Orange OR80 runaway bias problem

[ronco05]

A buddy of mine gave me a mid 70's Orange OR80 that was broken. He said it kept burning out tubes. I had a tech replace the tubes, all the caps and some burned out resistors. The amp turns on but when we go to bias the tubes the meter keeps climbing real fast and wont stop.We had to shut it down after it hit 100mA. The bias on this amp should be in the 31 to 35mA range. The plate voltage is 560v. Thats normal for el34's. Any help. We are running out of ideas. Thanks.

[Gtr_tech]

Is there bias voltage at pin 5 of the output tubes? Doesn't sound like there is. Check there first. You can do this with the tubes out and in standby. If there is stable bias there then look to the PI coupling caps, If they get leaky they can pass positive voltage onto the control grids and effectively null you bias voltage.

[ronco05]

Is there bias voltage at pin 5 of the output tubes? Doesn't sound like there is. Check there first. You can do this with the tubes out and in standby. If there is stable bias there then look to the PI coupling caps, If they get leaky they can pass positive voltage onto the control grids and effectively null you bias voltage.

the voltage at pin 5 is -45.2
pin 3 is 559
pin 4 is 557
I cant find anything wrong with the caps.
when i put the meter on to bias it runs right off the scale.

[52 Bill]

the voltage at pin 5 is -45.2
pin 3 is 559
pin 4 is 557
I cant find anything wrong with the caps.
when i put the meter on to bias it runs right off the scale.

Does the -45.2 volts remain stable?

How are you measuring the bias? Please describe your set up.

[ronco05]

Does the -45.2 volts remain stable?

How are you measuring the bias? Please describe your set up.

the -45.2 volts is stable. i am using a eurotubes bias probe and a multimeter. when i turn the amp on the mA runs right off the scale.i have to turn the amp right off because i dont want to cook the tubes. the tubes are jj el34's that are brand new from eurotubes.

[Gtr_tech]

Sounds like you may have leaky coupling caps if the bias is ok in stby, but drifts in play mode.

[Bruce / Mission Amps]

Pretty much an OR120 with only two power tubes.
Not saying this is your problem but a common failure mode on these amps is the two low voltage bias supply filter caps... yup they are kind of a pain in the butt to replace but a must do!!!
I have refurbished at least four or five dozen vintage Orange amps... those bias filter caps need to be replaced as soon as you get the chassis apart.
I had been using two 22uF-100v caps or equivalent.

[ronco05]

The only caps i didnt replace are the two large caps on top of the chassis next to the transformer. are these the low voltage bias supply filter caps?

[Steve Conner]

No they aren't.

Did you maybe put the new bias filter caps in backwards? The positive terminal goes to ground.

Bruce, are the bias filter caps hiding somewhere that they might have missed being replaced?

[ronco05]

Then what are the two large caps on top of the chassis?

[gijoe]

The two caps on top of chassis are high voltage DC supply filter caps for your pin 3 plate voltage.