pa amp distortion at low volume

blindbrett · 8th January 2008, 02:09 AM

I have been rebuilding an old fanon 65 watt pa amp and after recapping have serious distortion as soon as I try to turn it up. At about a quarter volume, the amp starts to distort quite bad and I don't seem to get any more volume no matter how high I turn it up. I am still learning and I don't have a schematic for it so I am just checking out what I know. the bias is ok and the tubes all test fine. I am wondering if I have an open cap or shorted resistor somewhere, I am just not sure what to test first. Any help would be greatly appreciated and I will try to be as informative as I can when describing what I find. thanks to all.

Best Regards

Brett

Giulio Maiocco · 8th January 2008, 01:19 PM

Hello,

I'm wondering if you have problems with the output transformer, in the past I had troubles with some amps distorting horribly and with low output volume, in all cases it turned out the output transformer was the culprit, having one side of the primary winding with shorted turns. An easy check is to measure the primary resistance between the center tap and each of the ends, a reading up to 10 - 15% of difference may be Ok.

Cheers

Giulio Maiocco

pedroskova · 8th January 2008, 01:29 PM

Was it working properly before hand?

When you say "recap" does this mean just the PSU caps or all caps including signal and feedback(if any) caps?

8th January 2008, 02:09 AM	#1
blindbrett diyAudio Member Join Date: Sep 2007	pa amp distortion at low volume I have been rebuilding an old fanon 65 watt pa amp and after recapping have serious distortion as soon as I try to turn it up. At about a quarter volume, the amp starts to distort quite bad and I don't seem to get any more volume no matter how high I turn it up. I am still learning and I don't have a schematic for it so I am just checking out what I know. the bias is ok and the tubes all test fine. I am wondering if I have an open cap or shorted resistor somewhere, I am just not sure what to test first. Any help would be greatly appreciated and I will try to be as informative as I can when describing what I find. thanks to all. Best Regards Brett

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8th January 2008, 01:19 PM	#2
Giulio Maiocco diyAudio Member Join Date: Apr 2006 Location: Fossano/CN	Hello, I'm wondering if you have problems with the output transformer, in the past I had troubles with some amps distorting horribly and with low output volume, in all cases it turned out the output transformer was the culprit, having one side of the primary winding with shorted turns. An easy check is to measure the primary resistance between the center tap and each of the ends, a reading up to 10 - 15% of difference may be Ok. Cheers Giulio Maiocco

8th January 2008, 01:29 PM	#3
pedroskova RIP Join Date: Dec 2002 Location: C'ville VA, USA	Was it working properly before hand? When you say "recap" does this mean just the PSU caps or all caps including signal and feedback(if any) caps?