What does this AB763 reverb mod do?

[Randall]

Rebuilding a 66 Super Reverb, migrating what I can salvage over to a glass epoxy board. Customer wants some mods he had done previously to remain. One of them I don't get. In the reverb driver stage he tacked in a 330K across the 1 Meg that goes from the grids to ground, effectively making it 250K. He also changed the 500pf to 0.02 and put a 380K resistor between it and the grid. Why?

[pdf64]

If I understand correctly, I think it will attenuate the signal into the reverb driver but make it a lot more bassy.
Is there also a master volume mod?
If so, this will help to make the clipping nicer, which may be why it was done.
May be a similar idea to SF reverb drivers, which have an unbypassed cathode.

[Randall]

There is no master volume mod, but the second cathodes are split between V1 and V2, with V2 going to ground.

[Enzo]

Agree with pdf, it is just a volume drop at the driver input. The two resistors make a voltage divider. 250k and 380k (now THERE is an odd value) makes 630k. So essentially they created a volume control at the drive input set at about 40%. The big input cap adds back all the bottom end the 500pf cap was designed to exclude. In my view it makes the reverb muddier.

If he likes it, no reason to change it.

[bob p]

Reverb circuits normally filter out bass frequencies because they give the impression of muddiness in the sound. Normally nothing lower than midbass (a few hundred Hz) gets passed. IMO it's the stuff > 1000 Hz that gives our ears the perception of ambiance. It's looking like your guy wants to have all of the LF content restored. Like Enz said, that would make things muddy.

So what's the point of the volume reduction at the reverb driver input? Is he trying to reduce the amount of wet signal with full pot rotation? I'm guessing that he prefers subtle reverb rather than deep reverb, and that he wants more pot rotation with low amounts of reverb, for fine-tuning at the low end. If that's not it then i'm lost.

[g1]

I like how you threw that trojan horse in there Bob.

[Enzo]

Which is different from a horse Trojan.

[g1]

Now that would be impressive in the shopping cart.

[Randall]

Customer is an old school blues guy, so maybe a muddier, less in your face reverb is what he likes. The mod wasn't done very well at all, but I will put it in the new board as was intended, just a little nicer.

Thanks guys!

[bob p]

Which is different from a horse Trojan.

I seeded the thread with the "Enz" comment hoping that someone would swing at it.

Another perfect strike ... another homer!

[Steve A.]

My guess is that the 500pF cap was changed first and when the guitarist thought it sounded way too muddy he had the guy with the soldering iron (I will certainly not call him an amp tech!) attenuate the signal going into the reverb driver.

I like to replace the 1M grid load resistor with a 1M linear pot wired as a variable resistor with a 510k resistor on the "tail" to ground to use as a Dwell control with a nice boost when you turn it up past 5. On vintage amps you could remove the ground switch and/or ext. spkr jack to add extra controls without having to take drill to chassis. Alternately you could just use a trim pot and set it to taste.

Steve A.

[nickb]

Rebuilding a 66 Super Reverb, migrating what I can salvage over to a glass epoxy board. Customer wants some mods he had done previously to remain. One of them I don't get. In the reverb driver stage he tacked in a 330K across the 1 Meg that goes from the grids to ground, effectively making it 250K. He also changed the 500pf to 0.02 and put a 380K resistor between it and the grid. Why?

[ATTACH=CONFIG]45841[/ATTACH]

My understanding of your description is this ( sorry about the crummy drawing):



Is that right? If it is, the 2nf /250K will have give a small reduction in gain and the 380K ( it isn't 380k really - right?) will lose some highs. I suspect it's really 68K or 33k in which case the effect would be a grid stopper.

[Randall]

Not actually, no. It's 20nF (.02uF), And my mistake what I thought was the 380K is actually 390K. I just measured it out of circuit at 387K. I find it hard to imagine this guy knows what he is doing, especially judging from the crappy work and soldering.

[nickb]

Not actually, no. It's 20nF (.02uF), And my mistake what I thought was the 380K is actually 390K. I just measured it out of circuit at 387K. I find it hard to imagine this guy knows what he is doing, especially judging from the crappy work and soldering.

Way too much bass leading to a muddy reverb. The 390K grid stopper together with the source impedance will roll off the highs above about 4Khz so probably won't be a very audible effect. I think you are right. He had no idea.