tube current problem

[srv69]

I recently checked the 5150's bias with a weber bias probe and I am detecting no current through one of the middle tubes... the rest are fine and that particular tube position seems to be the problem rather than the tube itself. Would you have any idea why I would be getting a full voltage reading on all the tubes and no current out of that one position?

[srv69]

I read somewhere that this type of thing could be a screen grid resistor problem... what value would they be and what location on the board would they be if this were the case?

[Jerry P]

Chances are real good if you have no current draw thru a tube you have a bad screen grid resistor. A few things to try first are swapping tubes around to see if the zero current reading stays with that socket or follows the tube. If it stays with that socket it's probably a resistor, if it follows the tube, you have a bad tube. If they are JJ 6L6's try wiggling them around to see if you can get noises and a reading. They have been using smaller base pins that are casuing problems with connections in the tube sockets.
The screen grid resistors are mounted on the bottom of the power amp circuit board. Each tube has a 100 ohm 5 watt resistor. To remove the board you have to drill out the rivets holding the tube sockets in. It's not a fun job. I've done enough of them that it's easy for me now. The first few times I did it, it wasn't any fun.
Jerry

[srv69]

I was just wondering if a 10 watt wirewound resistor would be an ok replacement or if the fact that the resistors are 5 watt is for safety?

[Jerry P]

I would stick with the 5 watt.
Jerry

[srv69]

Unfortunately it is very hard to locate anything other than 10 watt resistors. Will it make anything function incorrectly if I use this value? I know you responded that you recommend the 5 watt.

Thanks for all your time,
Sam