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Vibrato Project...


 :
12/27/1999 2:16 AM
Rob W.
 Vibrato Project...
Hello All!!!  
 
After having enough holiday cheer, I decided to venture back to  
the bench today... ;-)  
 
My recent project is to install a 6G16 Vibroverb style vibrato into  
my AB763 Twin. (The opto died recently, so i felt a change  
wouldn't hurt ;-)  
 
The install went off without a hich, the new bias modulating  
vibrato sounds much better, but I am encountering a small problem.  
I installed a SPDT 250K-L Intensity pot, so i can disconnect the  
vibrato effect when not in use. But, when I disconnect the circuit, I get this  
annoying sort of hum!!!! ;-( I have had no luck in trying to trace this  
problem down yet... I guess that I can leave the vibrato on all of the  
time, but that defeats the nifty disconnect pot.. ;-)  
 
( By the way, the disconnect circuit is designed to just pass the  
bias voltage around the pot, uneffected *in theory* from the  
vibrato circuit...)  
 
Suggestions / Ideas????  
 
Cheers!  
Happy Holidays!  
Rob W. :-)  
  
12/27/1999 10:43 PM
tboy
Rob,  
 
A defeat switch would have been useful for the original opto-electric tremelo that your amp started out with (to disconnect the gain-sucking intensity pot), but really has no purpose with the bias modulation type circuit, because the tremelo is effectively out-of-circuit anyway when the intensity is set to zero.  
 
If you really want to have a useful kill switch, get a pot with one of those pull-push switches, and just parallel the switch contacts across the tremelo footswitch jack. That way you can kill the effect from the front panel without disturbing your intensity setting.  
 
--tboy  
  
12/27/1999 10:56 PM
Scott Swartz
I can't tell exactly how you hooked up the switch from your description, but you may be switching off the power tube bias, causing the hum. Obviously this is BAD. If the switch was disconnecting the leg with the cap that supplies the modulation it shouldn't cause a hum.  
 
I would advise you to remove the switch you installed and kill the tremolo by grounding the phase shift network on the oscillator as is normal on Fenders. There is absolutely no tonal impact with the Vibroverb tremolo when the oscillator is grounded out, so there is no need for any type of disconnect switching.
  

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