Adding a reverb footswitch to Hot Rod Deluxe....

[GibsonGM]

I want to add a reverb footswitch to my Hot Rod Dlxe, which unfortunately doesn't come with one.  Ideally, I'd like to have a 2nd pot that can be switched in with a relay to let me boost the reverb.
This is how that section of the amp is wired stock:


I'm assuming that when the pot = 25k ohms (wiper at top), all signal passes and you get full reverb.  At 0K (wiper at bottom), you are sinking the signal to ground, thus eliminating that pathway from the signal chain.

My thoughts on this are to either switch in a pot in parallel with the existing pot at point "B", thereby allowing a change down to 1/2 the original resistance (probably would give plenty of reverb!), or to simply switch in a jumper from point "A" to above the 25K pot, giving 100% reverb - or using a resistance with the jumper to get a reverb level somewhere in between (a pot or trimmer) :



Has anyone else done this?  Just looking for a check of the logic.

Thanks,

~Mike 

[Paul Marossy]

I wouldn't mind adding a reverb switch to my Hot Rod DeVille. I'd be curious as to how your idea works...

[Mark Hammer]

http://www.fender.com/support/amp_schematics/pdfs/Hot_Rod_Deluxe_Schematic.pdf

Most straightforward method is to simply ground the return input from the reverb pan.  In this case, that means running a shielded cable out from the two ends of R35.  bridge them with a switch and reverb is effectively killed.  Open up the connection and reverb returns.  That's pretty much how they do it in a lot of the older amps.

[Paul Marossy]

http://www.fender.com/support/amp_schematics/pdfs/Hot_Rod_Deluxe_Schematic.pdf

Most straightforward method is to simply ground the return input from the reverb pan.  In this case, that means running a shielded cable out from the two ends of R35.  bridge them with a switch and reverb is effectively killed.  Open up the connection and reverb returns.  That's pretty much how they do it in a lot of the older amps.

Yeah, it would seem that simple. But my concern is about possibly getting a "POP" when you switch it on/off.

[Mark Hammer]

I can't see any reason why there would be.  No "hanging caps" anywhere.

If one is skittish about being stuck with the residual hiss of recovery amp U2B, you can always do what I suggested over at the reverb-pot side of C17.  That cap will always have a drain path to ground via pot R39, so grounding and ungrounding C17 should not result in any audible pop.

The initial suggestion still leaves your reverb level control passing whatever hiss might be incorporated with the reverb signal.  This alternate strategy will provide the same category of solution but with less potential hiss.

[GibsonGM]

Well, what I really want to do is to have a 2nd pot in there, so I can go from "2" on the 'verb to like '10', for the occasional echo-y solo.   I've heard of that being done with a relay, and I happen to have a DPDT 12v relay sitting there looking at me....
Hence the thoughts on a 2nd pot parallel with the one there, at location "B".
Think that'll work?

[Mark Hammer]

Running a cable out of the amp that is directly coupled to the audio path, even if it is well-shielded, risks picking up noise like any "wire antenna" would.  So, your instincts to want to use a relay are sensible.  My first suggestion to simply shunt R35 riskled noise because even when you aren't cancelling reverb, that's an "antenna" hanging XX feet outside the amp, and connected to the input of a recovery stage with a gain of just under 80x.

I mention this because I first considered suggestion a cable that selected between the stock value of R36 (220k) and a parallel combination of R36 and a second 220k resistor that would result in less recovery gain applied (hence less audible reverb signal).  Not so sure that running a cable directly out from the feedback path was such a great idea either.

That's what brings us back to relays.

[GibsonGM]

I hear you about the cable run, Mark...I had considered doing such a thing, but that antenna scared me so the relay came to mind.  I could probably tap into the trafo to get 12V for my relay and use a remote switch, mount the relay on a small PCB on the back panel of the amp or something with a jack...

So, do you think adding a 2nd pot, switched into parallel with the existing pot, would be a decent idea?  It could pop or make noise, but I just don't want to fry anything, he he he.
I could temporarily solder one in to see what the results are, I suppose...

[Morocotopo]

Just an idea: if you go with the grounding idea, an optocoupler would work, I think...
Or I might not have the slightest idea of what I´m talking about.

Morocotopo

[BrianJ]

I'm not really sure what you are intending to switch with the relay based on your first post.  Will the second pot be hard wired but the wipers switched by the relay?  That will pop if there is no pull down resistor near V3.  If the circuit really is an op amp I don't think you would lose much reverb signal by paralleling the 25k pot with another pot.  12k is still a good sized resistance for the low impedance output of an opamp - certainly not 50%.  Bypassing to point A should give full reverb as you said.

I would put a pot wired as a rheostat in series with the input of the existing pot.  The relay will bypass the new pot with straight wire.  The rheostat will dial in and attenuation of the higher reverb level.  A 25k pot with give 50% attenuation at maximum resistance, a 1M would be able to all-but-kill you reverb and reverb recovery noise.  Might have a funny taper though.

[GibsonGM]

I see what you're saying, Brian...I'm really interested in doing this as a "shock during the solo" type of thing...think of "Needle & The Spoon" by Skynyrd.  Or when playing a clean intro to a song, etc.  I currently reach over and crank the verb to 8 or so.  PITA.  It would be neat to be able to switch it, and the HRDx is my 'experimental amp'.    I thought of putting the rheostat (no wiper output) in para with the existing pot, but I think you are right, it won't offer enough 'change' to affect things, really...
 
I guess you're suggesting ADDING a rheostat-wired pot before the one already there, and using the relay to either jump it or put it in series - something to think about, and I bet it would work out OK.   Therefore, we'd be INCREASING the overall resistance with the rheo in - and switching it out will jump the reverb up.   I'll look into that - thanks!
 

[Bard Morons]

I didn't read through all of this, but I believe this gentleman did something similar to what you want by using a relay.  I didn't see anything about having two reverb levels available, but that shouldn't be too hard to tackle.

http://studentweb.eku.edu/justin_holton/laybuttmods.html

And this is the main page.

http://studentweb.eku.edu/justin_holton/

Definately worth a look through that site if you've got a Hot Rod Deluxe.  In fact, I'm going to do a few things to mine in the very near future.  Especially fixing that dang lead channel pot from linear to audio!  haha.  Hope that helps some!

[Mark Hammer]

Actually, Morocotopo's idea about the optocoupler is a very good one IMHO.

Your remote box could have a battery, and two current limiting variable resistances governing the illumination of an optocoupler back in the recovery circuit.  No "antenna" problems, and a pop-free guarantee.  Hell, you could even have a status-indicator LED in the remote box, all for the inconvenience of a 9v battery.

The question is where to stick the thing in the recovery circuit.  If you look at the circuit built around U2B, you have the choice of varying the value of R37 (2k7) or R36 (220k).  Making R37 larger in value, or making R36 smaller/lower, will reduce the maximum amount of reverb signal to blend back in. 

So, let's say you wanted to tinker with R37.  To my mind, you'd want to replace R37 with a 1k8 or 2k2 fixed resistor, in series with a 4k7 or thereabouts, for a total of 6k5 or 6k9.  That would reduce your recovery gain from 82x down to either 35x or 33x.  The LDR half of the optoisolator would naturally be in parallel with the added 4k7 resistor, and the effective parallel resistance would be something between 4k7, and something smaller that got you closer to a 2k7 target-value, once you add up the 1k8/2k2 fixed value into the mix.  WHERE it gets you in there, would depend on the value of the LDR, and extent of illumination.  Light up the LED in the optocoupler and the effective parallel resistance of the LDR+4k7 drops, thereby increasing recovery-stage gain.  The basic gist is that it gets you some value between the stock gain, and a lesser gain.  So, you'd set the reverb control on the amp for the most reverb you'd want, and the remote pedal could get you fractions of that, whether 7/8 or 1/5 or whatever.  If you dropped recovery-stage gain low enough (e.g., 10x) and had the amp control set to 4, my guess is you'd barely hear the reverb, so it'd be as good as "off".

The tricky part is that you need to work out in advance what the biggest drop in recovery gain is that you can tolerate and still have a workable reverb blend control.  So, if you wouldn't dream of ever having the reverb knob set above 6, then you have to figure out what a reasonable gain-drop is in the recovery circuit at that blend amount.  Certainly feasible, just a smattering of trial and error required.

[petemoore]

  Comment to design:
 Great idea...
 I have a reverb on/off switch, and like every one I've had, I wished that instead of on/off, it had say a "15% / 30%" function.
 Because I'm used to a preset 'basic' reverb tone [I dunno ~10'%' ?...or setting of '2' ?]...just a bit of reverb to wet everything up a touch...leaving it on the whole time...then adding echo because I need the 'dripping wet' sound to be only temporary and on a footswitch...all that could be narrowed down pretty good with a reverb 'amount' switch as opposed to a reverb 'on/off' switch.
 And the reasons are pretty obvious...it'd require one of the two switchable options be preset or would require another control knob or cost in parts/panel space IMO.
 One side being 'off' seems to be the convention that needs challenged I think.
  I'm starting a new thread...
 

[GibsonGM]

I hear that, Pete...Exactly how you said, I want to go from "2" on the 'verb to like "8".  I don't want "off".   I'm currently just figuring out where to mount the 2nd pot, where to get the 12V for the relay (heater tap?).    Adding a 2nd pot in series might make things 'weird' but once dialed in should be pretty simple....maybe a trimmer and small PCB inside the amp that also holds the relay, where you don't have to see the knob?  Pre-set and forget....

So on to the actual design of the thing, feel free to think something up! 

[Paul Marossy]

I don't know how feasible it is, but I think it would be cool to just take that reverb depth pot and make external to the amp via some kind of switching jack. Then it would be something that could be controlled with a foot pedal...

[GibsonGM]

...using a wah shell ;o)
But an optocoupler would still be necessary to adjust the resistance, as Mark Hammer pointed out - the cable run would be a huge noisy antenna.
   
So for your fan, you just paralleled your computer transformer to the power switch, eh?
I need to get 12v for that relay...that seems like the right way to do it.  Set up the board in there, and have a jack on the back panel for a footswitch.