Weber 6H50 "Hiwatt" style amp. Howl and presence control issues

[axeman1956]

I need some help here. I just finished a weber amp kit 6H50,

Schematic here: http://taweber.powweb.com/store/6h50_schem.jpg
Layout here: http://taweber.powweb.com/store/6h50_layout.jpg

When wired as per the layout I get a low freq howl when I turn the post master up over 60%. Turning the post master down below 60% the amp seems to work properly, including the presence control. I have tried moving wires around to eliminate the howl with no change whatsoever. I reversed the output transformer leads to the power tubes and the howl stopped completely, but now the presence control has no effect. The fact that the presence control now doesn't work makes me think that the OT polarity was not incorrect. When I initially swapped OT wires the I got some high pitched oscillations when turning the post master above 75%. I was able to eliminate that by moving some wires around.

I read elsewhere online that a Hiwatt manual states that you may get feedback if you use a 12ax7 in the phase invertor position. I tried a 12at7 there without success.

I don't have a scope. Is there any sure fire way to know that I have the OT polarity correct? The presence control actions tells me one way is correct, and the howling tells me the other way is. Any ideas would be appreciated. Thanks.

[Jazz P Bass]

To test the polarity of a negative feedback amp you must find the wire that goes to the feedback path.
(You really need a scope & a signal generator.)
Well let's try it with a meter.
Hook up your meter to the speaker leads to measure Volts ac.
Turn the amp up to a decent level & strum a chord a few times.
Note the meter reading.
Turn off the amp & disconnect the feedback path.
Turn on the amp & play exactly as you did the first time.
If the signal measurement goes up (negative feedback) then the transformer polarity is correct.
If it goes down, then you had positive feedback & the polarity is incorrect.

[axeman1956]

Thanks. That helped me narrow it down. Based on the test above, with the OT polarity correct, there is no howl, but the presence control does not have any effect. Now I can concentrate on that part of the circuit.