Tremolo pumping the speaker in Premier 120

[Rhodesplyr]

I've been working a bit at a time on a 1953 Multivox Premier 120, and I've noticed that, with the "Instrument" channel turned up high, the tremolo circuit tends to pump the speaker back and forth a lot, whether tremolo is turned on or not.

Are there any general approaches to tweaking tremolo circuits that help reduce the amount of LFO that gets into the audio?

The power supply decoupling capacitors in this amp are minimal with only 0.1uF used in conjunction with the tremolo circuit. I was thinking that perhaps a larger electrolytic might help isolate the LFO, but I'll confess that I'm not sure I fully understand all the aspects of the tremolo circuit in this amp.

[loudthud]

How is the modulation implemented? Got a link to a schematic? Some schemes depend on push pull amplifier balance to cancel the LFO. Is this such an amp?

[Rhodesplyr]

Here's the general schematic:

http://www.harpamps.com/premier/Prem..._schematic.gif

This schematic is not exactly like the one I'm working on; it differs in a few ways, but it's basically the same circuit. It has a 0.1uF capacitor from the high voltage side of the 100k LFO plate resistor to ground.

To answer my own question, however, I clipped a 33uF capacitor in parallel with this 0.1uF cap, and it solved the problem. Now, there is no pumping of the speaker with tremolo off, even if you turn the instrument channel up all the way. (On the one I have, the volume control is *after* the tremolo circuit, not before it.) There's still some speaker movement with tremolo on, but not as much, and its function is unaffected. If anything, the whole channel sounds better.

An extra benefit is that the added capacitor lessens the 'pop' of turning tremolo on & off. This amp has a switch for this on the control panel not shown on the schematic. I've seen several of these amps, and I've yet to see two identical ones....

[Rhodesplyr]

Also, for anyone working on one of these amps, I've found that, for some reason, how well the tremolo works is dependent on choosing a 12AX7 and 12AT7 that work well together. Weird...

[tubeswell]

Yes I was wondering about the role of tube selection, especially where the LFO stage is in the same envelope as one of the signal path's gain stages.

[davohilts]

I have the same issue with mine! Where is the cap you cut out in the circuit? Is it associated with the tremolo?
Thanks,
Dave

Here's the general schematic:

http://www.harpamps.com/premier/Prem..._schematic.gif

This schematic is not exactly like the one I'm working on; it differs in a few ways, but it's basically the same circuit. It has a 0.1uF capacitor from the high voltage side of the 100k LFO plate resistor to ground.

To answer my own question, however, I clipped a 33uF capacitor in parallel with this 0.1uF cap, and it solved the problem. Now, there is no pumping of the speaker with tremolo off, even if you turn the instrument channel up all the way. (On the one I have, the volume control is *after* the tremolo circuit, not before it.) There's still some speaker movement with tremolo on, but not as much, and its function is unaffected. If anything, the whole channel sounds better.

An extra benefit is that the added capacitor lessens the 'pop' of turning tremolo on & off. This amp has a switch for this on the control panel not shown on the schematic. I've seen several of these amps, and I've yet to see two identical ones....

[52 Bill]

I have the same issue with mine! Where is the cap you cut out in the circuit? Is it associated with the tremolo?
Thanks,
Dave

Re-read the post, he added a 33uF cap across the 0.1uf cap to reduce the thump.

[Bruce / Mission Amps]

I was wondering if the "pumping" is modulating the 6V6's screen voltage enough to cause the power tubes to conduct on and off ... kind of like screen modulation or "mixing" in old RF amps... etc.

[davohilts]

Re-read the post, he added a 33uF cap across the 0.1uf cap to reduce the thump.

oops! thanks Bill52, I misread the post, when I saw 'clipped' I thought 'cut' thats why I couldn't find a 33uF cap in the circuit!

[davohilts]

I installed the 33uf cap across the .1 uf cap. I tested the amp and didn't like the sound, it has always been very bass heavy and the thumping tremolo is a result of this. I decided to reduce the coupling caps on V1 from .02mf to .01mf and this helped some, I then changed out the caps between V4 and the power tubes from .02mf to .01. This really helped and made the amp much brighter and less bass heavy and as a result stopped the tremolo thumping in unless the tremolo is turned up all the way. It does make a ticking sound when the tremolo intensity is turned down almost to zero but overall reducing the cap values to .01 really improved the amps overall sound quality.