Turning a practice amp into a spring reverb tank?

[Marty]

What would it take to turn a squier champ 15g into a reverb tank?

[BAARON]

Well, considering that there's no spring reverb tank in a Champ 15G and that you're not going to get much use out of the components in the amp itself, I'd say that all you're going to get out of that amp is the cabinet itself.  There's not going to be much in there that you can use to build a reverb unit.

http://www.solorb.com/elect/musiccirc/reverb2/ is a solid state spring reverb project, if you're really desperate to build yourself a spring reverb.  This is probably the cheapest option, though I'm not going to say it would be simple or easy.
https://taweber.powweb.com/store/kits_weber.htm#Add-A-Verb is an all-tube reverb unit.  I wouldn't build this unless you already have experience building with tubes.

[Marty]

I was thinking  Since the speaker is 4 ohms and the input in the reverb assembly is also 4 ohms.  I could swap the two and make some sort of recovery amp.  Does that make sense?

[Mark Hammer]

I gather this is the schematic for the amp, or is VERY close to it:  http://www.fender.com/support/amp_schematics/pdfs/Frontman_15G_Schematic_68F.pdf

You will note that at the output of U1B, the signal splits into 2 paths, one of them the overdrive channel, and the other a clean channel.  Both channels are returned to the same common point at the input to U3B, though only one of those paths is fed to U3B at a time (clean OR dirty).  It is technically possible to reconfigure the amp such that the clean output of U1A feeds a reverb tank, which can then be blended back in with the dirty signal at U3B.

It is unlikely to be a simple mod, and will necessitate at least one more op-amp stage added between the reverb pan and the mixing point at U3B, but it IS do-able without having to be terribly invasive.  That same mod can also lend itself to having a patch point between the extra recovery stage and mixing stage (U3B) such that yu could send the reverb signal to another amp/location.  Alternatively, simply mix the dirty channel way back and crank the reverb, feeding its own speaker to provide a separate clean (and EQ-able with the amp controls) reverb signal with its own speaker.

[Marty]

Yeah that makes sense.   Could you un-overdrive the overdrive. What model reverb unit would you use for that? how would you wire in the op-amp

[petemoore]

  Do you want to know what it might be like ?
  The amp can certainly drive a dinky reverb coil, if 4ohms that should match.
  You can plug the amp in [turn it way down, increase very slowly, turn all the way down again until you get something, that amp might easily pop the tanks-ducer].
  It basically works like a speaker, needs power to get going, too much power makes it go no more, tttoooo much power and fzsst/done, you can hear what the zone of safe drive level sounds like, medium drive level I think is the preferred anyway.
  Then plug the other end of the tank into an amp, er that's how I was able to determine that both transducers worked, the tank had input and the output sounded 100% springs.
  Once the thing's 'workin', figuring out the recover/mix with opamps etc. is really just a boost and buffer stage assignment.
  I think a Stage Center Reverb Project or better might help with GAS though.

[Mark Hammer]

Here is a quick clip of the dirty and clean channels of the 15G.

"Normalizing" the dirty channel:
You can see that C9/C10 and R16/R17 form a notch filter similar to what one finds on things like the Superfuzz and Foxx Tone Machine,  It makes the bottom and tops more accentuated.  If you wanted your dirty channel to be more "normal" and recoup a bit of mids, you'd probably want to stick a small resistor (1k-3k9) between C10 and ground.

The gain adjustment is very reminiscent of the Marshall Bluesbreaker pedal in that the Gain pot simultaneously adjusts the feedback resistance of U2B and the input resistance of U2A.  The use of two sets of diodes in U2A's feedback loop is also very much like the Bluesbreaker ( http://www.generalguitargadgets.com/pdf/ggg_mbb_sc1.pdf ).  My own preference for cleaning that stage up to be suitable for use as a "cleanish" channel (but still capable of some dirt when pushed), would be to remove CR1, CR2, and R23, replacing R23 with something like a 6k8 resistor.  A 47pf-68pf cap in parallel with R22 couldn't hurt either if you find things too harsh and buzzy.

"Reverbizing" the clean channel:
U1A is set up for a gain of 5.6, but it also has some attenuation just ahead of it provided by R8 and R9.  If you look at the schematic for the Anderton Stage Center Reverb ( http://www.generalguitargadgets.com/diagrams/stage_center_reverb_sc.gif ), we see that the driver stage has a gain of about 21, and a treble rolloff starting around 1.5khz.  The driver op-amp feeds the reverb pan directly.  Assuming we have a suitable reverb pan,then we need to hike the gain and tailor the frequency response.

So here's what we'll try.  First, lets reduce the attenuation a bit by making R9 a little higher, like 220k-270k.  Second, we'll up the gain by making R11 a 10k unit instead of 47k.  Third, we'll replace C4 with .047uf, tie it directly to ground, and remove C5.  Fourth, C3 will be replaced with a more sensible 220pf to trim back the treble.  With those changes, U1A will now have a gain of 23x, a bass rolloff around 338hz and treble rolloff around 3.3khz.  If we lift R12, the output at pin 1 of U1A replicates our reverb drive signal from the Stage Center Reverb.

Now what's left to do is insert a single op-amp stage similar to the recovery stage in the SCR, and stick a pot after it before the signal gets mixed back in via U3B.

That's about as complicated as I want to make this post at the moment.  A lotta stuff there.  Note that since we're going to scrap the channel switching, this reconfiguration would mean C13 is wired directly to the gain pot and R14 goes directly to U3B.  That leaves us a spare switch which I'm sure we can figure out something productive to do with eventually.

[Marty]

wow thanks.  I'll give it a try and keep you updated

[Marty]

this is the actual schematic http://music-electronics-forum.com/attachment.php?attachmentid=3932&d=1230346488