Twin Conn Organ 7868 Rebuild

[scottpetersen]

The big thing with converting these is that you need s a scope and signal generator to properly adjust the FB loop
Is it a low frequency or high "buzz" An increase in sound when the FB is connected suggests that the phase inverter or output tube plate leads are reversed

 

[tizman]

Thanks for your reply Scott. I have not changed the leads from the original cofiguration. I took out all the tubes, components and wiring not directly associated to the power amp portion, and later moved one of the output transformers to the bottom of the chassis. The buzz, which is high pitched, was there before I moved the transformer when I tried putting the NFB back in. However, I haven't tried it with the capacitor yet. Unfortunately I don't have a scope. The amp works without the feedback network in place. It has a slight buzz that is only noticeable because the sets of speakers that I have range from 94 to 102 db in efficiency. Do you believe that the capacitor in parallel to the 18k resistor will solve the problem? I'm going to try it when I get a chance, but am wondering what you think. Thanks again.

 

[tizman]

When I put the the 18k resistor in as the only component in the NFB loop, I get a very loud buzz that doesn't vary with volume. Thanks again.

 

[scottpetersen]

Usually if the feedback increases the volume, adding the cap will likely make it worse.
Is it possible when you replaced the coupling caps from the inverter, you switched which output it was connected to?
because it is high pitched, it sounds like oscillation, This thread might be helpful:
http://www.diyaudio.com/forums/tubes-valves/145103-oscillation-tube-amps.html

 

[tizman]

I'm not sure. I'll try reversing the connections. Thanks again.

 

[tizman]

Hi. I let it sit for a while and then gave it to a friend to look over. The primary issue was in fact what scottpeterson last suggested. In addition, the power supply was a voltage doubler which I had mistakenly put different capacity caps into instead of matching capacity into the first positions. I got the feedback to work, but elected to not use it at the end because of the minimal buzz I heard after matching the first two caps in the voltage doubler. It sounds more alive and real without feedback. Sorry about the long delay in posting, and thanks for the help!