Can someone explain how this tremolo works?

[beasleybodyshop]

See attached schematic.

Im not sure how the tremolo actually functions in this amp. Is it feeding an oscillation into pin 9 of the power tube? I see that the v4a has its grid connected to its plate, so im assuming its just sending some kind of oscillation into the power tube?

[vibratoking]

V4a is an oscillator and the cathode of V1a is sharing the cathode of V4a. So the voltage on both cathodes is oscillating. V1a is a gain stage with it's bias varying due to that connection.

Although not the exact circuit you posted, this link should help:
http://www.valvewizard.co.uk/trem1.html

[Phil_S]

Over simplified, but essentially (guessing)...
The plate of the oscillator tube (V4A) is connected to 3 R/C "filters" each of which has a time constant that is affected somewhat by the pot. These things suck voltage from the circuit and then stop for a short interval and then do it again. This affects the B+ rail to the preamp. You can hear this.

I'm guessing. Put a meter on the B+ node at R10 where the trem circuit connects. Turn on the tremolo. See if the voltage wiggles. Let me know what you find out!

[shane]

Vibratoking has it spot on!

Cheers
Shane

[cbass]

Here's another take on preamp cathode wiggling.This one spounds pretty good

[beasleybodyshop]

Thanks guys, I got one of these here Kalamazoo Model II's on my bench and was curious as to how it only worked with half a tube.

Wiggle trem sounds alright. It fits for this little amp perfectly well.

[beasleybodyshop]

Wait a second....doesn't electron flow occur from cathode to plate? Why would the voltage wiggle if the oscillator is after the cathode?

[gui_tarzan]

It would still start and stop in a complete circuit.

[Smokebreak]

Funny , I just finished putting this Supro circuit in an old Bogen. Very similar preamp wiggle. I was a bit hesitant to install the intensity pot!

I don't mean to hijack, but since we're talking preamp wiggle, I have a question as well :

I built this up w/o trem first, and when I added the trem, I expected the sound/gain to change in the cathode biased input channel, but it didn't(I don't think). The oscillator and preamp cathodes are separated by the 15K and "on" just shorts cathode>cathode, same with the footswitch
So how does the preamp's bias not change even with the trem "off"? 15K?