bassman 135 feedback or parasitic

[sadgod]

I replaced the ot with a hammond equivilent and changed stock rectifier diodes with 1n4007s now when I turn it on it gives a loud high pitch almost feedback type of noise. I checked the hook up of the new tranny several times probed connections wiggled wires and I just cant figure out the problem. it does it at all volumes and settings. Anybody experience this?

[Enzo]

Perhaps you have the OT primary wires reversed? Swap places with the blue and brown wires - or whtever color the plate wires are on the new tranny.

[sadgod]

thanks enzo
thats one of the first things I tried

[Bruce / Mission Amps]

Make sure the primary leads are short and away from the circuit board and the secondary leads are not too close to the phase inverter tube socket and wires.
Keep them short enough to get to the output jacks and flat on the chassis.
If you have extra unused secondary leads, snip them off short and with shrink tubing on the ends so they can't touch anything and pull them out on the OT side of the chassis, not inside.

[sadgod]

I did that also. but thanks for the input. I noticed the output matching variable resistor on the back of the chassis and the master volume pot seam to effect the sound of the oscillation quite a bit. I recapped it about a year ago and all capacitors are visually fine I even replaced the spragues. I think if I link the master volume and the"matching" pots I should come up with a cure by examining the schematic. I was thinking about double checking the wall outlet to verify a ground, because I have often questioned this at other times with other gear being more susceptible to noise than usual.

[Steve Conner]

Try disconnecting the negative feedback altogether, does that cure it? The replacement OT might have more leakage inductance than the original, to the point that it screws up the feedback. If this is the problem, leaving the feedback disconnected, or making the compensating capacitor bigger should fix it.