terrain, and everyone should attempt to find his own best combination of tubes. To someone owning a box full of black anode RCA 6AU6 the acquisition of some Philips or Valvo 6201 (particularly at current prices, which inevitably increase in time), probably represents a nonsense – unless they are absolutely necessary.

It is quite obvious that the RH84-PPE lacks “universality”. You can use the xAU6 chosen, the output tubes are EL84, and the power supply as drawn requires a GZ34/5AR4 rectifier. But if your output transformers have 8 and 4 ohm taps, you can always play the game of “power vs finesse”! Provided your loudspeakers are a nominal 8 ohm load, if connected to the 8 ohm tap the anode load will be 3k, just as foreseen in the schematics. But if you connect them to the 4 ohm tap, the anode load will be 6k, just what is needed for a classic RH84. The schematics foresees the possibility to operate the amplifier with just one EL84 per side into 6k – with half the power: just unplug one EL84 per side before powering up, and you can check whether two tubes in parallel necessarily lead to a loss of focus, as some like to define it. This is basically the same game as “triode vs pentode RH mode” some used to play at the time the original RH84 was introduced. The output transformers will probably be quite large (3k at 100mA DC current across the primary) and thus represent an overkill for a single EL84 – if nothing else, power bandwidth should be at its best, and expected power output is just above 5W per channel. You could do it as well to consume less energy – up to 50W per hour less, as your contribution to a greener planet.

Operating the amplifier with just one EL84 per side will decrease the current draw for approximately 100mA, thus the B+ will be too high at approximately 360V. Therefore a change of rectifier will be necessary, and a 5R4 will provide the correct B+ value. Since the current draw will not exceed 125mA with one EL84 per side, a 5Y3GT can be used as well.

Last but not least, a few words about the power supply. It is kept deliberately simple to keep costs as low as possible, thus a simple C-L-C filter is applied. The first cap should be oil for the best results, while the second cap can be electrolytic. Any choke capable of approximately 200mA DC would do, preferably a 10H unit. Since the power supply is cap first, the choke will not be particularly stressed, and does not need to be particularly large or stiff (potted etc.).

CRITICISM AND SUPERFICIALITY

Since the DIY community has discovered the first RH amplifiers published on the net, back at the beginning of 2001, there has been a lot of appraisal by those who built with success the various amps (in particular the RH84 and RH807) – and a line of critics has emerged as well, which is both healthy and normal: without criticism, mankind would not have accomplished so much.

Most of the criticism falls in the following categories:

1.       RH amps are not original – there were a lot of amplifiers in the past that with plate to plate feedback (Schade and other examples).

2.       RH amps are not optimal – the driver should be a pentode (because most of the above mentioned sources say so).

A recent example can be found at: http://www.diyaudio.com/forums/tubes-valves/226544-rh-84-variation-direct-coupled-pentode-6.html#post3568423

There is a widespread misunderstanding among the critics that I claim the invention of anode to anode feedback, in spite of the fact that in the tube era the feedback path was known and applied in various forms. Just as usual (and how conveniently for the critics, if I might add) I cannot reply since I have no access to the forum – it is a fact that responding to harsh criticism, protesting copycats, and posting with arguments showing facts instead of empty words, leads to banning and expulsion. The conflict of interest between forum stakeholders – people who defend their arguments by saying, basically, “I am the authority because I say so”, or “because I build amps in that way (and I do it for money, so do not step on my toes)” is more than evident to all those who are willing to consider it.

While I have never actually claimed the “invention” of a certain feedback path, I was in all sincerity not even aware of the existence of the literature mentioned (like the RDH 4^th edition which is often mentioned, or the famous work of Schade for RCA, which is basically an addendum to the datasheets and application data for the RCA beam tubes) – and have read none of the works mentioned at the time when I designed the first RH amps. The well-fed cannot fully understand the hungry – or what hunger actually is: living in the USA or Canada, it is very hard to imagine that such (seminal?) works (most examples are actually very common or normal books, but tend to be exalted by those who find it is in their interest to do so) were neither available nor accessible in socialist countries! Furthermore, in 2000 we still had dial-up (expensive dial-up, if I might add) and downloading a couple MB worth of tube datasheets was considered a feast! Besides, I do not recall being able to download the RDH 4^th edition back in 2000, or the work of Schade regarding beam tubes. Thus, in a way, I had no examples to draw on for inspiration except my own ideas: that is maybe the reason why I keep defending this notion – my ideas are my own.

What I claim is that my designs are fresh and different, and up to the point when it comes to feasibility and results. The freshness and difference comes from the fact that before 2001 there have been no similar amplifiers published on the net, as far as I am aware (original designs done by the person sharing it, not books or magazines shared with or without the consent of the authors). The feasibility and good results come from what those who have built any of the published RH amplifiers have so far reported, both on forums and in direct e-mail communication. Due to the latter (reported good results), copycats started appearing since 2004, but became much more commonplace near 2009. The 2^nd generation amps, characterized by having only the Rfb resistor (the classic anode resistor has disappeared) were first published in forums around 2005, when I built the original RH88 breadbord. Fresh and different ideas, and designs with a high degree of feasibility and success in obtaining good results, that is what is mostly lacking to the critics and their friends – at least when shared and publicised work is concerned.

The latest post linked above mentions “Hugo Gernsback's 1947 Amplifier Builder's Guide”. I have never heard of it, so I did some searching and quite easily found references to it, and – like always when the zealous critics (whose identity in most cases remains hidden behind pseudonims) have something to point out – it is not exactly what they are assuming it to be. In this case, Hugo Gernsback is actually the editor, or publisher – the copyright holder, anyway – of the Radio Craft Library (No. 33): Amplifier Builder’s Guide prepared by the editors. The amplifier mentioned (PA 8W amplifier) is not by H. Gernsback, but rather by Andrew Tait… so much for the attention to detail.

I have so far avoided direct comparison with old designs (while I do sometimes point out the flaws or inaccuracies in copycat designs) because I both felt there was no need, and due to having respect for the work of people who lived in a different era. Like I already stated, the well-fed cannot understand the hungry, and just like RDH or Schade were SF to me before the internet era, and throughout the dial-up internet era – until the moment when kind people shared those works with us – spice simulation and computers were mere science-fiction to the designers of the tube-era (maybe even beyond the imaginable)!

Nevertheless, this time I am going to make an exception, both because I cannot reply otherwise, and because I guess most blog readers will find the comparison interesting. Thus I apologize to the (most probably) late Mr. Tait for dissecting his work in this manner.

The amplifier in question which allegedly resembles the RH amps due to the fact that it shares the same feedback principle is basically a combination of 6L6G output tube and 6SF5 driver. While I have no model available for the 6SF5 (and no intention of writing one specifically) I will use the ECC83 model instead, since the two tubes are very similar – the 6SF5 is (almost exactly) half an ECC83 placed in an octal envelope: so much so, that any good ECC83 would probably test just as good if it could be used as 6SF5 replacement. While the correct operating voltages are not mentioned, the 6L6G is the old 6L6 type tube with 19W dissipation, meaning that with a 200 ohm cathode resistor it will draw approximately 70mA of anode current and 5mA of cathode current with a B+ of 300V (expected anode voltage across the tube of 274V and anode dissipation of 19.18W – too much in practice, but just about adequate to show the principle at work). Maybe a power supply simulation might show higher B+ (leading to improvement in results) but it is quite unreasonable to assume 25 or 30W dissipation capability in a 19W anode dissipation tube.

As the simulation shows, the amplifier is capable of 1.5W output at 3% distortion, with an input sensitivity of 83.33mV RMS (120mV peak).

Pushing the volume up to the 8W limit (actually, the article states 8-10W), the THD is 7.7%, which is much more distortion than would be acceptable for a relaxing listening session in your living room?! . It is important to point out that this happens with an input of 199mV RMS (287mV peak), which you could probably achieve with a tuner or ceramic cartridge.

From the two simulations above, it can be deducted that while the feedback is really taken from the anode of the output tube and fed to the grid (as the author simplifies) by connecting it to the anode of the driver tube, it is not very effective… maybe that is because of the decoupling on the cathode resistor of the driver tube?

Well, after removing that cap the distortion is slightly lower at 7%, but still not adequate for your living room. On the other hand, notice that the input is now 722.22mV RMS (1,04V peak) which cannot be achieved with the above mentioned sources without additional gain (would be fitting for a reel-to-reel tape deck, actually, with a standard 775mV RMS output at 0dB… but I guess those were not available back in 1947?).

Now for a much different approach: this would be (almost) an RH amplifier, and more careful readers might have noticed that the resistors applied to the ECC83 (6SF5, for that matter) are now identical to the combination shown in the RH Universal version 2. All other elements are kept identical for comparison purposes. While some critics have tried to imply high distortion in RH amplifiers, or that RH amp fans are “distortion loving” (freaks?), that is nowhere near the case. I sincerely think that the two amplifiers (RH and original PA) are not the same, although sharing the feedback approach: it is not about the principle, but how well do you apply it. The difference in design is profound to me, from the driver operating point all the way to the feedback resistor and the lack of decoupling cap on the g2 of the output tube - if I went a few steps further, to me the amps would be so far apart that a real comparison in terms of simulation would be pointless - but that does not stop the critics from mixing apples and oranges. Thus the simulation comparison is basically limited to details of the driver circuitry.

At 8W output the circuit of the PA amplifier modified as per RH amplifier principles now has a much more acceptable (although still far from desired) distortion figure of 4.4% - but the input has to be approximately 1V RMS, which would require an additional gain stage in the amplifier to allow for use with the sources that were available at the time. The AC response has also improved in the transition from the 1947 PA amp to RH mod, although the coupling cap has been kept at (nowadays unnecessarily low) 50nF.

This is not an attempt to denigrate the amplifiers of the golden age of tubes: the sources were different, and spice models were definitively SF to the designers of that age. The components at hand were also far from what we have today, both as values and quality. It is often simplified that at a moment in time we have rediscovered tubes and SE amps, and some criticize this move as backward thinking, the senseless embracing of flea powered 50’s amplifiers – but the truth is that modern tube amplifiers have very little in common with their ancestors (less than catches the eye), and the less they have in common - usually the better they sound (cheap clones and copycats excluded). Components have gone a long way since the golden age of tubes, designing has become very easy with spice models and analysis – easy that is, for those who have good ideas and enough knowledge to transform ideas into designs.

Let me now address the other issue – pentode driver. Indeed, why not a pentode driver in RH amps? There has been so much criticism both on the choice of 12AT7 (ECC81) as “driver par excellence” in the RH amplifiers, and the fact that the driver must get out of steam because it is not a pentode…

OK – here is your pentode driver in action, applied to this PA amp. While most components remain with the original (nowadays pointlessly low) values, like the 50nF coupling cap – a few others have been replaced for different values that were not easy to apply in 1947. It is obvious that this amplifier outperforms the original design at 3% distortion for 7.3W of output – and the reason why a pentode should be applied is input sensitivity: at 290mV it is low-ish for the era, but still quite feasible! The larger coupling caps and the removed unnecessary cap from g2 to ground on the output 6L6G help creating a very good AC response – and since the pentode driver is less affected by the absence of “cathode degeneration” feedback due to the decoupling of the cathode resistor, input sensitivity is retained even with the much higher feedback involved.

This pentode driver modification is obviously unobtainium back in 1947, since 6AU6 was still not made – but similar results could probably be achieved with 6J7 or similar tubes. This should explain once and for all "why Langford-Smith (and all the Langford-Smiths that populate the dreams and reality of critics) preferred a pentode driver" (“…did Langford-Smith really understand how the circuit worked? And where did he get off on saying pentode is preferred in the V1 position, I mean really…”): input sensitivity was paramount in order to avoid an additional gain stage. They did not have digital audio and op-amps, the equipment at hand did not have a standard 2V RMS output! On the other hand, what good is 290mV sensitivity in power amplifiers today – unless you are “active-preamplifier challenged”, or prefer using a passive preamp with your analogue FM tuner (because you do not need that much sensitivity with your CD player)?

The pentode driver will actually yield no improvement in distortion at 8W power (compared to the triode driver RH mod of this PA amp), and while representing a viable alternative, it remains to be seen how good it would actually sound. Because, schematics do not produce sound – while the actual components do. Just like several different make or sub-type tubes from the ECC81 family will measure approximately the same but will have a different sound (just to mention a garden variety ECC81 compared to 6021) – how is the 6AU6 going to fare? How many 6AU6 types are there to be found and tried, compared to ECC81? The most important point is that that there will be no improvement in power output, because that will be limited by the output tube – how much power can you extract from an EL84 in SE mode? What is more limiting, the type of driver (relevant mainly to the nowadays unnecessary input sensitivity) or the output tube (where an EL34 with 25W anode dissipation will indeed have twice the output of an EL84 with 12W anode dissipation). Let me remind you that we are not talking about low to medium triodes with a bias of 80V, but pentodes and beam tetrodes with a bias voltage of 8-18V! Last but not least, what is the difference between 5W and 10W? If you own high efficiency speakers (at least 96dB/W/m) you might probably miss the difference… but if you own low efficiency multi-driver speakers (i.e. 86dB/W/m) it will mean the difference between listening to some music, and trying to listen at low levels! This fact goes a long way towards explaining the huge success of the 211 and 845 amplifiers at the beginning of the SE revival in Hi-End audio: at 20-25W output power, we are basically just enough on the loudness target even with the lower sensitivity Hi-End speakers of the 90s!

That said, maybe the critics should concentrate on claiming that a 45 (or 10Y for that matter) cannot be beat and what you need is a pair of efficient 104dB/W/m speakers. The correct reply would probably have to be somewhere between the price of the two music reproduction combinations, taking as well into account the space (cost of rental, or cost of additional mortgage funds related to a larger living room) needed to house a pair of Klipschorns.

Just a last remark on RHD 4^th edition and page 333 showing various anode-to-anode feedback arrangements… I wonder whether the critics of RH amps have actually done any of the math shown on that same page, or just strolled along with their eyes, finding similar looking resistors? Are they sure where would the math lead them?