CF 275 amp problem

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kve777
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Location: Wonderful South Jersey

CF 275 amp problem

Post by kve777 » Wed Jan 02, 2019 8:29 pm

I'm on my third amp build, but have a problem that's got me stumped. I get this:
IMG_1019.JPG
at C18/R32 when I get the Variac up to 80VAC. It's not constant, but a pulse that only lasts half a second, repeats about every two seconds. There is a corresponding jump in amperage draw and needle deflection. If I turn the Variac up any more it goes nuts. Haven't blown a fuse. Rechecked all components and solder joints in both feedback loops. It goes away if the 12AX7 is removed or if all of the KT120's are removed. Something is oscillating, but I can't figure out what. I tries different 12AX7 and 12AT7 tubes. All tubes test good. I've even tried just one KT120, but it's still oscillating no matter which position it is in. Any thoughts?
Kevin

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Ben D. Rinkin
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Re: CF 275 amp problem

Post by Ben D. Rinkin » Thu Jan 03, 2019 5:02 am

Kevin,

That sounds like feedback 180 out. Try flipping the phase by reversing the output wires on that channel.

BDR
Seeking audio quality that's so real that it can change your mood!

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UncleMeat
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Re: CF 275 amp problem

Post by UncleMeat » Thu Jan 03, 2019 7:11 pm

Try swapping the signal wires that go to the output transformer from the KT-120 outputs on the left channel. The schematic does not show any particular polarity of them but in order for the feedback to work correctly they cannot be 'backwards'. I had this problem with my amp at CF, I didn't see it on a scope but when trying to run it at Cabin 2 it was oscillating. Compared it to the working amp that was the source of the trouble.

This is similar to what BDR is suggesting, but it's not in the feedback, it's on the supply side of the output transformer.

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Ben D. Rinkin
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Re: CF 275 amp problem

Post by Ben D. Rinkin » Fri Jan 04, 2019 6:19 am

Yes, thank you Mr. UncleMeat, that's better said. The whole circuit is a closed loop when you close it with feedback. The point is it has to be negative, not positive, to avoid jumping immediately into wild oscillation. Without any documents to go on in front of me, I was having trouble visualizing the specifics.

Over the holidays my dear old Mother said, "Benny, you need to spend more time with your toys, the things you used to enjoy so much". And she's right of course. So hear I am, trying to follow her advice and get myself back into the audio groove again.

So, I would be interested to hear what is found.

BDR
Seeking audio quality that's so real that it can change your mood!

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