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What determines the size of that cap, out of curiosity? Also, would this work with DHTs? Not sure why it wouldn't, unless the filament current would interfere with the CCS operation?

I'd probably size it based on the effective DCR of the CCS under static conditions. As for DHTs, I don't see how it would matter, you hook the CCSs up the same way you'd do a cathode bias resistor.

Roscoe

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I can explain it to you, but I can’t understand it for you.

Not sure, but I think each cathode "sees" the effective DCR of the opposite tube's cathode in parallel with the cathode impedance of the tube (as in the output of a cathode follower). In that case, the CCS would be swamped by the few hundreds of ohms of the cathode impedance. But that's just a guess.

In any case, it is a high pass filter. Start with Richard's values and see where the low cutoff is. Adjust from there.

Yeah, what Stuart said....

Roscoe

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I can explain it to you, but I can’t understand it for you.

I had the chance to hear Jim’s push-pull 300B amps last Wednesday and they are truly remarkable. Like most 300B amps they provide an air of delicacy and detail, and like most push-pull amps they offer better dynamics and power, but unlike most other amps these amps throw a super wide and super deep soundstage that seem to significantly extend beyond the room boundaries. Grover can speak more about the amp design which I think were driven by one 6SN7 and 6J5 tubes or three 6J5 tubes by way of an adapter with two 6J5 tubes to replace the 6SN7 tube. The rectifier I believe is a 5AR4.

Jim and I first listened to his ProAc Studio 1 speakers that were very nice sounding with superb highs, image and balance. Then we listened to Jim’s OB speakers from Caintuck Audio, which were better sounding with some music but they have no bass without a subwoofer which I was all too happy to supply from my house. The Caintuck OBs were more revealing and the soundstage width and depth seemed to increase 50%. One problem was that these speakers can sound aggressive and harsh on certain music while very good on other tracks. Unfortunately I am not a fan of systems where one has to select the right tracks to hear good sounding music while avoiding other music that normally sound good on other systems. I suggested that Jim talk to Roscoe about what the Caintuck OBs may be doing wrong and if any baffle or driver mods might improve things. I also told Jim that he may want to wait until CAF to hear what better low cost OBs may offer.

I'm glad you got to hear them! Jim filled me in a bit on the session. Yes, it's a very simple circuit: 6J5 direct-coupled to a 6SN7 long-tailed phase splitter/driver. The 6SN7 can be replaced by a pair of 6J5s with an adapter (they sell them on eBay). I can post a schematic if anyone is interested. It sounds far better than it has any right to. I'm sure it could be improved as well, but for a butt-simple, inexpensive amp it's pretty darn good. I'm currently rebuilding mine using a pair of Tektronix 502a power trannies I found on eBay. These are fantastic, with tons of taps, including 800 VCT, four 6.3 v and a 5 v tape, and lots of other HV taps for bias/driver/negative/whatever supplies. They are built like tanks and were $50 each plus shipping. I got four. There might still be some listed, they guy seemed to have a lot of them.

I also finally figured out the output bias issue by going back and actually LOOKING AT THE SCHEMATIC of the Heath W5. The common cathode resistor and the grid resistors are tied to a balance pot that adds a positive voltage to the output tube grids to balance and stabilize them. Duh. It works well and sounds a bit better too. But I'm very intrigued by Richard's current source method and amp looking into that.

My friend Jeff has become fascinated with Richard's schematics and is planning to build his PP EL34 amp.