Tube Amp Squeel

[jshine]

I just finished building the S-5 Electronics K-16LS tube amp kit. It is in an aluminum chassis with Edcor CXPP30 output transformers. The problem is that as soon as the tubes warm up a high pitched squealing sound starts. This happens at every volume level, including zero volume. I have isolated the squealing to the preamp stage, since pulling the preamp tubes causes the squeal to stop. I have heard that this could be microphonics, but it seems strange that the preamp tubes for both channels would have this problem at the same time. I have tried tapping the preamp tubes, but this doesn't seem to change the sound, so I am not sure if microphonics is the problem. If anybody has any idea what might be wrong, I would be grateful for the help. Thanks.

[Soonerorlater]

Edcor CXPP30 output transformers.

Check your Output Transformer wiring to see if you have taken " positive " feedback off the OT secondarys instead of " negative " feedback.


Brgds Bill


ps, here is a link to ( I hope ) your amp schematic


Using Seperate Preamp to Drive S-5 Electronics K-16LS Integrated Amp?

Other thoughts,

1) According to the above schematic the negative feedback needs to be taken from the 4 ohm tap

2) Looks like the output transformer frame needs to be grounded

[Mickeystan]

Hello Jsine,

It could well be due to the amplifier's feedback being positive and not negative. To test for this try disconnecting the feeback loop and see if the problem goes away. If the amplifier has global feedback from the output transformer to the input stage and the squeel goes away when the feedback is disconnected. The problem was due to positive feedback and can be fixed one way or the other based upon the amplifier's design. Mickeystan

PS. Same perspective as Soonorlater posted. We must have been typing at the same time.

[jshine]

The squeal is gone. I reversed the plate leads on the output transformer primary and the squealing stopped. The PCB has the wire colors for the output transformers printed on the board, so I just hooked up my OPTs the way the originals were connected.

[Soonerorlater]

Enjoy