"Fender" remote reverb control Badabing ka-pow!

[NorCalTuna]

I'm puttin up my super cool mod to the board for critique. A bit of background and then the mod.

I play my pedal steel through my "deluxe reverb" in an Americana band, which is cool since it's got all the headroom I need for small gigs, and is small enough to not resent at shows where there's mics, monitors and the like. It's a "dr" in that it's a bit worked over rebuild of a RI chassis and cab, which is to say basically a worked over torres kit amp, which has functioned nearly flawlessly since fire up about 7 years ago. It's awesome. it's fundamentally ab763.

However, my band requires my frequent use of my "long" reverb tank. In practice, reaching back to turn the knob from 2 to about 8 and back is easy, and at a lil bar, it's doable- but at a club, can't do. Can't rely on the reach, and if the knob ends up at, 2.75, instead of 2 to 2.25, it's mud city. I need a remote reverb- and not just a pedal, but a control for the awesome reverb that's there, but one thats real simple, doesn't need a unique cable, and leaves the fundamental tone of the amp untouched. This is what I came up with last night at practice.

It's pretty obvious I'm either gonna need a pile of cables, or a way to control it with 2 wires- the vactrol on my desk seems to seem just right. So instrument cable, and leave it untouched when not there. I whipped up a box with 4-AAA, a 10k-a I found on an old sub amp chassis, a 120 ohm resistor and a single pole switch. The 10ka pot pulls a vactrol vtlc1 very smoothly through it's useful range, although i'm guessing a 25ka would let you get a little more useful range on the low.

First, I tried the vactrol across the wiper and top of the reverb pot. I knew it wouldn't be perfect, but it took about 2 minutes to try. works, sort of- got about 3 more db reverb out of it, according to the eye on the db meter method of measuring. No real point in real measurements, there was only one result that counted: reverb like no tomorrow. That's not gonna fly. So, i say, ok, i'll cheat the voltage divider 470K and..... definitely boost of reverb, but too much path to ground swamped the dry signal and it seemed like the reverb sounded meh. Lost overall volume and too much noise. Not pro. No matter what way I looked at it, that 100k pot, and it's position interacted too much with the signal. (in retrospect, i may have got some traction by simply swapping the wiper and top of the pot around)

So, I pulled the signal off the .003- the reverb recovery plate cap, where the signal comes from. I formed a voltage divider with the vactrol and a 1M resistor, with the signal to the center tap. That kept the change in output impedance minimal on the reverb stage. I finally had a variable reverb signal that didn't fuck up the initial tone! To save some of your time (you're still reading?) I simply disconnected the lead from the "normal" jack (it's a grounding jack, and with a dummy plug in it hummed) and connected my signal to a fresh 47k grid-stopper at the input triode of the first stage. I popped a 220k grid resistor on it after I decided 1M was noisy. This part may be redundant, and the 220k probably changed the reverb circuit impedance somewhat but I couldn't hear it. It definitely quieted down a bit with the change.

BADABING! first shot, "functioned" exactly as hoped! An hour of tuning, futzing and lead dress later, sweet sweet remote knob, but sounded much better, with the additional EQ!
I drew y'all a picture!

iMod reverb.pdf

[cbarrow7625]

Interesting solution. It's always fun to find a way to utilize the unused channels in these amps for something creative. I have a couple of questions about this implementation.

1.) Since the "louder" reverb is going through the tone stack, I assume the reverb signal is now getting a "mid-scoop" EQ from the stock tone stack. Is that desirable? I suppose if it sounds good, it is good. I'm just curious if you notice any difference in tonality. Since the reverb signal is mostly upper mids & highs, I suppose it probably doesn't make a lot of difference.

2.) The "louder" reverb side chain rejoins the dry signal in the opposite polarity of the dry signal & stock reverb signal. The stock reverb & dry signal go through the 1 additional mixer stage, the boosted reverb goes through 2 stages and are mixed in after the mixer stage. Can you hear any cancellation or comb filtering from the polarity differences between the signals? Maybe with the reverb it just doesn't matter, although I doubt it. I would also worry that at higher stage levels the polarity flip of the boosted reverb signal may lead to unexpected feedback when it is engaged (you would hate to find that out in the middle of a gig the first time out). Changing the polarity will change the peaks (& dips) on stage that lead to feedback.

To eliminate the polarity reversal, you should try just tapping the vactrol right into the reverb pedal jack & grabbing the signal right from the reverb tank to send back to the "boost channel". That would also completely eliminate any interaction with the output impedance and voltage dividers at the reverb/dry summing stage. the other advantage this would have is that it would allow you to keep this reverb booster device completely external to the amp. You could just plug it into any old stock fender & get your reverb boost! No need to modify anything inside the am at all! Maybe you could sell this "invention" to other Fender users?

It would probably sound better as well (and possibly quieter) because the polarity reversal will be eliminated and you have one less gain stage of noise. I imagine it would give the "Boost" channel volume control a more useful range as well since the gain will be significantly reduced. Another cool benefit will be that you will have slightly more range of tones from the tone control since you will be eliminating the high pass at the output of the reverb recovery stage; you'll be able to dial in just a little more low end in the reverb (not that that is really useful, just interesting!).

If you do the above, you will need to shield EVERYTHING extremely well. Using a metal box for the vactrol & knob is probably the best idea. Make all of the cables in and out of the box as short as they can possibly be as well. Last but not least, if there is some noise picked up or injected into the external (passive) circuit, make sure that you have enough slack to rotate the box around a bit and change it's position so that you can get it in the best position to reject as much electromagnetic interference as possible.

Since you already need a little bit of power for the vactrol, why not add a transistor buffer/line driver (emmiter follower) to the input and/or output? That would immediately help you to reduce noise pickup while adding longer cables to make it easier to keep the control near you.

Now that I think about it, you could probably accomplish all of the above by just plugging a passive volume pedal into the reverb switch jack (used as the reverb signal output in this case) and route the output back to the unused channel input (as long as noise doesn't become a huge problem). Now I'm going to have to try this one myself....

[NorCalTuna]

Great suggestions, and I also thought about using an "ice box" kind of approach, and even rigged it up with a little fet buffer I have that ranges from .75 to 20 db ish gain. I decided to do what I did for a few reasons, and then we'll return to the nuts and bolts. Primarily, it's for steel- sitting- with 4-9 pedals I can't see already there with the volume pedal. I grab the wrong pedal, mehhhhh, so I still ahve to build a box with a knob of some sort. And it's for pro use, I have to be real about it. It's not a lot of money but it's my job. Less cables on the stage is a great thing.

I didn't want a volume pedal, and an extra 40+ feet of cable in my reverb circuit to deal with, every night I play hell no. I love amps, I love pedals and effects but rigs and gigs do not play well until you have a truck, roadie and labor. I'd rather spend my time warming up so I can play well. Also, since my amp started life as a RI, it has a TRS style footswitch, one more thing to rig around or deal with using that jack. Also, if I lost the box I could rebuild it in less time than it took to drive to the radio shack to buy the parts it would be made from, give or take, anywhere in the usa. This formed a strong, serviceable and flexible installation. It could easily be adapted to volume-pedal use.

One, cheap cable that carries a few ma of DC please! Replaceable by any cable around. I tried 2 20 footers hooked together, it works just the same. I could have applied this control approach to the "from the tank" topology instead of what I did but I seem to remember something odd (although perhaps it was from the jumper lead) I didn't like about coming straight off the tank, I think it seemed to lose dwell iirc.

Yah, the mid scoop is cool with the long tank. You can boost the highs just a scooch and keep the lows at bay. I don't know about the bouncier, livelier "medium" tank.

So, the phase issue. Yes, when the two are set at coincidental volumes there is some cancellation. However, the "knob" is sort of set up to never play that low. I can trick it into canceling, and it actually kind of sounds neat- you get the differences. Filters of any sort also have a relevance to phase, so it's not as simple as "triode triode flop flop." The "normal" channel in that amp was built conservatively and is almost noise free. There is only extra shot noise (although I also wonder if the tank is making some of the hushhhhhisss) when things are at totally loco levels and the input is quiet, like most effects that are set a lil loco. It's simply turned down or off as the effect fades, like any other part of the song. The switching is completely silent, and I tested it at dumb monkey levels. At least in this room, there's no surprises waiting in the box.

The idea was to have the amp's reverb on like, 2, and only turn it off when needed with the stock switch. The circuit works just fine with the reverb knob on 0, but the minimum "boost" setting corresponds to about 3. that way, I have no reverb on a switch, 2 reverb on a switch and then 3+ reverb on a switch that can be altered with the knob. And the stock pedal kills it instantly. I still have to check if the stock pedal cable extends well or if I have to work around that.

There is the possibility with the long tank of feedback, but there always has been around 7+ on the reverb knob and big volume. It's far worse with my archtop than the steel, which tends to have a very high resonant freq and higher Q. Just something to be mindful of, that any delay can feedback.

The mod was obviously inspired by the old RCA->phone plug, but I couldn't deal with the cables, and the potential unreliability. Thanks for your input! I'm definitely thinking of moving the PSU to internal and doing so other things with low voltage switching and adjustments in the AB763 platform now. The batteries are the next least reliable part.

[cbarrow7625]

Cool. Thanks for the detailed reasoning and explanation of how you perceive and deal with the questions I asked. With one level of reverb much higher than the other I can understand that you would not get any cancellation of the reverb under normal conditions. I suppose you really can't perceive much, if any, comb filtering when mixed with the dry signal because they are not "equal" signals. I can flip the polarity of my guitar around a bunch but it won't ever cancel or comb with the singers vocals! +1 on the phase relationship of filters vs. polarity flip of gain stages. I thought about getting into that but just wanted to keep my question simple; you answered it anyway, well done. It actually could be that the EQ circuit gives you the phase shift necessary to be closer to 90 degrees out of phase rather than 180 (at least at some frequencies), so you wouldn't get very much cancellation anyway.

Sounds like you have it set up exactly the way you need it! Thanks for the neat idea and getting my thought process going.