I noticed yesterday a 7591 power tube plate was glowing red.
I quickly connected my meter to the cathode 10 ohm resistor and saw .650v so I adjusted it down to .360v with the IBAM pot. But it ever so slowly continued to creep up. Swapped tubes around, same location was creeping up still. Verified the 10 ohm cathode resistor is good. Took some voltage readings and pondered over the schematic.
I realized a leaking coupling cap from the prior stage could put positive DC into the negative bias of the power tube control grid and cause the bias to go more positive. I replaced the coupling cap and problem solved.
I tested the removed cap on my Sencor checker. It went from original value of .068uF to .077uF and it had 9uV of leakage. I check all these Russian caps when I got them new, all were in tolerance. Installed them about 3 weeks ago.
Anyone had any issues with the Russian PIO caps?
Jef
I quickly connected my meter to the cathode 10 ohm resistor and saw .650v so I adjusted it down to .360v with the IBAM pot. But it ever so slowly continued to creep up. Swapped tubes around, same location was creeping up still. Verified the 10 ohm cathode resistor is good. Took some voltage readings and pondered over the schematic.
I realized a leaking coupling cap from the prior stage could put positive DC into the negative bias of the power tube control grid and cause the bias to go more positive. I replaced the coupling cap and problem solved.
I tested the removed cap on my Sencor checker. It went from original value of .068uF to .077uF and it had 9uV of leakage. I check all these Russian caps when I got them new, all were in tolerance. Installed them about 3 weeks ago.
Anyone had any issues with the Russian PIO caps?
Jef