Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ronco05 on March 16, 2012, 07:42:13 am

Title: Orange OR80 runaway bias
Post by: ronco05 on March 16, 2012, 07:42:13 am

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.

Title: Re: Orange OR80 runaway bias
Post by: rzenc on March 16, 2012, 07:52:00 am

Quote from: ronco05 on March 16, 2012, 07:42:13 am

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.

Check bias rail. You should get a stable reading before going on.
Post a schem. Is there a bias pot on your amp?

Title: Re: Orange OR80 runaway bias
Post by: ronco05 on March 16, 2012, 08:34:42 am

there is a bias pot but when i turn it nothing happens because the bias is running up so fast.. I dont know to how to upload a schematic but its at http://www.orangefieldguide.com/. (http://www.orangefieldguide.com/.) its the 1974 schematic.thanks.

Title: Re: Orange OR80 runaway bias
Post by: phsyconoodler on March 16, 2012, 01:13:46 pm

You need to rebuild the bias supply.There us a bad component,likely a diode or capacitor.Just replace the whole circuit and you will be back where you need to be.It's not at all hard to do.

Title: Re: Orange OR80 runaway bias
Post by: stratele52 on March 16, 2012, 04:01:00 pm

I can found your schematic . Don't put power tubes on their socket until you can read a good negative bias voltage on pin 5 on EL34. You'll save your tubes.
Repair your bias circuit. After that turn the bias adjustment pot for full negative voltage on pin 1 at least minus 25 volts DC . Put tubes on circuit and test bias tube's current and adjust.

Be sure your EL34 are not short before. It could if they run more than 100 ma

Title: Re: Orange OR80 runaway bias
Post by: jjasilli on March 16, 2012, 08:36:15 pm

Also, make sure ea power tube grid is connected to ground (through its grid leak, or mixing, resistor).

Title: Re: Orange OR80 runaway bias
Post by: ronco05 on March 19, 2012, 12:40:47 pm

I finally got it to work. The bias pot and a resistor were bad. Thanks for the help.