AB763 - options for reverb both channels

[Geezer]

I am starting a Twin Reverb rebuild/modding project and have questions about the best way to have reverb on both channels.

here's the original schem




here's (what I believe is) the traditional way to do the reverb mod




here's an option I thought of, moving the Vibrato channel 220k mixing resistor from after the reverb, to before the reverb (like the Normal channel mixer is on the traditional mod)




And then a 3rd option, leaving the 220k after the reverb, AND adding the Vibe channel 220k mixer before the reverb



I guess another option would be to add a 220k to ground (voltage divider) to the 220k AFTER the reverb, to keep the signal approx the same as original (from the reverb recovery into the phase inverter)



Is the 1st "moving one wire" mod good enough, or ar there better ways to do it?

Any thoughts or input appreciated

[floyd]

You've probably seen this...

[sluckey]

And here it is with all the verbage...

[jim]

better mousetrap.......it gets its own mixer instead of sharing.  Less wire is always good.  Jim

[tubenit]

I like option #2.    

Just sent you another email.

With respect, Tubenit

[tubenit]

IF one was going to use a "traditional" master volume (vs. PPIMV) .........................

Would you want the reverb done prior to the master volume OR  after the master volume?

With respect, Tubenit

[The_Gaz]

You would want it after the master because in practice there would be less change of overdriving the reverb, which does not sound very nice.

[Fresh_Start]

IRRC Option 2 will have unexpected results.  That 220K resistor between the .1uf and .001uf going into the phase inverter is part of a voltage divider with the lower leg being the Intensity pot.  I forget the details right now, but Doug's version of the AB763 with bias vary tremolo maintains the 220K resistor and substitutes a 47k resistor for the Intensity pot.

Floyd/sluckey's way works.  Just tie the two coupling caps after the plates of V1b and V2b together.  Some guys just tie the two plates together and share a common coupling cap but I prefer to have different values so that the two channels have somewhat different characters.

This is NOT the only way to get it done and it uses bias vary tremolo, but it does work well.  (The original schematic I drew did NOT work.)





The "Normal" channel was designed so that the Bright switch actually is a guitar/harmonica switch.  The amp's owner likes it.

Cheers,

Chip

[Geezer]

Thanks for all the info guys!

After careful consideration, I have decided to do a total rebuild of the SFTR using a different approach to the reverb altogether.

I will be doing a Hoffman-style board (at least leaning to that style at this point) with footswitchable discrete Fender-ish clean (Sluckey-style 3-stage)  and Dumble (Robben Ford ODS #102) OverDrive channels.

I'll start a new build thread when I have the schematic and layout finalized, and once I get started stripping out the old board.

Thx again!

G