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previous: Mike B Hi,I've been thinki... -- 10/12/2000 11:56 PM View Thread

Re: Fender Brownface "Harmonic Vibrato" (6G12A)

10/14/2000 1:39 AM
Re: Fender Brownface "Harmonic Vibrato" (6G12A)
I've spent a lot of time with that circuit.. build the '-A' version with the 2.5 tubes. Don't change a thing, save for keeping the B+ on the oscillator as high as possible (makes the osc work better). Your analysis is right, and that mixer tube with it's 4.7k Rk is the main piece of the puzzle. The osc is non-linear, the signals are freq adjusted (phase altered) and the mixer is non-linear too.. so you get an almost Leslie like effect. Think of it as a tube univibe (sorta!)  
IMHO the best way to hear it is to build the whole concert amp (in vivo as it were). 2 reasons it's hard to work with as an outboard effect:  
1. S/N stinks in no small part due to 4.k Rk - but it's the key to the effect. ;)  
2. Due to mixer's non-lin mixing (and osc non-lin LFO signal) you have lots of LF artifacts in the output signal. You don't hear them in the amp, but in a signal chain situation (like a studio) you need a good balsy HPF to kill off that stuff lest it toast yer woofers.  
Hear it once and you'll never go back!!

Peter S These are the only type of amp trem... -- 10/14/2000 1:55 AM
steveR Once you go brown, everything else ... -- 10/15/2000 3:45 AM