July 18th, 2020, 12:31 pm
July 18th, 2020, 3:01 pm
July 18th, 2020, 6:27 pm
July 19th, 2020, 1:00 pm
July 19th, 2020, 8:01 pm
July 19th, 2020, 8:07 pm
David McGown wrote: with an additional feedback loop and a weird resistor bypassing the coupling cap to each output tube
July 19th, 2020, 8:43 pm
David McGown wrote:Grover,
I never could get the Williamson version to stop oscillating and overload the output section. The problem was the amp pulling so much current that I feared I would redplate the tubes and/or burn up the output transformers. Therefore, never really could run it more than a few seconds at each change I made, which was not enough time to really do any live troubleshooting or using a scope to determine what the oscillation was and how to defeat it. I decided to build the other monoblock using the modified Mullard circuit, and it worked great at first turnon, absolutely stable. I then rebuilt the front end of the non-working Williamson to the Mullard and called it done. Hooked them up, and outside of upgrading coupling caps and some tube rolling, they have been in my system ever since.
I do not know what my problem was, but Dynaco modified Williamson circuit was more complex than the classic Williamson, with an additional feedback loop and a weird resistor bypassing the coupling cap to each output tube. Perhaps the classic circuit you used, with vintage iron, 6L6s, and cathode bias would have been the way to go. There is no doubt the Mullard is the more forgiving circuit, though it does have a good amount of gain, I have had to make adjustments in my sources (setting the Chord Qutest down to 1V output, and some tube changes in my phonostage). I wish I could find an input tube that sounds as good as the Genelex 12AX7 (it is REALLY good), my idea would be something around a gain of 40 to 50. Perhaps try a 12AY7...
David
July 19th, 2020, 9:35 pm
Roscoe Primrose wrote:David McGown wrote: with an additional feedback loop and a weird resistor bypassing the coupling cap to each output tube
With that resistor bypassing the coupling cap, are you sure you actually had a negative voltage on the grid?
Roscoe
September 6th, 2020, 3:25 pm
David McGown wrote:Yes. Global feedback, around 20 dB or so. Basically using the feedback network from the Dynaco Mark 3 and it seems to work fine. Made sure the divider at the cathode of the input tube was the same, and used a bypass cap around the rest of the cathode resistor needed to bias the stage. I probably should fine tune it, but the amps are so sounding fine as is, been hesitant to tweek them any more.
DavidRoscoe Primrose wrote:David McGown wrote: with an additional feedback loop and a weird resistor bypassing the coupling cap to each output tube
With that resistor bypassing the coupling cap, are you sure you actually had a negative voltage on the grid?
Roscoe
I tried it with and without, and had the same problem. That had me running in circles for a while.
David
September 6th, 2020, 3:41 pm
Roscoe Primrose wrote:David McGown wrote: with an additional feedback loop and a weird resistor bypassing the coupling cap to each output tube
With that resistor bypassing the coupling cap, are you sure you actually had a negative voltage on the grid?
Roscoe