Brown era Harmonic Trem; 4- vs 5-triode circuits?

[JazzGuitarGimp]

I am wondering if anyone has experience with both of these circuits:
The 4-triode Harmonic Tremolo (a la 5G13) and the 5-triode Harmonic Tremolo (a la 6G8).

Do both these circuits sound the same? It looks like the magic happens primarily in the two final triodes, with the hi- and lo-band filters at their inputs. The other two (or three) triodes appear to be the LFO and phase-splitter to get two equal, but opposite-phase waveforms to drive the final stage.

I have no idea if the two oscillator circuits produce the same wave shape, and I assume this could account for some of the magic as well.

Is there anyone who has heard both circuits who could give me a blow by blow comparison?

Also, are there any advantages to one of the two circuits, such as stability, LFO Speed range, etc?

Many thanks,
Lou

[JazzGuitarGimp]

Is there another forum where I might post this question?

Thanks,
Lou

[martin manning]

Hi Lou, I meant to comment on this and then forgot. I think the later circuit with the oscillator driving the harmonic vibrato through a CF stage is seen as an improvement, for example see 6G4 vs, 6G4-A. Note all of the Fender bias-varying tremolos went to that configuration before the introduction of the opto-isolator version. Perhaps the lower output impedance of the CF is better able to drive the rest of the circuit.

[JazzGuitarGimp]

Thank you, Martin.

I had noticed the five-triode variants are usually the -A version, so also assumed it was to be considered the 'preferred' version. I am working on a layout for a 40W 'ultimate' amp with 4 6V6GT's, reverb, tremolo, active effects loop, and D-Style switchable OD. I am using a Weber 5E3x2 chassis, which is cut for 6 preamp and 4 octal sockets. Using the Accutronics 'Brick' for reverb eliminates three triodes. If I were to use the 4-triode term, I'd have 6 preamp tubes but with the 5-triode term, I'm going to have to add a seventh 12AX7. It's already getting tight in that chassis (at least on paper), so I am trying to avoid adding another tube.

I am wondering if the CF in the 5-triode trem could be replaced with a MOSFET source follower? Seems like this should be feasible. I don't imagine it would have to handle more than a few milliamps of current, so it should be a relatively low-power application. Not sure how to bias the MOSFET, and also don't know about gate requirements. Maybe I should breadboard the trem circuit to see if I can do this before I commit to it in the amp.

[kiwiandy]

Hi Lou,

If you want to save on tubes and don't mind a bit of solid state circuitry outside the signal chain, you might want to think about using an opamp based oscillator. I've used the oscillator from the Electro Harmonix Wiggler effect box on Vox AC30-type vibratos and it works well. It has a greater speed range and no pesky rev log pots to find. Fender harmonic trem amps use a similar type of modulator so it should work fine maybe with a little coupling resistor adjustment. I don't have a Wiggler schemo at hand but it's easy to google

[JazzGuitarGimp]

Hi Lou,

If you want to save on tubes and don't mind a bit of solid state circuitry outside the signal chain, you might want to think about using an opamp based oscillator. I've used the oscillator from the Electro Harmonix Wiggler effect box on Vox AC30-type vibratos and it works well. It has a greater speed range and no pesky rev log pots to find. Fender harmonic trem amps use a similar type of modulator so it should work fine maybe with a little coupling resistor adjustment. I don't have a Wiggler schemo at hand but it's easy to google

Hi Andy,

SS doesn't bother me if it isn't passing the guitar's audio. To that end, I've been thinking about replacing the first three triodes with mosfets, and keeping the last two triodes intact. Of course, this would still require RA pots. Thanks for the suggestion, i will look into it. When I get to the point where I need the circuit, i plan on building a prototype on its own chassis, starting with the five triode version as i want to be able to document the LFO waveform as a point of reference before I try to shoehorn the mosfets in. At that point, I'll also breadboard the Wiggler oscillator for comparison's sake.

Many thanks for your reply and suggestion,
Lou

[tictac]

You can use a 6AT6 7 pin tube for a triode if need be...

TT

[mwillums]

Curious how this project turned out. What were your findings? I have a weber revibe (4 triode harmonic trem) that I'm thinking about changing to the 5 triode trem.

[pompeiisneaks]

Curious how this project turned out. What were your findings? I have a weber revibe (4 triode harmonic trem) that I'm thinking about changing to the 5 triode trem.

It's not super common for people to know much about a thread that was last updated over 6 years ago. Sometimes it's better to start a new thread explaining and linking to this one with new questions so others can get up to speed etc.

I think some of the people in this thread are still around but I don't recognize some as well, so it's possible there's some info, OP is definitely still active, though so that may be helpful

Sadly I know nothing of the thread myself.

~Phil

[SoK66]

I've had several brown amps over the years. The early 1960 five tube preamp 5G* center volume amps became the 6G* when the volume control was moved to the first in line, and the earliest of those retained the same circuit. Beginning roughly Nov. '60 the 6G* amps got the earlier first stages and PI cap & resistor values, but added the 6 tube (actually just 1 triode) preamp with the later 6G*-A vibrato. (The only place I've seen this circuit documented is for the 6G8 Twin.)

The vibrato on either of them is very nice, but you can't top the -A's swirl. As opposed to the BF/SF amps that came later, on brown amps you'll get the best tone from the Normal channel rather than the vibrato channel, which tends to be rather bright. The brown / blonde Fender era is a fascinating look into Leo experimenting as he went. Probably the highest quality amps Fender ever made.