mod AB763 to include Vibro Champ tremolo

[bent765522]

Hi all, I'd love some input on this - can I delete the tremolo section on an AB763 circuit and paste in the tremolo from a Vibro Champ? I've attached a modified schematic for reference.. the red highlights are where the old circuit is cut, green highlights are where I've attached the Vibro champ circuit.

I've taken a guess at where the B+ connection might go.. would the Vibrochamp trem take a wide voltage range?
I haven't worked out a layout yet I'm still thinking about options.
Thanks all! also just as a note, I'm not really going for power tube bias trem or modifying the existing trem circuit.. I'm specifically interested in preamp bias trem.. I love the sound of the Vibro champ tremolo but I've also considered tweed tremolux 5E9-A but that looks a little less cut and paste.. any input appreciated! thanks!

[sluckey]

You should power the trem circuit from node B (screen supply).

Also put a 47K between red dot (junction of .1uf and 220K) to simulate the removed 50K INT pot. Otherwise gain of vib channel will be hot. Better yet, make the 47K switchable for a gain boost feature.

[bent765522]

thanks sluckey,

updated schematic attached, look correct?

Do you think ~ 460v from the B node would present any problems for the vibrochamp trem circuit? The vibrochamp schem shows ~360v to the trem circuit, would a voltage dropping resistor be required there? Or could I use the C node? Not really sure what I'm doing there.

Also yeah thanks for the advice re 47k resistor.. I've become used to it being disconnected there for a hotter signal but yes would be good to have that option on a switch, I like that idea.

thanks!

[Stevem]

Is your main reason for wanting the Champ tremolo is because how slow it can go?

[bent765522]

Is your main reason for wanting the Champ tremolo is because how slow it can go?

Hi Stevem, ab763 tremolo is just too square wave for me and I never vibed with it.
I really love the sound of vibro champ tremolo.. also Princeton reverb trem.. I dig slower smoother tremolo tones. I really don't want to pursue power tube bias tremolo so though I'd check out preamp bias tremolo options.. might check out tweed Tremolux 5E9-A as another options but the VC seems like it would be easier to fit in existing ab763 amp layout..

[Ten Over]

The way you have it drawn will put tremolo on both channels because the cathodes of V1B and V2B are connected. If you only want tremolo on the Vibrato channel, then you need to separate the cathodes.

Deluxe Reverb Preamp Trem 1.png

The tremolo is ahead of the reverb which may or may not be desirable. The reverb will tend to carry-over the quiet phase of the tremolo which might sound a little odd. The AB763 designs have the tremolo after the reverb so that everything wobbles. In order to have the tremolo after the reverb, you might try applying the tremolo to the mixer stage.

Deluxe Reverb Preamp Trem 2.png

[Ten Over]

If you turn the Vibro Champ tremolo off with the foot switch and turn the Intensity control up, it fouls-up the bias on the pre-amp tube which causes distortion reminiscent of Clapton's tone on certain recordings. If you don't like this distortion, and I don't, you can minimize it by wiring the foot switch differently.

Deluxe Reverb Preamp Trem 3.png

Making the grid-to-cathode voltage zero maximizes the voltage drop across the 470k plate resistor which minimizes the DC voltage on the cathode of the cathode follower.

[bent765522]

If you turn the Vibro Champ tremolo off with the foot switch and turn the Intensity control up, it fouls-up the bias on the pre-amp tube which causes distortion reminiscent of Clapton's tone on certain recordings. If you don't like this distortion, and I don't, you can minimize it by wiring the foot switch differently.

Deluxe Reverb Preamp Trem 3.png

Making the grid-to-cathode voltage zero maximizes the voltage drop across the 470k plate resistor which minimizes the DC voltage on the cathode of the cathode follower.

Thanks for this! Super interesting.
Yes I see what you mean re separate the cathodes.. I was thinking about that and my plan is to have tremolo only on the trem / verb channel, so yeah I'll need to separate those.
Re tremolo before verb, noted, I often patch pedals this way and dig it so all good there.
As for powering the trem circuit, if I connect it to the B node and I'm getting say 460v there, is this acceptable? Vibrochamp schem notes 360v powering the trem circuit.
Ten Over one last question if I may.. if I turn the intensity control to zero, will the preamp bias on the vibrato channel be the same as if it wasn't connected at all? wondering how the trem circuit would effect the tone on the vibrato channel vs not connected at all (in other words, if the intensity control is set to zero, it's not effecting the vibrato channel tone correct?).
thanks!

[Ten Over]

As for powering the trem circuit, if I connect it to the B node and I'm getting say 460v there, is this acceptable? Vibrochamp schem notes 360v powering the trem circuit.

The absolute maximum anode voltage for a 12AX7 is 550V. The Fender AB763 Super Reverb put 460V on the tremolo circuit. The AB764 Vibro Champ put 410V on the tremolo circuit.

[Ten Over]

if I turn the intensity control to zero, will the preamp bias on the vibrato channel be the same as if it wasn't connected at all? wondering how the trem circuit would effect the tone on the vibrato channel vs not connected at all (in other words, if the intensity control is set to zero, it's not effecting the vibrato channel tone correct?).

When the intensity knob is set to zero, the cathode resistance is just 25k in parallel with 1.547k which is 1.457k. This is only a 5.8% difference from the cathode without any tremolo circuit. The bias is going to be very nearly the same and you're not going to be able to tell any difference in tone.

[bent765522]

awesome! thanks! very much appreciated

[romberg]

ab763 tremolo is just too square wave for me and I never vibed with it.
...
I dig slower smoother tremolo tones.

Unless you are dead set on playing around with a bias tremolo circuit, you may want to first try out slucky's Trem-o-Nator:

https://sluckeyamps.com/trem/trem.htm

It is a fairly simple modification of the stock ab763 trem. So, you can just use that layout. It can also run at a slower speed and is much smoother than the stock fender setup. You can see some scope shots of the oscillator here with my build:

https://ampgarage.com/forum/viewtopic.php?f=8&t=35989

The stock fender oscillator puts out kinda triangular or sawtoothed waves. The trem-o-nator looks as close to a sine wave as you can get. Just make sure to not use opto-couplers from Amazon .

Mike