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RE: Questions about 6BG6G amp build (schematic) Grommes 216BA

You've received a lot of good advice and suggestions so far. I'll throw my $.02 into the ring at this point...

1. "Are there any modern but simple changes that would improve this 1950s design?"

I suggest replacing the original output tubes with 6L6GC or 6P3S-E. That will eliminate the high voltage wiring above the chassis. Replace the 12AU7s with 'SN7 octals. I would use a solid state power supply for better regulation. Also, the filter design needs to be qualified for output impedance over frequency. Some older designs perform poorly in this regard. I never buy parts or cut the chassis before analyzing a proposed PS filter with SPICE. Several people here can do that for you if you want.

2. "What does that connection w/ 220K R from 6BG6 plates to the cathode of the driver 12AU7 accomplish?"

It's a nested feedback loop. I would eliminate it and use one loop only. Including the power transformer in the loop does no harm.

3. "Since I will be using OPTs with 8ohm speaker tap only, how much would I adjust the feedback resistance? I have modern speakers. My OPTs are 5.5Kohm, (w/ no UL taps) though this design calls for 7Kohm. (If I can find 6.6 - 7Kohm OPTs I'll use 'em)"

That's one of the most difficult tasks you'll face in terms of optimization. I generally apply about 12dB NFB to pentode amps, then work it from there using square waves, distortion measurements and various loads (including capacitors). OPT primary impedance isn't critical for pentode use; your 5.5K transformers will be fine.

4. "Should I use the triode connection shown or supply G2 from a separate ~400V DC source off the B+ line? I am using tube data from old sources, tdsl.duncanamps."

I would not make it switchable like the original design unless you plan to use this as a guitar amplifier. For hi-fi use, it's not possible to optimize for both modes in the same amplifier. Triode operation can bring sonic benefits, but only if the support components are very high quality and your speakers are efficient enough that you don't need the additional power provided in pentode mode. Also, the OPTs need to be greater than 5.5K for best performance in triode mode. Regarding screen voltage, the most common method is to use a dropping resistor and good quality bypass. Regulation can be beneficial, but I wouldn't include it in a first-time build.

I'm sure you realize by now that there are many ways to skin this cat. There's nothing particularly outstanding about the Grommes design, and it shouldn't be considered an immutable standard for your work going forward.


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