How does bias mod trem work?

[jzucker]

Looking at the 6G16 schematic, it appears that the oscillating tube is varying the grid by sending A/C through the circuit. Could someone explain how that works? Am I correct in thinking that because it's modulating an A/C Component it's not really changing the bias which is what I initially thought it was doing?

 

[Jeff Seal]

The 6G16 and the RI Vibroverbs do exactly what you originally thought, modulate the actual negative voltage. Both are notorious for hitting limitations that exceed the user controls. (especially in the RI's, turning intensity to max often results in a very sharp "pop" that the Output tubes do not like.)
Basically, a 55v peak to peak(max) AC Oscillation is injected into the existing bias circuit through the .1 uf coupling cap. The intensity controls either fixed or the variation of the fixed bias..
Yes, this does throw the tube well into crossover notch territory and also into the over 60 ma range... Probably explains why this circuit was not utilized more, it's range is not quite as "effective" as the opto-coupled designs.
The Black Princetons, (AA964 and the later AA1164) and the early 5E11 and 5F11 Vibrolux's where the only three designs to use this circuit..(that I know of..)

If you're looking for really neat Vibrato, check out the Magnatones. (They actually DID pitch shift)...Fwiw

Jeff Seal

 

[jzucker]

Thanks Jeff. What would you recommend to insert into the circuit which would sound better? I wasn't crazy about the opto-coupled tremelos either....

 

[Jeff Seal]

I'm afraid I can't really recommend anything that would work better other than making sure the circuit is well within tolerance. Sad to report it's not very flexible, and given it's nature, can't really be altered without some severe surgery. Or are you just looking at design's?

Again the Magnatone circuits are incredible..

hope this helps..
Jeff

 

[Jeff Seal]

You could also try the Vibrosonic's, no opto's just simple amplitude modulation pre PI.

Jeff Seal

 

[jzucker]

Originally posted by Jeff Seal
You could also try the Vibrosonic's, no opto's just simple amplitude modulation pre PI.

Jeff Seal

Thanks. Is there a specific circuit I should look for? Vibrosonic Reverb or just plain Vibrosonic?

 

[Jeff Seal]

Almost any of the Brown series..Tremolux, Pro, Super.. any of the 6G's with tremolo, obviously except the 6G16.. should work.

 

[jzucker]

Originally posted by Jeff Seal
Almost any of the Brown series..Tremolux, Pro, Super.. any of the 6G's with tremolo, obviously except the 6G16.. should work.

Don't those need more than a single ax7 for the trem?

 

[Jeff Seal]

Yes, they do.. obviously not feasible with the 6G16 circuit. The only one I know of you could try would be the AA764 Vibro Champ circuit... FWIW
Jeff

 

[jzucker]

Originally posted by Jeff Seal
Yes, they do.. obviously not feasible with the 6G16 circuit. The only one I know of you could try would be the AA764 Vibro Champ circuit... FWIW
Jeff

Well, the bias mod trem sounds good at low volumes. I guess I'll just leave it and get a pedal if I need to use it at louder volumes.

Thanks for your technical advice!