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Bass pot causes major prob. in 66 Pro-Rev

9/8/1999 3:35 AM
Bass pot causes major prob. in 66 Pro-Rev
still going crazy over this weird problem. In the Vibrato  
channel only, with volume on 7 or more, guitar plugged in  
and volume full up, I thump the low E string, then when I crank up the Bass control (stock 250K Aud -with 6K8 resistor) past '5' this low freq. thrumming' pulsating  
effect occurs, (the power tubes pulse blue light during  
this) - If I turn down the Bass pot or the volume all the way down - then back up it stops.  
I have swapped out every component in the signal path from  
input jack resistor to grids of power tubes - including all the tubes. I even ran the preamp tube for the Vibrato channel thru the normal ch's tone stack (disconnecting the Norm tone wiring o'course)- and seperated the common cathode resistor/cap circuit that pins 8 of the preamp tubes share.  
but nothing stops this. I am going to decouple the plate rail and see if that helps -  
anyone have any ideas or seen anything like this?
9/8/1999 1:03 PM
J Fletcher
Sounds to me like there's some motorboating going on, due to poor decoupling in the supply, as you suspect, or perhaps some positive feedback due to stray capacitance or resistance between components. I'd try moving some of the leads or components to try and get some seperation, physically, between them, as the problem is occuring. Sometimes there's a problem with the fibreboard card underneath the circuit card, becoming "live", that is I've seen them become microphonic and cause weird problems. Same for the circuit card. Just a couple of guesses..Jerry
9/9/1999 12:57 AM
Re: Bass pot causes major prob - no More!
thanks Jerry,  
it was poor decoupling! I installed an additional  
22 microfarad/450vdc on the vibrato chan. rail  
and wa-la - weird motorboating stopped! I had  
considered the fibreboard card, but as u could imagine  
was saving that tedious task as a last effort.
9/9/1999 1:00 PM
Ron Orlando

I had this problem with a 3-10 bandmaster clone that I built. It turns out that I had a cold solder joint where the B+ goes to the power tubes (after the choke). You might want to check and recheck all your connections.
9/9/1999 6:25 PM
J Fletcher
So was the original filter cap bad? I don't see why you'd need extra decoupling...Jerry
9/11/1999 6:25 AM

original filter cap was fine- i installed an additional  
22/450 cap hanging off the rail feeding the 2 100K  
plate resistors on the vib. ch. preamp tube. I dunno  
about why I needed a 'extra' cap. I have a couple of  
Kevin O'Cconner books and he recommends and design with a 22/450 de-coupling cap on EVERY single plate. I do not  
really understand this whole power supply business with  
respect to caps. Like some Fender Tweeds used 8 uf caps,  
then onto 16's, BF mostly 20uf et all. I have a Mesa  
SOB and it uses 2 200uf and 2 300uf ..... is there any  
good reading on this with a focus on guitar amps?  
9/13/1999 3:08 PM
J Fletcher check the decoupling resistor
If the value of the decoupling resistor has dropped, it would have a similar effect as the cap going bad. The older designs that used an 8 uf decoupling cap generally used a higher value decoupling resistor. Say 10K vs 4K7. Increasing the value of the decoupling resistor has a similar effect as increasing the value of the decoupling cap, but the available high voltage drops...Jerry
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