OT transformer feeding back

[jreese2001]

Hey I have a strange thing happening with my '66 ab165 bassman that I've been rewiring. Note that the phase inverter is basically the aa864 version.

First of all, the amp has about half power, based on voltage measurements on the speaker jack and by ear. I've made significant improvements on oscillation in the preamp, because previously the sound was very distorted and very quiet. Now the guitar sound is clearer but still not as loud or as clear as it should be.

Here is the strange part:
I input a signal generator (from my PC), a 1000Hz sine wave, about 150mV. I monitor the output on the oscilloscope, with amp connected to dummy load. If I start turning up the volume, I get to the point where the waveform on the scope starts to clip. At the very same moment, I start hearing a vibration in the output transformer, which intensifies slightly as I turn up the volume even more. As it clips more, the waveform on the o-scope shows more or less a square wave with a pretty sharply defined crossover notch. I tried a 600Hz sine wave as well, same result.

I am thinking this probably has something to do with the lack of power in the amp, as it seems I might be losing energy in the OT due to this oscillation or whatever it is.

When converting the PI back to aa864 specs, I made sure to switch the secondary leads to the speaker jack. Is there something else I should do?

Is this a sign that the OT is damaged? Any thoughts would be greatly appreciated.
Thanks,
Josh

[Twist]

With a dummy load I wouldn't be surprised to hear some noise from the output tranny. Just make sure your testing with the right impedance load. Also while your looking at the waveform on your scope, hook up an AC voltmeter to the dummy load and make note of the AC measurement just prior to clipping. Square that value and divide it by the resistance of the dummy load and that will tell you roughly how much clean power your amp is producing. The crossover notch should just barely appear prior to clipping if the bias is set correctly. Don't worry too much about it beyond that. Don't be surprised if your output tranny has partially failed. I'm assuming this is the original speaker transformer with the correct primary impedance for the tubes your using.

[jreese2001]

Twist, thanks a lot for your reply.

OK I did some more power tests and it looks like the amp is giving me full power. With my voltage-averaging DMM (and sine wave input) I got about 14 V at the output. 14^2 / 4 ohm is close to 50W.

I won't worry about the crossover notch, it sounds like that is normal.

One interesting thing is that today I was taking some voltage measurements on the preamp plates. For some reason, everytime I touched the DMM probe to these points (and also the grid of the second stage preamp tube) I could hear in the transformer sort of a clicking (arcing?) sound. This does not seem normal to me, has anyone else experienced this?

Thanks,
Josh