Can't properly bias JJ EL34 tubes

[guitarist1977]

Hi,

I have recently completed the 50 watt build. Yesterday I got 2 JJ EL34 tubes and tried to bias them. I turned bias pot all the way up however I get nothing more than 27ma per tube?

Are the tubes bad?

Before I had a pair of EH EL34 and I could bias them for higher current, but they were slightly mismatched. 37ma on one and 33ma on another.

Thanks,
Alex.

[danman]

It looks like you just happen to get a set of tubes that bias up on the cool side. You can lower the value of the resistor that is inline with the bias pot and this should allow you to get the tubes biased up properly. If you are currently using a 47k in that position, you should be good replacing it with a 33k. It might be wise to check pin 5 of each power tube socket first to make sure that you are getting good negative DC bias voltage at each socket.

[guitarist1977]

Thanks danman. It is a good idea to check that. BTW what's the range for negative DC bias?

[danman]

I can't remember off the top of my head what the range was on my 50 watt build. If I get some time soon I will try to check it for you. The main reason that I mentioned it was just to make sure that you were seeing a good voltage range from one end of your bias pot's sweep to the other. If that looks good then you should be able to get into a better range by dropping the resistor value one step.

[guitarist1977]

Thanks danman. Changed resistor to 33k and was able to set bias properly. Voltages are all ok.

[danman]

Excellent!!