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In Reply to: RE: Questions about 6BG6G amp build (schematic) Grommes 216BA posted by PeterI on May 22, 2017 at 21:02:33
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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Buy Chinese. Bury freedom.
Follow Ups:
"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."
Two things; one, Global loop NFB is the worst for creating stability issues, and IME it sounds the worst. This NFB is also balanced, and reduces the output impedance directly which helps with bandwidth in and of itself. It also linearizes/reduces distortion faster than SE on a per dB application.
Second, why would you put the power trans in the loop? I suspect you meant OPT...LOL
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Placing the power trans in the loop allows you to modulate the AC mains. Your neighbor can enjoy tube sound too!
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Buy Chinese. Bury freedom.
Thanks for your advice on this, in particular about triode operation. I have built several amps inc 6L6GB/C and mention them below, but certainly don't expect anyone to dig thru all that. :) The plate caps are sort of the whole point of this build. :)
I once made a preamp around a beautiful cast alloy Vol knob from a 1948 EH Scott amp.Seems like another way to treat screen voltage is with a 15H 40mA choke, instead of resistor. Unfortunately, R is high in those chokes.
I could do screen regulation; I have room for OBx type tube regulator, since I'm doing this old-school. They will handle the current but not sure about 250-300V.
Researching screen regulation on this august site shows lots of fights and opinions. Consensus is rare. But I guess that keeps it interesting. :)
Thx
Edits: 05/25/17 05/25/17
I don't think a screen choke is useful for push-pull. The simple RC approach has proven to work well. If you want to regulate, a series pair of 0A2 or 0D3 will get you to +300V.
OK, I understand the aesthetics you're after. If I were going down that road, I'd consider the 1625/807. A quad of those has a nostalgic look that's hard to beat.
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Buy Chinese. Bury freedom.
That's good advice - i will keep screen connections simple. Probably can convert to Triode later to see what that's like. I don't think I need a ton of power. My only triode amp (my first build) was w/ EL34s.
Funny you should mention 1625 and 807s - I have good quads of each of those. BUT you have to commit to the oddball 5 and 7 pin sockets; can't plug in alternatives.
I believe those numbers are maximums. That's a lot of juice, will try to stay in the moderate zone.
Datasheets indicate that this tube type is rated for 20W maximum anode dissipation. TDSL shows that as well in the abbreviated specifications. However, TDSL shows AB1 operating conditions with 450V at 116mA idle current. That's over 26W per tube. Operating at full power, or 210 mA, pushes that to 47W.
TDSL shows these same operating conditions for the 6L6 and 6L6GC. In fact, the latter is the only one in this series that can be run at those power levels.
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Buy Chinese. Bury freedom.
Yeah, I knew that current draw was crazy high, but I'll have adjustments to each tube, at least that's the plan. I will measure mA at each tube.
OK, sounds like you've got it under control. Have fun with the build!
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Buy Chinese. Bury freedom.
Sorry, I had forgotten that the 6BG6G is the ST bottle. It's also one of the nicer looking tubes. Guess there's not much reason to use the less common sockets, although the 1625/807 do have a higher dissipation rating. Speaking of which, watch out for the limit if you're building a supply over +400V. The 20W rating will be easily exceeded at higher voltages.
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Buy Chinese. Bury freedom.
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