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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
With 400VAC to rectify, and a DH rectifier, root2 times your AC voltage is a good, absolute minimum capacitor rating. It will be over 500V...:)
The plate to cathode connection is NFB...:) I'd bump the current in their circuit( lower value resistor) up and drop the global NFB connection.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Follow Ups:
Doug
He may not be able to totally dropping the NFB because the transformers have small iron content a lot of time.He could add a 100 resistor in series with the 1k and reinsert the loop between them and add a bypass lytic. That would lower it quite a bit but being the amp also has that 220k to the driver,that tells me they probably did a lot of tweaking to make the amp stable with the existing iron.I'm speculating but it makes sense.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
That is, of course, the problem (of several) using old schematics as a guide. One can't fully know the old iron used, and what compromises were made to work around the iron's specs/shortcomings. A picture of the amp shows BIG OPTs. Mine are smallish, from Hammond organ amp. I used them in a previous build so they sound full range.Tube data shows 6BG6G in class AB1 - 450V anode, 400V grid2, -37V grid1 bias, and 5.6Kohm impedance on OPT. So that was sorta was I was going for.
Thanks for info,
Pete
Edits: 05/23/17
For a given amount of FB, the longer the path the less stable. The RCA SP10 and 20 used a similar topology, and had no global loop.
Aside from the idea of building with 12AU7 because that is what is on the original, bean countered schematic...well, good luck wid that. Or, build, learn, and then decide how to apply what you learned for further improvement...LOL
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I agree but look at the iron in the SP20.It is humongous and the amp is low power.All I'm saying is, Grommes used that output trafo with GFB and loop FB to the driver and much of that was probably to compensate for the output trafo. I havven't seen the OPT trafo in the amp but if he runs it at full power and bandwidth,he may run into problems.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 05/24/17
This is what I'd do. You may end up liking the 12AU7.
I may. A surprising number of high-end preamp/amp manufacturers still use 12AU7 tubes. They aren't doing it because they're beancounters.
Thanks for "input" :)
In a preamp they are fine but in an amp where the driver is much more demanding,there I would use a 6CG7.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
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