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Fender Brownface "Harmonic Vibrato" (6G12A)

10/12/2000 11:56 PM
Mike B
Fender Brownface "Harmonic Vibrato" (6G12A)
Hi,  
 
I've been thinking about experimenting with the brownface trem circuit, the one called "harmonic vibrato" by Fender. First of all - any opinions on this circuit vs. output tube bias modulation? What are the tonal advantages? It seems like a lot of extra circuitry just to implement tremolo, although I've been told that the results are worth it...  
 
Now on to the technical questions...The 6G12A circuit requires 2.5 tubes. Is the cathode follower that buffers the output of the trem oscillator really necessary? It would be nice to get down to 2 tubes even. I haven't spent too much time analyzing the circuit but it appears that the main feature is the splitting of the preamp signal into 2 separate frequency components via filtering. Then these two signals are modulated with the trem oscillator signal and recombined. The result would be some interesting phase cancellations due to modulating different portions of the guitar signal frequency spectrum. I would imagine that a sort of phase shifter effect is the result. Is this quickie analysis correct?  
 
I would appreciate hearing from anybody that has actually built this thing or tinkered with it.  
 
Thanks a lot,  
Mike B

 
Replies:
Don Symes No experience with the circuit, but... -- 10/13/2000 2:24 AM
SteveF Hi, Mike.I've got a... -- 10/13/2000 6:52 AM
SpeedRacer I've spent a lot of time with that ... -- 10/14/2000 1:39 AM