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Pro Rev distortion


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8/9/1999 6:04 PM
Steve W
Pro Rev distortion
Hi everyone  
 
 
 
I have a late 67 or early 68 bf Pro Reverb I am working on. It works OK with any single coil pickup guitar, but when it's played with a Gibson LP, the amp distorts at even low volume. I have checked the obvious things such as bias,power tubes, all preamp tubes, speakers, etc. Nothing seems to make any difference. I am assuming that the increased output of the humbuckers in the LP are in some way causing the distortion. The distortion is not like the usual power tube or preamp tube distortion....it reminds me of a cheap SS preamp distortion. Any thoughts?  
 
 
 
Thanks  
 
 
 
Steve W  
 
 
 
 
 
 
 
 
 
 
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8/9/1999 7:57 PM
Jim S.

Have you checked voltages, especially on the plates and cathodes? What about the cathode bypass caps -- are they original? Maybe one or more of them have gone bad and shorted out (if so, you'll see a cathode voltage near 0 volts). I've heard that the black & red plastic insulated 25x2 caps Fender used during this period were not very good quality to begin with. An out-of-bias preamp tube could easily cause excessive distortion even with normal signal levels.
 
8/13/1999 1:51 AM
Steve W

Thanks Jim  
 
 
 
I have checked all the preamp cathodes/caps/resistors/voltages and replaced a few questionable ones.....no change. I should have tried this first, but I have now found out that it occurs on both channels (normal and vibrato). The only tubes common to both channels are the power tubes and phase splitter. These all seem to check out ok, i.e. voltages, bias,grounds,screen resistors. Any other thoughts? Could it be a bad OPT?  
 
 
 
Steve W
 
8/13/1999 3:44 AM
SpeedRacer
- is this one of the 8-ohm amps, and if so, have the speakers been replaced with the wrong imp? (some 68's have a pair of 16-ohm speakers stock.. very wierd stuff)  
 
 
 
- is this one of the "hybrid" bias set ups? (you have small cathode resistors, & a non-pol cap connecting the cathodes)? This set up sounds extra lousy when overdriven.. follow the BF schematic & layout to fix many tonal problems.  
 
 
 
- I know this is a lame suggestion, but it *works* a lot of the time.. have you used the #2 input for your higher output guitar?
 
8/13/1999 3:40 PM
Jim S.

You say it sounds fine with single-coil pickup guitars. Are you sure the problem is not with the Les Paul? What type of pickups are in there? Mega-super-duper-distortion humbuckers? Active EMGs with a weak battery?  
 
 
 
Just throwing out some ideas...  
 
 
 
Normally, it takes a VERY hot signal to significantly overdrive the input gain stage. If the amp is being played at living-room volume and there is distortion, then it's probably happening in the signal chain up to and including the input gain stage, but not after the volume control (which sits between the input gain stage and the 2nd gain stage).
 

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