Marshall silver jubilee clone problem

[ericlen838]

I just finished building my first amp and it has a few issues.  Both channels have a slight buzz and if I turn the presence up or down there is a very loud squeal.  When I play power cords this loud sound starts building and won't stop until I adjust volume up or down.  HELP!!!

[DummyLoad]

disconnect the NFB (wire to presence pot from the OT) if the squeal stops, then reverse the OT wires going to pin 3 of the output tubes. that should take care of the squeal and the swelling note issue.

can't tell you how to fiix buzz without looking at some detail of the build. IOW, pics needed to have a look at your grounding.

--pete

[eleventeen]

Ambitious project for a first build!


Agree w/DL, almost any time you build a push-pull amp that uses negative feedback (which would be 99+% of them) you have to resolve the issue of OT phasing, eg; which wire (usually blue/brown) connects to which output tube. One way will be clearly, blatantly obviously wrong and if that happens you just flip the output transformer primary wires.


On a one-off project, this is considered nearly impossible to predict, which is why it is reco'ed to NOT cut the OT wires to the tube plates to length, nor dress them neatly until you confirm proper phasing. Just leave them long and sloppy and tack them into place to test phase.


Leo Fender et all DID NOT have to worry about this on an amp-by-amp basis because he was using all one-kind, one manufacturer OT, all of which were phased the same.

[ericlen838]

Thanks for the advice I will try that an let you know.

[ericlen838]

I switched the OT wires and that fixed the problem.  The amp still has a hum and the power switch light will not come on.


Thanks guys!

[sluckey]

Do you have a Marshall style lighted rocker switch? If so, look at this...

http://el34world.com/charts/images/mar_switch.gif