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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
 
10/13/2000 2:24 AM
Don Symes

No experience with the circuit, but an IRF540 makes a nice cathode-follower replacement (technically it's a source-follower with the MOSFET).
 
10/13/2000 6:52 AM
SteveF
Hi, Mike.  
 
I've got a Vibrasonic, circa 1960 that, if  
I'm not mistaken, uses this vibrato circuit.  
It's the circuit with the tapped treble pots.  
 
This is just my opinion, but...  
I'm not real crazy about the vibrato effect.  
It seems very abrupt to me, almost choppy. Could  
be just this particular amp, but I much prefer  
the blackface type tremolo. One other thing I've  
noticed is that when the intensity is set to max,  
the speaker cone whomps around a lot in sync with  
the vibrato. I wonder if that can be good for the  
speaker.  
 
Part of the whole deal is I've never much cared for the overall voicing of the amp. The vibrato circuit might sound great in a better sounding amp.  
 
Re: your speculation that it might sound like a phaser - mine doesn't. One peculiarity of the  
amp is when you turn the speed control down the effect diminishes dramatically and disappears altogether below a certain setting.  
 
Regards,  
Steve F.
 
10/14/2000 1:39 AM
SpeedRacer

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!!
 
10/14/2000 1:55 AM
Peter S

These are the only type of amp tremolo/vibrato effect I can stand. I really do not like the amplitude tremolo effect at all.....Vibrato RULES!!!  
 
Peter S  
 
10/15/2000 3:45 AM
steveR
Once you go brown, everything else will let you down!  
 
Speed turned me on to to the brownface circuit and it ROCKS. If you've got to have a Fender type trem, then this is the one to do. Yup, do the -A version. You don't have to waste the half stage either. On my recent project I used it as a post trem stage (actually the driver, but it's SE so I didn't need a phase splitter.) If you have a lot of preamp distortion before the tremolo it can have a kind of phaser effect, or like Speedracer said sort of Leslie-like. If you drive it really hard it seems to just get swamped out and go away. This is really nice for controlling dynamics and gain from the guitar. Back off the volume a bit and you get lush swirly tremolo. Crank it up and you've got a great lead tone with a touch of "phasing".  
 
My only wish with this circuit is that it could go slower. I've increased the values of the oscillator caps , and that helped a bit, but it has a tendency to just go away and stop oscillating if turned down too slow.  
 
The 3 meg RA pots are available from Ampwares, Hoffman, Clark Parts, and Angela, but good luck finding the 10meg RA. I found one at a surplus electronics store. It was actually s-taper which I didn't really like so I built my own "stepped" pot with a single pole twelve position rotary switch.  
 
good luck,  
 
steveR
 
10/15/2000 7:06 PM
Mike B

Hey guys, thanks a lot for all the info. I've been intrigued by this circuit for a while and I haven't been real crazy about the results I've gotten from basic amplitude modulation. Looks like I've got another winter project to work on :)  
 
Thanks,  
Mike B
 

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