Paging Glasman: Mixing Reverb in PI

[ElectronAvalanche]

Hi Gary,

I have a question concerning your Reverb mix schematic.

Do you ground the 22nF PI input cap of your modified PI?

If I understand it correctly, you take the signal from the Relais or Master Volume, then part of the signal is send to the Reverb input cap (550pF), the Dry is routed via the 100k resistor to the PI, the Reverb signal comes in after the 100k via 470k.

The combined signals go to the 0.1uF cap of the X2 half of the PI.

The 22nF input cap of the X1 half of the PI seems to be floating in your schemo. Should this cap be grounded (as per TUT?).

Thanks for any insight on this.

Electron

[Funkalicousgroove]

Hey Electron Avalanche,

What happened to the SSS files you said you were going to post? I'd love to have them for sure!!

[glasman]

Hi Gary,

I have a question concerning your Reverb mix schematic.

Do you ground the 22nF PI input cap of your modified PI?

If I understand it correctly, you take the signal from the Relais or Master Volume, then part of the signal is send to the Reverb input cap (550pF), the Dry is routed via the 100k resistor to the PI, the Reverb signal comes in after the 100k via 470k.

The combined signals go to the 0.1uF cap of the X2 half of the PI.

The 22nF input cap of the X1 half of the PI seems to be floating in your schemo. Should this cap be grounded (as per TUT?).

Thanks for any insight on this.

Electron

The 22nf is the input to the PI. I will say I have NEVER tested this approach and it may not work due to the low impedances created by the negative feedback. This was something I was toying with a couple of years ago, but never tried. It would probably require at least 60dB of additional gain to work correctly. This circuit idea came from a Matchless Chieftain schematic I looked at a couple of years ago. The Matchless did not use NFB. I would say this idea would be best implemented on an amp that did not use feedback in the power amp. AC-15/AC-30 style.

The second drawing you posted is what I have been using (or slighly modified) in my amps. This does not give a Fender style of reverb (boingy), but adds depth and richness. The tone is also a bit "darker" than a Fender reverb. I would love to add another gain stage and will have to try it in the future. I would try type 9 med tanks. The long tanks have a bit too long of decay (right dogears?). A type 4 might also be an option.

The two schematics are NOT related to each other.

Gary

[ElectronAvalanche]

@Funkalicousgroove,

I am working on a schemo of it. Will post that.

@Gary:

now I see...

Thanks for the fast reply.

I plan on doing the following:

1 hole AT7 driving standard Fender Reverb tranny.

1/2 of 12AX7 being a CF driving FX loop.

1/2 of the same 12AX7 being the Recovery amp for the Reverb AND loop return.

I am still wondering how to mix the Reverb/FX with the Dry signal. The PI mixing should work, but you are probably correct, that the NFB will rob to much signal.

I have CFs after the PI, so I could probably get away with a medium db drop. The CFs will drive the Output tubes nicely.

Think SVT or SSS for the CF.

Have you tried to put a cap across the 470k to get a bit more high freq from your Reverb?

Thanks again,

Electron

[glasman]

Have you tried to put a cap across the 470k to get a bit more high freq from your Reverb?

Thanks again,

Electron

Yup, I actually did something like that (across the pot) on the last couple of amps.

The cap would do about the same across the 470K. The big thing is the 2Meg control adds a lot of series resistance that cuts the top end. Actually I like the effect of the reverb. It does not get in the way of the amps tonal signature. I find myself setting it almost off in most cases. This type of amp can get away with no reverb. But of course coming fro the old school, I JUST HAD TO HAVE REVERB .

As case as the SVT/SSS thing. Yup, you need a good supply of current to make that type of power amp work. In both cases as I recall the tubes use a type of current source for drive signal.

Gary