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DIY Bassman: located the distortion

8/10/1999 2:15 PM
mosrite man
DIY Bassman: located the distortion
I scoped the whole signal path in my 5F6 Bassman clone, and I found that the signal begins to distort (deform?) immediately after the 68k resistors that are attached to the grid of the first preamp tube. It happens with both channels in an identical fashion. Since there is no voltage applied to this grid, except for the signal, can I assume that this distortion is due to some lead dress problem? I've checked and rechecked connections, voltages etc., and everything else seems to be within parameters, save for a high plate voltage on this first 12ax7. This is my last hope!!!!  
By the way, I do measure a 13mV AC voltage on the grids of the first preamp tube when the amp is idling with no signal applied; is this normal?
8/10/1999 5:38 PM
Steve A.

    A bad solder joint can act as a diode. Maybe resolder the joints on your jacks and cable, and try a different guitar?  
    Another thought: maybe try isolating the ground on the input jacks (they could be picking up some crap from the chassis). Did you wire up the amp using star ground techniques? One great tip I picked up here is to run the PT CT directly to the negative ground of the main filter caps and then a ground wire from that point to the star ground. By doing that you remove a lot of the crud that can otherwise cause noise and strange happenings. You may also have a coupling problem in your power supply (too many stages on the same ps pole) but I think I mentioned this before.  
Steve Ahola

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