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

Posted by Michael Samra on May 23, 2017 at 03:07:41:
Pete
You dont have to change the circuitry for the 12AU7 but I would use adapters and run 6CG7s because the difference in distortion and bandwidth is noticeable and measurable.Steve is right in that you want to beef the bias supply. You can use a 20uf film cap there but a lytic there is fine.
Why do you want to cut the charging cap to 20uf off the 5U4? I would use a 50uf film cap there and keep the existing choke because if you go to a 6Hy,that can kill dynamics and roll off the high end.What is the factory value of the choke? If it is 6Hy,then you should be ok because it was probably calculated.The 220k resistor from the 6BG6 plate is a loop feedback resistor for the driver.One more thing,if the output trafos in the amp are small,you may have to use some global NFB to keep the amp stable.