Brown Pro tremolo circuit

[Randall]

Can someone explain how the trem works in this amp? Seems to me it takes two 12AX7 tubes and no photoresistor or bias wiggle? Or is it wiggling the bias on the second trem tube?

http://www.thetubestore.com/lib/thet...-Schematic.pdf

[The Dude]

Yep. You got it. The left tube is an oscillator. It modulates the signal at the second tube.

[pdf64]

I think that on those few brown models, they got the terminology right. Rather than amplitude, it’s the signal phase that gets modulated, and phase modulation equates to frequency modulation aka vibrato.
Unfortunately I’d need a somewhat better grasp of the theory to explain how it actually works.

[bob p]

I think that on those few brown models, they got the terminology right. Rather than amplitude, it’s the signal phase that gets modulated, and phase modulation equates to frequency modulation aka vibrato.
Unfortunately I’d need a somewhat better grasp of the theory to explain how it actually works.

The Fender brown circuits get a lot of reverence, partly because Leo used them in the older amps and partly because their operation seems mystical in comparison to the more simple AM tremolo of the BF era. To get a better handle on how these circuits work it might be helpful to think about how a phase modulation transmitter works, as that's what these sorts of vibrato are derived from. Like Pete said, they are actually phase shifters, and phase shifting amounts to FM. The key to understanding this type of circuit is to understand the quadrature relationship of the signal voltages.

In a classical PMT the signal first goes through phase inversion, then the two oop signals are fed through an array of RC filters that comprise a phase shift network. The phase shift network creates signals that maintain constant 90 deg separation. The resulting two quadrature voltages are fed to a mixing amplifier where they are mixed and modulated by the output of an LFO.

Perhaps the best example of the adaptation of the PMT shifting idea for the musical instrument application is the Wurlitizer organ circuit, or the AC30 Vib/Trem circuit. Fender utilized these types of circuits, but continually redesigned them to lower the parts count. (Leo was a bean counter.)

5G13 / 6G5 circuit was the most complex Fender implementation. It replaced the typical 6 RC filters with a twin T type of phase shift network, but the basic approach is the same. The 6G13 circuit took the bean counting even further by using the source impedance of the driving stage to replace some of the resistors in the phase shift network. That resulted in cost savings and further obfuscated the function of the circuit to the point that very few people interested in guitar amps understand how it operates.

IMO the key to understanding the circuit is to recognize the quadrature nature of the signals.

hth