express bias too hot

[toner]

ive had a express up and running great for a while. tubes biased great in normal range with 220k resistor. i left the amp running for a couple days to "break it in" plugged to cab with a signal going thru it. didn't have any problems. now i went and put my old el34's in and the amp wouldn't bias the same set of tubes i used before. it was around 80ma now with the 25k pot all way down. then i turned it off and went tru it, couldn't find anything... turned it on again.. bias was at 60ma now... checked voltages and everything seemed pretty normal but it couldn't hurt to go over it again. turned it off... then back on a bit later and bias is now at 50ma... what could be doing this? my bias supply with no tubes was -34v. could a power supply cap be going bad from the stress? the tubes are still matched as far as bias so thought that would leave out a coupling cap going bad. i tried another set of power tubes and same result. thanks for your help in advance, if anyone has any ideas
thanks!
bry

[Richie]

it was around 80ma now with the 25k pot all way down

So, are you useing a 1 ohm resistor each socket or pair, or a bias meter?
One tube or both?

If so, check the 1 ohm resistor/s,make sure it hasn't changed value.

Using that method,You should bias the amp without any signal,and volume pot down.
A few things could cause the change. but first, is the neg voltage changing? or the neg voltage supply changing? If it changes, look at maybe the bias caps. If the neg voltage supply stays steady, i would suspect the 1 ohm resistors, bad meter your using to check with.. bad tube/s A leaky coupling cap, but that would be for one tube, what ever side was bad..

Just a few things. maybe check a few things, and let us know if the neg voltage supply changes or not. Is the bias pot ok ?

An oscilation can also cause the tube to run away.

[toner]

thanks for your help... allesandro bias meter on both tubes at once.. i just fired it up again.. 40ma.. dam! thats good, and it works fine now but just makes me worry its going to cause problems later. it creeped up to 42ma after warm up for a bit. it makes me think a electro cap is going bad somehwere, and getting better by turning on and off... i don't have a in circuit cap checker.. any ideas what would of done that?
thanks again!

btw, the bias meter is old.... i guess i could check it out but im not sure if thats a possibility.

[toner]

sorry forgot, bias pot is fine.. bias voltage was steady at around -35.5v (on the two resistor and cap junction)

[Structo]

To me that sounds like a possible bad solder joint.
Such that it changes with how warm the amp gets.

Did you check the plate voltage each time you checked the bias?

Have you compared any other biasing method to rule out the bias probe as the culprit?

[ckpop]

I would agree with looking for a bad solder joint in the bias supply first.
Make sure the bias supply on the output tube sockets is rock solid if not I can think of a couple of things

Cold solder joint
Bad bias trim pot
Bad caps in the bias supply
Check the wire in your bias supply that go's to ground

If you don't have a cap leakage tester just replace the caps in the bias supply.

Start with that

[SkinPimp]

Check the wire in your bias supply that go's to ground

I am not the grammar police by any means, But I love this.. I'm going to use it.. ha ha

[ckpop]

You got me guilty as charged with bad grammer

Don't do what I say, just do what I mean

[SkinPimp]

You got me guilty as charged with bad grammer

Don't do what I say, just do what I mean

Oh I am one of the worst with grammar. Anything important i have my wife proofread ..LOL..

Sorry to de- rail...

[jaysg]

dying cap or cold solder joint