02-23-2018, 08:10 PM
Hey AMPREPAIRJN,
Thanks for your suggestions!
This issue was the reason I started rebuilding the output section. Those .33uf caps were the first thing I replaced. I did double check them for a DC voltage and they were clean. Also, all resistors in the bias supply checked out, including R35 and R36 (2x100k) which I had already replaced.
I did notice something interesting:
-the AC signal through the .33uf corresponding to the pair of tubes that slowly red plate (when driven very hard) is always 10v higher than that of the other .33uf. It's as if the phase inverter in unbalanced.
As part of this rebuild, I had already replaced the PI plate resistors and I double checked them tonight. C12 was also replaced at some point. The plate voltage of the PI 210vdc on each and I double checked the 12AU7 on a curve tracer to make sure the triodes were matched. I also checked R27, R29, R28, R32 and R33 - they all checked out. Cathode voltage of the PI is 9vdc.
All voltages on the power tube sockets check out. I did go back though and meticulously checked the work I had done. It seems to be good.
I will leave you with this:
I again attached a meter to the control grid on of one power tube on the "bad" half and another meter to the control grid of one power tube on the "good" half. At idle, each grid measures -60vdc. As I increased the amplitude of a 1k tone, the "bad" tubes bias voltage reached -75vdc where as the "good" tubes bias voltage reached -85vdc.
Here's a chart:
VAC VDC
R37/R33(pin3) 70 -75
R38/R44(pin3) 60 -85
After all the checking and double checking I have done, I'm dumbfound as to why these voltages are asymmetrical.
Thanks for your suggestions!
This issue was the reason I started rebuilding the output section. Those .33uf caps were the first thing I replaced. I did double check them for a DC voltage and they were clean. Also, all resistors in the bias supply checked out, including R35 and R36 (2x100k) which I had already replaced.
I did notice something interesting:
-the AC signal through the .33uf corresponding to the pair of tubes that slowly red plate (when driven very hard) is always 10v higher than that of the other .33uf. It's as if the phase inverter in unbalanced.
As part of this rebuild, I had already replaced the PI plate resistors and I double checked them tonight. C12 was also replaced at some point. The plate voltage of the PI 210vdc on each and I double checked the 12AU7 on a curve tracer to make sure the triodes were matched. I also checked R27, R29, R28, R32 and R33 - they all checked out. Cathode voltage of the PI is 9vdc.
All voltages on the power tube sockets check out. I did go back though and meticulously checked the work I had done. It seems to be good.
I will leave you with this:
I again attached a meter to the control grid on of one power tube on the "bad" half and another meter to the control grid of one power tube on the "good" half. At idle, each grid measures -60vdc. As I increased the amplitude of a 1k tone, the "bad" tubes bias voltage reached -75vdc where as the "good" tubes bias voltage reached -85vdc.
Here's a chart:
VAC VDC
R37/R33(pin3) 70 -75
R38/R44(pin3) 60 -85
After all the checking and double checking I have done, I'm dumbfound as to why these voltages are asymmetrical.