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In Reply to: RE: D3A (6J52P/6Ж52П) Class A2 driver/buffer posted by Galin on December 13, 2015 at 05:19:54
Read the attached article . Note the section on "Source Follower Drive for Output Tubes" A MOSFET source follower flat out works and drives A2 better than anything else.
Read this one too http://tubelab.com/designs/845-se/
The Shishido amps are also an option, but I could never get the designs to flow much current into the A2 grid: http://www.single-ended.com/direct811.htm
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Say what.
The point I think he was going after is that a MOSFET used as a source follower does not provide any gain and is very non-intrusive into the sound of a Tube amp. When you start getting into gain is when the amp become hybrid and takes on the characteristics of solid state amplification.
Yes, bi-polar transistors can make beautiful sounding solid state amplifiers.
Thanks for the links. I'm aware of R.G.'s article and at some point I was considering a MOSFET buffer however currently the amp is set up like on the schematic so I would like to try some direct replacements if possible and keep it as simple as possible.
Actually it works now with a triode connected 6J52P and 6N1P but the sine wave I have at the output doesn't look symmetric. I'm getting good sine wave up until the SV11-10 grid. It's running at ~105mA which kind of high even for those tubes.
You really need to post all the voltages that you have on your schematic so we can get an idea about what the operating points might be.
You can just put different tubes in and expect a circuit like this to work, they are far more complicated than you are giving them credit for.
It is just physics. You need to be able to have a super low grid impedance and source many watts of current. Yes, it works now, but obviously not very well.
Actually I have only one SV811 per channel (I borrowed the preamp/Cf part only). The triode connected 6J52P should have low enough output impedance to do the job. Obviously the circuit will need some more tweaking.
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