Can I mod my Ampeg j12t to add a footswitch?

[littleman]

First post here, I came across this forum via Google and am hoping you guys can help me out. Sorry if I'm a bit noobish, I'm pretty good with a soldering iron but my understanding of amp circuitry is a little basic.

I have an Ampeg J12t that I absolutely love, but the tremolo and reverb circuits aren't footswitchable so they're not too useful in a live setting, especially the trem. How would I go about modding this amp to add a stereo footswitch jack to it that would control the tremolo and reverb? Can it be done?

Thanks in advance for any help.

~Mitch

EDIT: I found a link to the schematic, if that helps.
http://www.schematicheaven.com/ampeg...12t_jet_ri.pdf

[Steve A.]

I have an Ampeg J12t that I absolutely love, but the tremolo and reverb circuits aren't footswitchable so they're not too useful in a live setting, especially the trem. How would I go about modding this amp to add a stereo footswitch jack to it that would control the tremolo and reverb? Can it be done?

~Mitch:

You might try connecting shielded cable to the wipers of the reverb and intensity pots and then switch them to ground to kill the reverb or tremelo. I'm sure that it would "work" but it might add hum to the amp.

Another suggestion: perhaps you can find a similar Ampeg amp that has the dual footswitch and see how they wire that up.

Good luck!

Steve Ahola

[pdf64]

A switch that shorts TP5 to ground should cut the trem out.
A switch that shorts J12 to J13 should cut the reverb out.

[Bruce / Mission Amps]

~Mitch:

You might try connecting shielded cable to the wipers of the reverb and intensity pots and then switch them to ground to kill the reverb or tremelo. I'm sure that it would "work" but it might add hum to the amp.

Another suggestion: perhaps you can find a similar Ampeg amp that has the dual footswitch and see how they wire that up.

Good luck!

Steve Ahola

Grounding any of the feedback caps or the 1m grid load resistor would stop the vibrato circuit from oscillating... a shielded cable from the tank output to a shorting switch will kill reverb.


Steve don't hear much from you anymore. Where you been hiding out lately?

[Steve A.]

Steve don't hear much from you anymore. Where you been hiding out lately?

Shhhhh... I've been in the witless protection plan because I testified against the LA Mafia. Hey, I thought that they were just musicians- how did I know that they were "connected"?

I gotta go- I think that this phone line is tapped.

Ste..., er, Roger- yeah Roger it is.

[littleman]

Thanks for the help guys! One concern I have is that if set up the footswitch to open/close the circuit and short stuff, then I'll end up routing the signal through the footswitch and back into the amp, that will 1) add noise and 2) keep me from being able to use the trem or reverb if I don't have a footswitch plugged in. If I ground one of the parts of the tremolo circuit, I won't be sending the signal through the footswitch, right? Is there a method to have a footswitch control a separate switch inside the amp or something like that? Or if I ground the reverb send at J11 or J13, would that work to cut the reverb?

Or am I making so little sense that there's no way I should be messing with the inside of an amp?

[Bruce / Mission Amps]

Thanks for the help guys! One concern I have is that if set up the footswitch to open/close the circuit and short stuff, then I'll end up routing the signal through the footswitch and back into the amp, that will 1) add noise and 2) keep me from being able to use the trem or reverb if I don't have a footswitch plugged in. If I ground one of the parts of the tremolo circuit, I won't be sending the signal through the footswitch, right? ...

Well, in a word no, but noises do get through anyhow sometimes.
It is an oscillator that shouldn't really be sending any noise to the amp... The reverb is in the audio chain and you can get noise pickup from cables but it is relatively easy to fix that, especially if you use a pedal like the Fender one where one cable is shielded... the reverb on-off is the one that will make noise and needs shielding.
Just go look at how the old Fender reverb amps were wired up.

[littleman]

Finally got around to doing this. I decided to just do the tremolo, I really don't need to switch the reverb. I did it by grounding TP5, the center wiper of the tremolo intensity pot, to the chassis.

Problem is, it zaps me. Not too bad, but it's a nice little jolt if I let the amp stay on with the tremolo switched off for long enough. Not really sure why, as far as I can tell there's no difference between what I did and turning the intensity all the way down.

[francuccino]

Vib and Rev switching

Finally got around to doing this. I decided to just do the tremolo, I really don't need to switch the reverb. I did it by grounding TP5, the center wiper of the tremolo intensity pot, to the chassis.

Problem is, it zaps me. Not too bad, but it's a nice little jolt if I let the amp stay on with the tremolo switched off for long enough. Not really sure why, as far as I can tell there's no difference between what I did and turning the intensity all the way down.

Looking at a J20 vibrato switch circuit, this should work without zapping you. I am going to do both Rev/ Trem and use a fender 2 button stereo footswitch and install a stereo jack in place of the Accordion input