Visualising Tremolo

[johnberg]

Spending the last few weeks poring over Fender schematics with the intention of building an amp for myself sometime in the near future, I've come across a schematic for the 5E9-A Tremolux, Fender's first punt at tremolo, and this has got me interested in including tremolo in my future build.

5E9-A Schematic.png

I have spent some time reading over Fender's patent for this particular tremolo circuit and tremolo circuits in general, so I feel that I have a mediocre understanding of how things work. However, I was hoping someone could help to clear up a question I have regarding how the phase splitter processes the audio signal for this particular circuit.

Waveforms.png

Attached is a photo of a few waveforms I whipped up showing how I imagine an idealised signal flowing through the 5E9-A phase splitter to look like. Grid 1 is the signal which exits the tone control circuit. Plate 1 is the signal exiting the plate; amplified, modulated with tremolo and inverted. Grid 2 is the signal entering the phase splitter side, attenuated from the voltage divider. Plate 2 is the signal exiting the plate, amplified and inverted.

When the signal passes through the phase splitter tube (Grid 2 & Plate 2), am I right in thinking that because the tremolo circuit modulates the bias voltage, the signal passes through Grid 2 without any 'double down' from the tremolo modulation as I've showed in the waveforms, or is my thinking here completely wrong?

[Doug R.]

Go view Uncle Doug's video on Youtube...he does a great job explaining that circuit. I cannot remember the title...sorry. Basically, the three capacitor networks change the phase about 60 degrees per network so what you end up with is close to 180 degrees out of phase and injected back into the input. One network (speed) can be adjusted via a variable resistor.

[sluckey]

The trem circuit modulates the bias of both triodes. Since the input to the grid of the second triode is already modulated it's plate signal will effectively be double modulated.

[martin manning]

The LFO signal shifts the cathode voltage of both PI tubes in phase, but the second triode gets an approximately equal version of that signal at its grid, so the result is that the modulation of Vgk is about the same in each.