# help with trouble shooting



## washburned (Oct 13, 2006)

My Trainwreck inspired PA based mod is complete and now the fun stuff begins: Jason has rerouted several leads to quiet things down, changed a couple of components and now we are left with this: at low volume the amp makes a sound Jason describes as "like a truck running", a constant hum, but not 60 Hz. Anyone seen/heard such thing before? any ideas where we might look?


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## dtsaudio (Apr 15, 2009)

Two things come to mind. One is motorboating caused by an improperly grounded or defective filter cap. Two - is your output transformer wired correctly.

Just guessing here. Can you supply a sound bite.


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## washburned (Oct 13, 2006)

Not motorboating, and OT wiring hasn't been touched....thanks for the try, though. I know it's tough to diagnose even with the amp.....just hoping someone might have come across something similar.


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## washburned (Oct 13, 2006)

Found the hum....had to change a resistor value in the phase inverter. Now all that's left is getting rid of a few nasty oscillations by playing with lead dress....seems like this will be a kick a## amp!


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## JHarasym (Mar 27, 2007)

Care to provide some details on your conversion in case someone else wants to give it a try? Say, donor amp and mods.


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## washburned (Oct 13, 2006)

The amp was a '60s Precision Electronics PA amplifier running 12AX7 preamps and EL84 x 2 output...about 20 watts. After an initial mod that we weren't happy with, I asked for suggestions. Wild Bill recommended a 'wreck style amp and we found a schematic we liked: amp since then has been gutted and rebuilt. We added a presence control circuit. If Jason has any thing drawn up I'll see if he will let me post. He has some wicked home made stuff and mods: one of the best is a 100 watt Fender PA he worked up as a two channel, one Marshall and the other an old bassman circuit..deadly amp.


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