Tremelo schematic - correct?

[lowell]

Can anyone tell me if this tremelo circuit would work as drawn? I have overhauled this whole entire amp and am stuck on the trem. The DEPTH and SPEED knobs are working more like reverse volume controls. There is no trem happening. The tube is new and good. There is also a footswitch, and it connects/disconnects supply F to the bottom of V6 Rk. I don't believe this is correct.

http://music-electronics-forum.com/a...r_120r_sch.pdf

[Chuck H]

That doesn't look like any oscillator circuit I've ever seen. And it doesn't look like it would oscillate. I suspect the reason your knobs are acting like volumes is that they are changing the bias of the stage/s being tremolo-ed, but without tremolo-ing them. I don't even see supply F associated with the trem circuit. So why the footswitch is attached there and the resistor was clipped out is a mystery. As is the idea that someone wanted to run the HV through a footswitch. Thoroughly confusing. Can you tell or do you know if this amp has been F'd with by someone?

[52 Bill]

That doesn't look like any oscillator circuit I've ever seen. And it doesn't look like it would oscillate.

There is no ground path for the tube cathode.

Maybe the depth control should connect directly to the cathode and the other end of the 1K should be grounded.

[lowell]

I agree that the footswitch arrangement is f'd. I'm sure it's supposed to ground something instead... not sure what, but yah my guess was the 1k Rk on V6. I tried grounding that point and it didn't make it work. I don't understand what the oscillator stage is oscillating...? Is it changing the grid bias on V6A? It's worth noting that if V6 Rk is grounded, the depth control will be grounded too thus defeating any interaction between the oscillator stage and V6A's grid.

BTW this is a Premier 120R and is the only schematic I've been able to find.

Maybe I can wire the oscillator per your typical fender circuit. Maybe it'll only need some slight value adjustments to work with V6A's grid?

[Chuck H]

I agree that the footswitch arrangement is f'd. I'm sure it's supposed to ground something instead... not sure what, but yah my guess was the 1k Rk on V6. I tried grounding that point and it didn't make it work. I don't understand what the oscillator stage is oscillating...? Is it changing the grid bias on V6A? It's worth noting that if V6 Rk is grounded, the depth control will be grounded too thus defeating any interaction between the oscillator stage and V6A's grid.

Yes. And the cathode bias of V5B. That trimmer is probably to adjust the min or max amount of effect relative to the depth control.

I think there may have been changes made to the power supply. Ultimately this may need to be addressed. I had to get my thinking cap on to see it... But try this (changes in red):

[g1]

Different model, but does this help?

[Chuck H]

That helps a lot actually.

I found this very similar Multivox Premier 120 trem circuit.



If you hybrid it with the one g1 posted and then reason where something might have gone cattywhompus you might get something like this (changes from original in red again):



Which looks like it might work. And it isn't a stretch to think it may have been miswired the way it is now accidentally.

[lowell]

Thanks so much. Wiring it now. So the switch should be wired so it grounds the 220k junction to turn it on correct?

[Chuck H]

Well the switch certainly shouldn't be wired to the power supply anywhere. It's possible the switch was stopping the oscillator by lifting it's cathode from ground. But I doubt it. So I would say yes to the 220k resistor. The switch will ground or lift the 220k. Return the ground from the switch to the same point as the rest of the phase shift nodes. Don't share a preamp node on the board or it may tick tick tick tick tick. If the jack is grounded to the chassis then that'll probably be ok too.

[lowell]

You guys rock, thank you. So, vibrato works. It didn't at first but I moved the 220k below the speed knob back to where it was, the bottom of V6 Rk. So it works now, but ONLY if the speed is all the way up. It's fast too. When I turn the speed knob down at all, even a tiny bit, the vibrato stops altogether. Turning the speed knob up means the speed knob is 0ohms.

The switch is indeed connected to the 100v supply. When the vibrato is OFF, there is 100v on the center lug of the speed knob. Weird, but it works.

[lowell]

Whoa... I thought maybe the 47M was supposed to be 4.7M so I began clipping in leads to add a 4.7M in parallel. Once the 2 clips were in, with NO 4.7M, the vibrato came alive and worked on all speeds. I left both leads unterminated, it works great! Some stray capacitance is making it work!!! This is crazy. So now I'm wondering if maybe the .01 or .02 caps are open, or maybe their value has drifted. Will report back after replacing them.

[lowell]

Yep! Replaced the .01, boom, fixed. So, now another odd thing happening. Channel 2 works fine all around, vibrato, verb etc.. Channel 1 only passes signal when the vibrato is ON. Also, if the vibrato depth is turned down it acts like a volume control on Channel 1.

[lowell]

Correction to last post: both channels produce no sound when the footswitch is OFF and the vibrato DEPTH is up all the way. The sound comes back when the DEPTH is reduced.

[Chuck H]

Is there a reduction in volume before the preamp output vanishes? How is the footswitch hooked up now? Did you try the 6.8k resistor back on the bottom of the 1k cathode of the oscillator after the cap replacement?

I REALLY think we need a new schematic drawn as the circuit is now. I'm kind of lost so I can't make any suggestions. I only need the trem circuit and footswitch.

[lowell]

This is what I settled on. I added a cap to the depth pot to separate DC from the oscillator and the bias of V5/6. The vibrato wasn't enough so I changed the 6.8k to 47k. The overall signal wasn't loud enough compared to the unaffected signal, so I added the bypass cap on V6A.

The footswitch is as drawn in upper right corner. The 'E' supply gets connected to the wiper of the SPEED pot.

Everything works and sounds great!! THANK YOU!

EDIT: ignore the voltages listed. Those were from a while back before it was all fixed.