Sound City PA50+ low volumen...

[chocopower]

Hi there!

My old Sound City PA50+ is doing some strange things.

The volumen is not enought for a 50 watt amp and it distortioned al very low volumen.

Negative voltage at the grid is OK. (-32volts )
New filter and bias caps installed.
Plate voltage is Ok. (430 volts).

Before asume a semi-dead O.T. i´ll like to try some more options..

the schematic is for the guitar version, but the power amp secction is the same...

schematic

Any idea?

[Mars Amp Repair]

hello,
So many things could be causing this issue. Without having some measurements all I can say is I've had a few of these amps with open 1meg resistors in the phase inverter stage R36 & 33. I've only seen the 100 & 120 watt versions. they're slightly different in this stage.

I've also had issues with that pot with the sensitivity switch across it. BTW, in the high sensitivity position, these amps without mods, will always be very noisey. Suggestion is to only use in the lower sensitivity position.

Do you have a scope & audio generator that you can do some signal tracing with?

Also I believe there is a forum dedicated to the Sound City amps, too. Might be more experience there. glen

[chocopower]

hello,
So many things could be causing this issue. Without having some measurements all I can say is I've had a few of these amps with open 1meg resistors in the phase inverter stage R36 & 33. I've only seen the 100 & 120 watt versions. they're slightly different in this stage.

I've also had issues with that pot with the sensitivity switch across it. BTW, in the high sensitivity position, these amps without mods, will always be very noisey. Suggestion is to only use in the lower sensitivity position.

Do you have a scope & audio generator that you can do some signal tracing with?

Also I believe there is a forum dedicated to the Sound City amps, too. Might be more experience there. glen

Hi!

you are on right.
The 1M resistor was open! i changed it last week. More volumen, but still not the normal volumen for a 50watt amp...

I re-chek this area..

No.... i haven´t a scope.....

I post the question in the sound city forum and recomendme chek the Output Transformer... Is done... and everything seems to be ok..

Thanks!