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 4th 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 4th 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 2nd 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 4th 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?