Idle current going way too high

[jussi]

Hey

I have built this amp and again I have a problem I couldn't solve myself. The idle current of my 6v6s is going up to 80mv and plate voltage is around 450V.. The tubes are JJ so they should handle that plate voltage but the idle current seems to be going up and up so I always turn the amp off after <10 seconds.

I think the problem is in the grid bias supply because the voltages are never under -3 VDC. I've marked the measured voltages with red color in the schematic. PSU is taken from this schematic: http://mhuss.com/Plexi6V6/Plexi6V6c.gif

I have checked all the resistors and they seem to have the right resistance. The bias pot doesn't have much effect but it is wired right and works. Any ideas? Could it be broken diode or caps?

Schematic: http://jussii.tk/tmp/6v6volt.JPG
Layout: http://jussii.tk/tmp/layout.png
High-res picture: http://jussii.tk/tmp/IMG_4239.JPG

I also get no sound and the amp starts to hum (maybe 100hz or something) after it has warmed up but those might be from other problems.. I think it would be wise to fix this first.

[eschertron]

Remove the power tubes, turn it on and start troubleshooting the voltages on the bias circuit. That way the tubes are safe, and you can keep the power on for more than 10 seconds.

[g1]

The 2 bias caps (47uf) are installed backwards. The + end should be grounded as they are filtering negative voltage.
You should replace them with new ones as they may have been damaged from the reverse voltage.

[Bruce / Mission Amps]

Possibly you have wired in the first 47uf filter cap of your bias supply with reverse polarity... positive is to be grounded.
If yes, replace both the bias supply filter caps.

[Bruce / Mission Amps]

Oops... 2 mins late... ha ha

[g1]

Also, you should correct your layout drawing so it agrees with the Huss schematic. You have drawn those 2 caps backwards which is probably why this happened.

[jussi]

Oops... Good it didn't blow up. Thanks! Now I've replaced the caps and I'm getting 33 mV idle current which is still very high. hmm

[g1]

33mV is the lowest you can get? What is the DC voltage on pin5 of the output tubes when you have it set this way?
Have you tried other tubes?