Help needed with uneven bias on Fender Blues Junior

[jonradford]

Hello everyone,
This is my first post here. Hope I can learn something!

I have a Fender Blues Junior I'm servicing for a buddy. When I got the amp I noticed the bias was very uneven and figured it was probably the output tubes. So anyway I went ahead and installed a bias trim pot as these run really hot! I put in some new matched output tubes and re-biased the amp. All should have been ok but I discovered the bias on each tube is very different. 25ma on one and around 40ma on the other.
I tested all the voltages in the bias circuit and output tubes and they all match up. I tried switching the inputs to the OT (NFB disconnected) yet the high bias stayed in the same place. Tried yet more tubes with no difference. I replaced the coupling caps between the PI plates and the Output tube grids.
I then tried removing the PI tube and checking the bias gain, this time they matched. Both 25ma.
So before I start replacing every component in the PI stage I thought I'd ask around and see if anyone knows what is going on here?

Cheers,

Jon

[sluckey]

Removing the PI tube should not affect the output tube bias in a properly working BJ. Monitor the grids of the output tubes with a scope to see what's going on when you plug in/remove the PI tube.

[jonradford]

I just went to test the grids and everything seemed ok. I checked the bias again and all is well. The amp had been on for a few minutes. I decided to turn theamp off and on again and I'm back to the original problem.

I will try again!

[jonradford]

I'm not really sure what I'm looking for here? The bias voltage on the output tube grids drops a fraction on both sides when I plug the PI tube back in.

[sluckey]

You would be looking for some high frequency parasitic oscillation that might cause the current of one tube to increase.

How does the amp sound? How are you measuring the tube current? Have you put 1Ω resistors on the cathodes yet? Your meter may be lying to you???

[jonradford]

I use the shunt approach with my ammeter paralleled across each side of the OT. It's a bit tricky to install 1Ω resistors on the cathodes of Blues Jnrs.
I've tried using two different DMM and they both get the same readings.
The amp sounds ok.
I will check with the scope again but didn't see any parasitic oscillation before.

[jonradford]

Ok, I just checked again and there is something on the side where the bias current is lower.

[sluckey]

I was expecting something on the side where the current was higher.

[jonradford]

Ok, I've just taken some more readings and discovered there is 59vac on the plate of the tube which has a lower bias. There is 1.2vac on the other tube.
(Only when the PI tube is in)
When the PI tube is removed they both read around 1.3vac

[defy555]

Stumbled on this post while trolling through looking for something else, but thought I'd add my 2 cents worth in case it helps others...

I've had several Blues Jnrs come through my shop with this problem of uneven/unstable bias on the power tubes.  Everything checks out ok, but one tube of a matched pair runs waaay hotter than the other.  Problem stays with the socket, not the tube.  I think the culprit is parasitic oscillations.  Bump the grid resistor values up and it stabilises.  You can bump them straight up to 10k or play around with some values in between, but I have found 10k is always stable, and has negligible effect on tone.  Hope that helps.

Stay safe out there.