Good day all. I recently built a single channel version of an Ampeg B15N68.
http://www.prowessamplifiers.com/schematics/ampeg/B15N68_Portaflex.html
The only differences are that I used 12at7 input, a 5U4 rectifier, a 33uf-450V (this could be part of the problem) instead of 30uf-600V first cap, and moved the standby from the CT to the classic fender position between the Rectifier and the 33uf Capacitor.
When I turn it on it yells at me. Like a deep(ish) (not a squeal) hum, but really loud. I tried switching the plate connection on the OT with no change. There is also no change when I adjust volume or tone. There is actually no change when I plug in an instrument either, in fact there appears to be no input coming from the instrument at all.
All the tubes are new except 5U4 (which works in another amp) and the 12at7 which I tested in other amps as well as a tube tester and it checks out.
There is no noise when the standby is on, however when I turn the power off, the noise comes back and then fades out again as (i assume) the caps drain... all while the standby is on.
I have only checked the bias voltage (-49) so far. I'll check the rest of the voltage points tomorrow for reference, but I figured i'd throw this thread up first in case anyone had any ideas.
I am totally stuck. If anyone could give me some direction I would be very thankful.