AB763 Trem

[trem]

Can somebody give me an explanation of how the tremolo circuit operates? I do not have enough knowledge to figure out the path of travel the electrons take through this circuit, especially those three capacitors.
I am assuming the negative bias supply will turn the trem off ( perhaps I even have that wrong), but what occurs in the circuit while the trem is operating?
Thank You

http://www.el34world.com/charts/Sche...VERB_AB763.pdf

Maybe this is a tall order, requires a lot of typing and time.....I don't know. I would be happy to read a link, or something else that would help me understand electron flow through the trem circuit.....thanks again

[loudthud]

The circuit is known as a "Phase Shift Oscillator" (you can find out more in most text books). Normally feedback from plate to grid would be 180 degrees out of phase (ie negative feedback). Although it attenuates the signal, the three capacitors and associated resistors to ground advance the phase 180 degrees and cause the tube to oscillate (it's now positive feedback). The phase shift is 180 degrees at only one frequency, but you can adjust the frequency by means of the 3 meg pot. You could get 180 degrees of phase shift from just 2 capacitors, but the amplitude would be almost zero at the grid. You could use 4 capacitors (and 4 resistors to ground) if you wanted to.

The negative bias holds the tube off until the footswitch shorts it to ground. This affectively kickstarts the oscillator by injecting a pulse of voltage. Without the kickstart, the oscillator would start slowly, the amplitude would start small and take several cycles to build up, and all this would depend somewhat on the position of the Frequency control. The other half of the tube acts as a buffer to drive the neon lamp. Light from the lamp changes the resistance of the Cadmium Sulfate cell and thus the amplitude of the guitar signal via the Intensity pot.

[Enzo]

Oh, cool. I always looked at that negative bias strictly as an oscillation killer. Never occurred to me it was also a kick starter. Learn something new every day. Thanks.

[trem]

The circuit is known as a "Phase Shift Oscillator" (you can find out more in most text books). Normally feedback from plate to grid would be 180 degrees out of phase (ie negative feedback). Although it attenuates the signal, the three capacitors and associated resistors to ground advance the phase 180 degrees and cause the tube to oscillate (it's now positive feedback). The phase shift is 180 degrees at only one frequency, but you can adjust the frequency by means of the 3 meg pot. You could get 180 degrees of phase shift from just 2 capacitors, but the amplitude would be almost zero at the grid. You could use 4 capacitors (and 4 resistors to ground) if you wanted to.

The negative bias holds the tube off until the footswitch shorts it to ground. This affectively kickstarts the oscillator by injecting a pulse of voltage. Without the kickstart, the oscillator would start slowly, the amplitude would start small and take several cycles to build up, and all this would depend somewhat on the position of the Frequency control. The other half of the tube acts as a buffer to drive the neon lamp. Light from the lamp changes the resistance of the Cadmium Sulfate cell and thus the amplitude of the guitar signal via the Intensity pot.

I will research the PSO further. At least now I have a general understanding of the trem circuit.....
Thank You

[Wombaticus]

I will research the PSO further. At least now I have a general understanding of the trem circuit.....
Thank You

Two great articles that you should check out:

Designing Phase Shift Oscillators

The Valve Wizard- Tremolo Oscillator

[überfuzz]

You could read a line or two about Hartley oscillators as well.

[trem]

Two great articles that you should check out:

Designing Phase Shift Oscillators

The Valve Wizard- Tremolo Oscillator

Will do, I always forget about Merlin, and I don't know why....
Thanks

You could read a line or two about Hartley oscillators as well.

Hartley.....I will do a search
Thank You