Hello,

I'm restoring a tube amp/preamp from an old stereo console. Vintage and model completely unknown. I've replaced all of the electrolytic caps but left the ceramic caps and resistors untouched. I have replaced all tubes with verified tubes. The problem is that I'm getting distortion in one channel. Swapping tubes around has no impact. I realize this is a broad/vague question but any suggestions on how to troubleshoot? I have a quality multi-meter but no capacitor checker, tube bias tester etc. and no schematic to test specific voltages. Any suggestions appreciated.

 

since you have stereo unit, you can check and compare left and right channel.
at least check bias voltages of the tubes. also you should be able to read part numbers of the tubes. that should help in finding possible schematic at least for individual stages. in general tube amplifiers are simple (few components). i would not worry much about ceramic caps but would check resistors. usually old models had wide tolerance (20% for example). one more possible culprit is tube sockets (oxidized or mechanically loose contacts).

 

Tube amps are notorious for bias problems. That ends up causing clipping on one channel as it walks the operating point up or down.

You troubleshoot by isolating individual stages and injecting an audio signal and looking for distortion at the output of that stage.

 

Can you post the tube ID Numbers?
This can help in that it can provide a reference to get an accurate tube/Amp schematic for each tube.
..
May not be the exact schematic for your device - but tube amps are pretty defined when it comes to what schematics work with what tube.
..

 

Hello,

You might contact the people at UK Vintage Radio Repair and Restoration:
http://www.vintage-radio.net/forum/index.php

If you post some pictures over there, they might recognize the model.

Bertus

 

You should also replace paper and molded paper caps.
http://www.antiqueradio.org/recap.htm