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About the Vibroverb: are the tapped treble pots necessary ?


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4/16/2000 3:51 PM
EmScott
About the Vibroverb: are the tapped treble pots necessary ?
The 6GL6 (or is it 6G16 [sixteen ?]) Vibroverb  
tone controls used a 70K-tapped 250K audio taper  
pot for the treble pot.  
 
However, Doug Hoffman's AB763 circuit board (see  
layout diagram at http://www.hoffmanamps.com/ab763.htm)  
appears to use a Brownface-era tremolo, i.e.,  
without an optoisolator like the BF/SF tremolos  
used.  
 
He told me once on the phone that it modulated the power tubes bias, like the brownface tremolo did.  
 
However, there doesn't seem to be a  
tapped pot (or a 68K resistor used as a "work-  
around") anywhere near the treble pot on the  
Hoffman diagram.  
 
Does that mean that you can just use a  
"regular", salt-of-the-earth 250KA pot in the  
treble position? What about the connection at  
the tap on the Fender diagram--can one just  
IGNORE ?  
 
I'm just not able to quite get it when I look  
at the 6GL6 circuit diagram and the Hoffman  
diagram--thanks in advance to anyone who can  
help.
 
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4/17/2000 9:32 PM
Yossarian

I was wondering about this tone stack too. I'm building a Tremolux 6G9, and I ended up substituting a Blackface tone stack. Could someone explain that treble pot, and what the difference in tone would be? Thanks.
 
4/18/2000 11:19 AM
Andrew

It 6G16 (sixteen). You can find info and a schematic on the Fender Amp Field Guide:  
 
http://www.ampwares.com/ffg  
 
The treble pot is actually 350k with a tap at 70k.  
 
You're right about the Hoffman board using the bias modulation for the tremolo. Dunno why he'd do that, but I seem to recall that lots of folks prefer it to the roach. That makes his board a bit of a hybrid.  
 
I'm not sure why you would have expected to find the tapped pot on Hoffman's AB763 board/layout, since that's not really the circuit in the brownface Vibroverb, but anyway...  
 
You can just use a blackface tone stack in your amp, if you can't source the tapped pot. Compare the 6G16 schematic with the AB763 for the 'how to'. You could also fit a 10k (mid) pot instead of the 6800 resistor.  
 
I had a thread going here a week or so back about this exact same thing. One poster commented that there was a sudden/radical change in tone as the wiper traversed the tapped position. I'm going to give it a try, since I have two of these tapped pots sitting here.  
 
HTH - Andrew
 

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