Tone King MOSFET Trem

[lonote]

Looking at different SS circuits for a small add-on trem board for some already-built amps. The Tone King Imperial SS trem is one I thought I might try to prototype.

While the LFO section is pretty typical, it looks like that circuit applies a V- to the 6V6 grids through the usual grid leaks, though the power tubes are already cathode biased. There is also a second switch on the -50V secondary "bias" supply, shunting V- to ground, that I don't understand the purpose of.

Insights?

Tone King Trem bias wiggle.JPG

Tone King Power Amp.jpg

[R.G.]

While the LFO section is pretty typical, it looks like that circuit applies a V- to the 6V6 grids through the usual grid leaks, though the power tubes are already cathode biased. There is also a second switch on the -50V secondary "bias" supply, shunting V- to ground, that I don't understand the purpose of.

Tubes can be cathode self-biased, or external biased, or any mix of the two that gets the nominal "right" reverse voltage on the grid compared to the cathode. It's usually done completely self bias or completely external (and usually fixed) bias, but mixes work. I think that is what is going on here. The cathode bias sets part of the necessary reverse grid bias, and the negative bias from the trem circuit adds more. It's likely that the bias needed for tremolo operation needs to be different from the no-trem value, so the switch messes with part of the fixed reverse bias voltage.

[lonote]

Thanks RG, that was kind of where my thinking was headed.

That second switch didn't seem to be labelled & I thought it might be ganged with another pole somewhere else, but I didn't see where that was the case.

Anyway, people seem to like the sound of that circuit, which is why I thought it would be worth trying out. I'll put one together as-drawn & find out what that switch does.

[maxkracht]

Pretty sure I have gut shots if that would be helpful. Has been a few years since I worked on one of these so I don't remember much. It's a pretty wacky design, if this is the tone king i'm thinking of.

[lonote]

Pretty sure I have gut shots if that would be helpful. Has been a few years since I worked on one of these so I don't remember much. It's a pretty wacky design, if this is the tone king I'm thinking of.

Thanks for offering the pics, I should be good for the time being. That design comes from the TK Imperial.

I have never heard one in person, but while searching for SS trem circuits, mentions of that one kept popping up, which piqued my interest. It is a bit wacky, maybe a little more interesting that way..?

[maxkracht]

I meant wacky design more in the physical construction. PCBs are a stacked triangle sandwich on each tube with components spanning between two boards, chassis is a weird flip top floating on some rubber grommets for vibration damping. Was a real pain to get out of the cabinet. Attenuator using a custom OT with a bunch of extra taps is cool, though overly complicated. Decent sounding amp, but seemed like they were doing weird stuff for the sake of doing weird stuff. I think they could have achieved the same results with more conventional means and wouldn't have construction mistakes that I had to repair.

[lonote]

I meant wacky design more in the physical construction. PCBs are a stacked triangle sandwich on each tube with components spanning between two boards..

Got it.

I have seen pics/&or a vid of those. Looked like it would be a huge PIA to work on.

[stratomaster]

Thanks for this post. I'm contemplating how to meet the requests of a client with a late 70s Princeton (non reverb) who really likes the sound of a Princeton Reverb, but never uses amp reverb, and doesn't want to spring for a vintage Princeton Reverb--and hates the reissues.

I'm contemplating repurposing the tremolo oscillator triode as a 3rd gain stage ala Princeton Reverb and using a FET to drive the tremolo oscillator. This was useful as a proof of concept, though I think I can repurpose most of the circuitry already there.

RG's excellent FET oscillator circuit on GEOfex is a bit too complex for my tastes, so to see a bias tremolo with a single FET is encouraging. FETs are outside of my wheelhouse, and my only experience with them so far is adding a source follower to a Marshall Origin 50 to mimic a cathode follower driven tonestack.