Tuesday, January 8, 2013

RH Universal

After the success of the 1st generation of RH amplifiers (RH84, RH807, RH34), I have decided to create a new generation of amplifiers: simpler to build, less simple to understand (maybe) - and better, possibly.

Initial work and ideas were done in early 2005, creating the basis for two amplifiers - RH88 and RH300B. The RH88 was immediately transformed into breadboard, and it remained in function for the better part of the year. Due to objective reasons, work on the 2nd generation of RH amplifiers was halted, and it was not until the summer of 2012 that the breadboard emerged from a box.

Initial publishing of the schematics for the RH88 has shown that DIY-ers are interested, but fear the high prices of tubes, OPTs, chokes, and other parts associated with this amplifier. In particular, NOS tube prices have soared during the last decade, making both KT88 and 6550 NOS tubes almost collector's items with high price tags.

It is difficult to judge the prices involved when building a 5W class amplifier against the prices of building a 10W+ class amplifier. One should keep in mind that while the RH84 could be built with great results using parts from old console amplifiers, and improved using new parts - it's competitors are maybe some 2A3 or 6B4G amplifiers. The RH88 was conceived to compete with 211/845 amps (in class A1 - which most of those are, although many commercial products are marketed claiming class A2 output power) and easily exceed the power offered by the 300B SE amps... as such, it is still a relatively cheap amplifier to build, depending on the parts used.

The RH88 was initially conceived as a 6550 or KT88 amplifier, and later some DIY-ers asked for a customization to use 8417. While the latter could be inserted in the socket and used without problems, the output power would be lower, and the sound inferior, due to a lower current draw in the same circuit. I was also considering whether EL34 and 6L6 types could be used as well, and eventually decided that the amplifier should become universal - allowing the user to insert almost any given tube which is compatible with the pinout.

The principle is quite simple: regardless of tube type, the cathode current draw remains fixed at 100mA. Since output tubes vary in anode dissipation, with a fixed current draw the voltage across the tube must be variable to allow for different tubes to be used. The variable B+ is achieved by changing rectifier tubes (in this case, to simplify everything and avoid mistakes, rectification is done by hybrid Graetz bridge). Basically, a 5AR4 (GZ34) will give approximately 45V of DC more than a 5R4, which creates a difference of, roughly, 4.5W of dissipation. Some tubes draw less current through the anode, since they draw more current through the second grid (i.e. EL34)... which creates a further difference in anode dissipation (anode current x voltage across tube). Last but not least, tubes will differ in the cathode voltage at the same current draw (EL34 and 8417 approx. 10V while 6550 will have approx. 22V) which creates a further difference in anode dissipation (different voltage across tube).

As a rule of thumb, in the above circuit EL34 can be used only with 5R4 rectifiers, while GZ34/5AR4 should be used only with tubes capable of more than 30W anode dissipation (6550, KT88, 8417). It goes without saying that nothing forbids the user to combine the latter tubes with a 5U4 or even 5R4 - where of course the lower voltage across tube will result in a slightly lower output power. It can be thus said that output power varies between 8W per channel and 12W per channel.

The above picture: 7027A RCA tall bottle in action with 5AS4 GE and 6201 Philips driver.
In order to use EL34, it is necessary to connect pins 1 and 8 - which does not represent a problem to most tubes, except the 7027 which has a second connection to grid No. 1 on pin 1. A switch will allow the user to connect or disconnect the two pins on the output sockets (part of the switch can be seen on the above picture, choosing between the "7027" and the "EL34" setting.

Besides allowing the user to use a wide variety of tubes (basically, what is at hand), the amplifier is particularly suited for playing with tubes (tube rolling). While the basic character of the sound remains the RH signature, additional nuances can be had from different combination of output tubes, rectifiers, and even different drivers (ECC82 can be used with 8417 and EL34, but not with 6550 and KT88).

I prefer to let others speak about the sound of the RH amps...


  1. Dear Mr. Alex,
    I admire the simplicity of your designs and I folow the posts for a long time ago, I built the RH84 and RH34 with good results even if I use it with insensitive speakers. I was looking for further improvements, than I found RH88. I wonder if there is more space for simplification. I wonder if the two power pentodes can share the bypassed cathode "resistor"? May I use the same local feedback , in the driver section also? Is it possible to share with me the spice model you were using?

    With many thanks,
    Kis Csaba from Hungary.

    1. Dear Csaba,
      Please write an email with your questions, I will reply as soon as I can.
      Thank you,

  2. please take a look of my RH84: https://picasaweb.google.com/102637958124895318523/Erosito?authkey=Gv1sRgCN_cxu6Wu56NxwE

  3. I wonder in the RH Universal v.2 – Totally Universal
    if your reference for the tubes is in reverse ?

    1. Yes it was, although it did not matter since one is a triode and the other a pentode and the symbols are obvious.
      Nevertheless, it has now been corrected.

  4. RCA Receiving Tube Manual, 1960 series, page 29 under "Electron Tube Applications" illustrated the same feedback topology as your RH circuit. Plate-to- plate inverse feedback. It is certain that The RCA manual was published way ahead of your time of research, and the RCA engineers have figured it out before you did. With all due respect to you guys for rediscovering, let us give credit to where credit is due.

    1. 1. Back in 1960 I was not yet born - but people designed amplifiers already: should I give them credit for being born before me?

      2. Back in 1999 internet in Serbia was dial-up and we were lucky to download a tube datasheet: even if available, the Receiving Tube Manual at 20MB was out of my reach. I never read it. How can I give credit to something I have never read?

      3. Nowadays 20MB is a joke to download, thus I checked it out. It is not page 29, rather 27 - and the picture you are referring to is No. 29.

      4. If you were reading the mentioned text more carefully, and you had more knowledge about the issue, you would not have missed that the whole text is very vague, and even the references/circuit names are vague: anode to anode feedback is just called "inverse feedback" (a.k.a. negative feedback, which is a general term), and this particular type referred to as "constant current inverse feedback", probably implying that it is a feedback in current, not voltage (while the "other" similar type of feedback, which is attributed to Schade - an RCA employee, for authoring the application note for beam tetrodes - at least as explained by the author, is purely voltage feedback, applied between the anode and control grid of the same tube, with a capacitor in the signal path).

      5. Beside being vague and mildly informative - lacking any practical information on how should this be applied (formulas, etc), the author further distances himself stating that "inverse feedback is not generally applied to a triode amplifier, such as the 2A3, because the variation in speaker impedance does not produce much distortion in a triode stage having low plate impedance". With this words, the author of the text shows poor understanding of the topic (of anode to anode feedback as a type of negative feedback loop) and the influence feedback has on active elements in a circuit and on the total outcome of such circuit.

      Even if I knew of the example mentioned, I would never credit something that has nothing whatsoever to do with my designs, except for a schematic without values or explanation that looks similar (if you were to take a good look at the schematics mentioned, you would notice that similarities are limited to a non-better-defined-nor-explained resistor between two anodes...).

      I guess anode to anode feedback was done much before 1960, but it is difficult to credit someone for inventing hot water while you are taking a shower...

      I would also like to draw to your attention that in contrast with the ideas of the text author, I do apply this type of feedback to power triodes (RH300B, RH2A3) as well as power tetrodes/pentodes, with excellent results. Obviously, distortion and its causes in SE circuit are not limited to "loudspeaker impedance variations", and there is nothing wrong in further lowering the output impedance of a power tube, regardless of type (triode, tetrode, pentode).

      I do not know about the "other guys" you are mentioning (I do not even know who are you referring to), but I have not rediscovered anything, thus I cannot credit anyone for that.

      My later research has shown that the mentioned type of negative feedback has found very little use in commercial applications, and from my current standpoint and knowledge I can only guess that the reason might be a difficult and complicated calculation involved in the optimization of the circuit - which optimization I have never been able to find thoroughly explained nor effectively executed in historic literature.

      I have published and replied to this comment as an act of transparency. If you have further ideas or comments to discuss, please do so by e-mail.

  5. hello, Alex, I would like to thank you for your guidance in helping me construct 2 versions of RH universal, and I am extremely satisfied with the results. The first version is a "low" power version which can use 6V6 or 6L6 tubes. I estimate power as between 5 and 8 watts. The second is built exactly like the schematic except that I am using a 6SL7 as the driver tube. This amp is putting out at least 10 watts of sweet single ended music. Everyone who hears it comments on how clean and lifelike the music sounds through it. Well, now that the Uni2 is a successful build, I am eager to pursue my next project. And so, I come to ask you if you would like to help me construct an RH PSE KT88 as my next project? Thank you for getting back to me . Dak

    1. Thank you for your kind words and the appreciation of my work.
      Please contact me directly via e-mail (find it in the "about me").

      Regards, Alex