Doing repair on SF Pro Reverb...need advice

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5/29/2000 3:59 PM

jason

Doing repair on SF Pro Reverb...need advice I've got a late 70(?) SF pro reverb in to go over. I can't find a schematic on this particular model (has master volume). Last tech who looked at it did a cap job (PS filter caps anyway-not bypass and bias caps, I'll get those), he also unsoldered the 5 volt filament winding and taped it up, broke a pin off the rectifier socket and solder a pair of diodes across the back of the rectifier socket. He also changed the bias match to a bias adjust(which doesn't match a Fender bias adjust still...I'll fix that). My main question is, the PI plate resistors are 47k each, should I change these to 100k & 82k? Any sonic benifits from doing it, or any draw backs.   No UL taps involved.   Any help appreciated.   jason

 

5/29/2000 4:17 PM

Ray Ivers

Jason,     Are you using a 12AT7 driver tube? If so, I think I would leave the 47K's in there. If you're going to change to a 12AX7 driver tube, the 100K/82K (or 91K) values you mentioned should be more appropriate. I think the tail and bias resistors would also stay the same with the 12AX7 swap, if I'm not mistaken, but you will probably want to adjust (increase) the feedback resistor to taste to compensate for the added open-loop gain.     Ray

 

5/29/2000 5:07 PM
jason	The BF pro reverb usd a at7 PI tube also but used the 100k/82k resistors. In fact, all resistors in the PI are the same exept the two plate resistors.   jason

 

5/29/2000 5:50 PM

Carl Z

Jason;     These amps are pretty straightforward but the master volume is pretty odd though. I'd leave the 47k resistors in and keep the at7. Look at the Bandmasters for some ideas.     With regard to the bias, just match the circuit to the standard BF and you'll be good to go. One little note, odds are that you've still got that 70 volt mallory in there. Replace it with a 100/100v cap just to be on the safe side.     Second thing; fix the rectifier socket. These amps sound mcuh better with the tube rectifier.     Carl Z

 

5/29/2000 5:55 PM
Peter S	The higher plate resistors on the PI will make the amp a bit more dynamic with a little more output stage distortion when cranked up(master truned up too). This is just a matter of personal taste.   PS

 

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