Oscillation with high NFB on 5F6a

robrob

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Something may be wrong with this amp but I am not going to rebuild it. I will look into the NFB wire routing but it sounds pretty good as-is.
I'm not suggesting you rebuild it, just verify all the component values and continue to troubleshoot. Find the problem and make the amp hole without the use of bandaid caps.
 

Antoon

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I'm not suggesting you rebuild it, just verify all the component values and continue to troubleshoot. Find the problem and make the amp hole without the use of bandaid caps.

It is a possibility. But analyzing each individual component requires them to be removed from the amp, piece by piece. Also many components like the OT are original/vintage, that I only want to replace if they are really toast. Connections, solder joints have been gone over many times so only a very deep and meticulous analysis might reveal anything new. I have been working on this amp off and on for more than 15 years, like a never ending project to 'improve' it. Each time with smaller or larger modifications. It was a 50+ years old molded bag of parts when I got it. In some sections it has become one big spaghetti wired monster with many switchable options, additional circuit boards installed and taken out again, etc.. Much of the circuit is full of hot melt glue applied in the past to combat resonating components. It has been (partly) rebuilt several times. The few new amps I have built were easier, cheaper and quicker to assemble, and they worked without any weird noises. With this particular amp I would need an oscilloscope to really understand what is happening I think (I don't have a scope). And again, it sounds really good, it is a loud as it should be etc.. The oscillations occur only in very specific situations, so I can live with it.
 




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