AB763 preamp tremolo

[Esquirefreak]

Have any of you rebuilt the tremolo to wiggle the cathode of a preamp tube in an AB763 type amp? Didn't ampeg have a roach trem that worked (kinda) that way? I need someone to ease my curious mind.

(Yes, I know that Sluckey's TON works very well in place of the original roach tremolo). Im just asking out of curiosity 

/Max

[separateness]

Like the VibroChamp?

[sluckey]

Have any of you rebuilt the tremolo to wiggle the cathode of a preamp tube in an AB763 type amp? Didn't ampeg have a roach trem that worked (kinda) that way?

I haven't and I'm not aware of any Ampeg with a roach that wiggled a preamp cathode.

[Esquirefreak]

Yeah, like the Vibro Champ.

The opto tremolo still uses both tube triodes. I wonder why they didn't use that tube for an oscillator and a CF buffer instead.

@Sluckey: Yeah, I was pretty sure I'd gotten that mixed up with something else I read about Ampeg and tremolo. Sorry.

[sluckey]

The opto tremolo still uses both tube triodes. I wonder why they didn't use that tube for an oscillator and a CF buffer instead.

Leo didn't need a CF buffer. He needed a driver circuit that could flash the neon bulb in the roach.

[Esquirefreak]

I know he didn't need the buffer for the opto tremolo. I was talking about the buffer setup in the Vibro Champ, with the trem then injected in the preamp. Is there any reason it wouldn't work? The setup in the bigger amps looks awfully complicated in comparison.

[Willabe]

Have any of you rebuilt the tremolo to wiggle the cathode of a preamp tube in an AB763 type amp? Didn't ampeg have a roach trem that worked (kinda) that way?

Some old Gibson's did that. There's a few, not just the amp link below. I think I remember 1 that they used a 12AX7 to shake a 1st stage 12AX7's K.

https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga_40t.pdf

[sluckey]

Most Supros and Valcos wiggle the first preamp cathode. A few of those only use a one triode oscillator, but most use an oscillator plus a cathode follower.

The Marshall 1974 18W amp uses a single triode oscillator to wiggle the first preamp cathode.

[tdvt]

Have any of you rebuilt the tremolo to wiggle the cathode of a preamp tube in an AB763 type amp?

I have wondered about this myself a few times.

As a side project, I recently mocked-up a MOSFET version of the Fender trem circuit, using a Reverb Champ I built as a test subject. I was getting a pretty nice result but, was still fooling with the circuit, pot values, insertion points,  etc.  so the project isn't completed.

But I did try it briefly, a week or so ago in a Bassman I am working on for a friend & had a rather anemic result, so I moved on for the time being.

I think that it would work, but would need a bit of adaptation.

[sluckey]

Some old Gibson's did that. There's a few, not just the amp link below. I think I remember 1 that they used a 12AX7 to shake a 1st stage 12AX7's K.

https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga_40t.pdf

The GA-40 does not wiggle any cathode. It uses a LDR to shunt the signal at the wiper of the volume pot.

[Willabe]

The GA-40 does not wiggle any cathode. It uses a LDR to shunt the signal at the wiper of the volume pot.

Yeah, I figured it's close enough to look at, has LDR/roach. And to look for any other Giby's that do work with the K.

I should have been more clear.

[PRR]

> The setup in the bigger amps looks awfully complicated in comparison.

Injecting lopsided DC wiggle in a low-level stage is prone to distortion and thump. An opto-resistor OR a push-pull high-level stage will handle AC/audio cleanly low-thump.

> The setup in the bigger amps looks awfully complicated in comparison.

It's only complicated once, at the factory. The added cost and maintenance are minor. And while I would not call "Crimson and Clover" complicated, Link Wray's "Rumble", "Born On The Bayou", "Born to Run", or Pink Floyd's "Money" are not simple. Wray (the speaker slasher) could get away with a crude tremolo, Money would not be the aural treat if it reeked of hammered triode.

Also: how much? A slight shimmer is not so hard. Making deep trem without ugly artifacts is hard.

[Esquirefreak]

All very good points and I think I understand the pro's of the opto tremolo now. Thank you.

/Max