Fluctuations in Bias on One Tube

[sixpakldp]

I was doing some tube swapping on my plexi and when biasing the tubes I noticed that v4 would go to zero and then come back up to set point. So I went through the amp looking for suspect parts and also decided to replace the screen resistor on V4 for good measure as well. I had to bend the eyelets a little on the tube socket to get the old resistor out and the new in. Went to rebias and now I have 0 mV on V4. Think it could be a bad tube socket? Just curious before I make any conjectures of my own. BTW it's not the tube as it occurs at the same socket regardless if you swap tube positions.

[danman]

It sounds like you may be losing connection at pin 8 (cathode) or one of the heater pins. Either one of those could stop the tube from conducting and drawing current. A bad ground or faulty 1 ohm biasing resistor on pin 8 could also stop conduction. It would be a good idea to check the negative bias voltage at pin 5. This will be a DC voltage you are looking for. Usually, a problem with the bias voltage will cause excessive current draw and not a zero reading like you are seeing though.

[sixpakldp]

Thanks danman. I think you may be right on the pin 1/8 resistor. The other paired tube that it works with had one go.