1961 Zenith Stereo Amp Problem

[davet753]

The electric company just replaced the transformer on the pole in my back yard a few months ago. The voltage at my outlets is pretty steady at 124 volts. That is a bit high for the design of the amp, but I don't think that those extra 4 volts are enough to cause the B+ to be 30 volts off. The factory service manual says it's designed for 110 to 120 volts.

I know my speakers are good, because I've tried running another set and have the same outcome.

I'm wondering if the fault might be my output tubes. None of the tubes in this set have been replaced (with the exception of my substituting the 5U4 to see if there was any effect on setting the output tube bias). I never placed much stock in tube testers, and got rid of mine years ago, but I do wonder if these original output tubes have good emission levels now.

[Electronic M]

Originally Posted by davet753

The electric company just replaced the transformer on the pole in my back yard a few months ago. The voltage at my outlets is pretty steady at 124 volts. That is a bit high for the design of the amp, but I don't think that those extra 4 volts are enough to cause the B+ to be 30 volts off. The factory service manual says it's designed for 110 to 120 volts.

I know my speakers are good, because I've tried running another set and have the same outcome.

I'm wondering if the fault might be my output tubes. None of the tubes in this set have been replaced (with the exception of my substituting the 5U4 to see if there was any effect on setting the output tube bias). I never placed much stock in tube testers, and got rid of mine years ago, but I do wonder if these original output tubes have good emission levels now.

In the schematic you are getting the 'correct' B+ values out of do they list the line voltage they took that data at? If it was 115V, your line is running 9 volts higher, and the B+ winding of your power xformer/rectification scheme steps the line voltage up by a factor of 3 or more then that would be (3*9)=27 volts of extra B+ right there.

[davet753]

The schematic I have is a Photofact, and it says line voltage was maintained at 117 volts for the readings. Zenith rates the chassis at 110 to 120 volts. Thankfully, a generous videokarma member has offered to scan and email me a copy of the factory service manual It'll be interesting to see what Zenith has to say.

I turned it on this afternoon and let her play for a couple hours. I noticed that after it had been on that long, the volume could be turned up much higher before distortion sets in than it does after only being turned on a shorter amount of time. To be honest, it just sounded better in every aspect after being on for a few hours. I know weak output tubes can sometimes perform better after being ran for a while, and weak output tubes could also be the reason for the bias being a little off. I guess it would also likely have an effect on adjusting the current balance pots for the grids, also.