How to add Tremolo Depth control to this old Beltone tube combo?

[waspclothes]

Hey everyone, a friend brought over this old little Beltone combo hoping I could add a control for the tremolo depth (it currently just has a speed control). I figured it would be easy, but looking at the schematic the tremolo is like nothing I've seen before.

I'm not even sure what it's doing - varying the voltage at the first preamp section's plate?

[Helmholtz]

It varies the cathode voltage (bias) of the input tube (point D).

It's a very basic circuit and I don't have a good idea for a depth control. Especially regarding increase of depth.

Reducing depth by increasing the cathode impedance of the LFO tube bears the risk of stopping the LFO..

[dmeek]

Try this

[waspclothes]

Try this

Hi demeek, thanks for the suggestion but I'm pretty sure I tried an equivalent of this circuit when I removed the power switch and 35k resistor that bridges it, replacing with a 50k potentiometer. Turning the potentiometer basically either completely killed the LFO, or had it start up at it's normal depth. So it didn't attenuate at all unfortunately.

[Helmholtz]

Turning the potentiometer basically either completely killed the LFO, or had it start up at it's normal depth. So it didn't attenuate at all unfortunately.

That's what I meant in post #2.
The LFO tube needs to see a low impedance between its cathode and ground. Otherwise its gain drops and the oscillation stops.

[dmeek]

How about using a dual pot like this.



Or this with a single pot

[Helmholtz]

While the proposals above might work to some extent (the LFO might need an own cathode bypass cap), both will considerably vary the biasing of both tubes.

[Helmholtz]

If you just need to reduce the tremolo depth, you might try to increase the value of the 10΅ cathode cap. This should smooth out some of the modulation.

[waspclothes]

I just wanted to report back that I tried all suggestions, including a few guesses of my own but none worked. The tremolo oscillator just dies completely when trying to reduce the depth in any of these ways. Anyways, this is beyond me - I told my friend it's not an easy mod like I had hoped, he's not concerned. For what it's worth, this amp sounds great.. but I agree with my buddy here, the tremolo depth is way too deep by default.

[Helmholtz]

I just wanted to report back that I tried all suggestions, including a few guesses of my own but none worked. The tremolo oscillator just dies completely when trying to reduce the depth in any of these ways. Anyways, this is beyond me - I told my friend it's not an easy mod like I had hoped, he's not concerned. For what it's worth, this amp sounds great.. but I agree with my buddy here, the tremolo depth is way too deep by default.

Did you try to double the value of the 10΅ cathode cap?
I'm sure this won't stop the LFO.
I've actually seen circuits that use a 20΅ in this place.
If 20΅ or 22΅ is too much, just add smaller value caps (2.2΅or 4.7΅) to the 10΅.

[waspclothes]

Did you try to double the value of the 10΅ cathode cap?
I'm sure this won't stop the LFO.
I've actually seen circuits that use a 20΅ in this place.
If 20΅ or 22΅ is too much, just add smaller value caps (2.2΅or 4.7΅) to the 10΅.

I did, i swapped it out with a 25uF cap. I couldn't see a difference on the scope really, but I got the feeling it sort of helped (it didn't make it worse at least!) so I left it in.