1976 Fender Twin Reverb - DIY Vibrato footswitch

[timhypo]

Hey guys, I'm new to this forum and have found a ton of useful information here, so I think this might be the right place to ask these questions. If I'm wrong, please let me know where this should be posted and please don't reply telling me to use the search function - I already have and haven't found specific enough information to risk blowing up my amp.

As stated in the Title, I have what I believe is a 1976 Twin Reverb, but I might be off by a few years in either direction. If I need a more specific year on the amp to answer my questions correctly, let me know, I'm pretty broke, cannot afford to replace it, and must have it in playable condition at least once a week for practice and once a week for shows. Anyhow, I'm in a garage/surf/rockabilly-type band and think I would love to be able to use the Vibrato built into this amplifier, but it didn't come with a pedal, so I haven't been able to try it. Essentially, there are two places to plug in the pedal - one labelled vibrato and the other labelled reverb. I assume, based on how it sounds, that when there is no pedal plugged into either the reverb is always on and the vibrato is always off. My bandmate has an 80's Fender solid-state amp and I tried to plug the pedal from it into the vibrato plug and was surprised to find that it's smaller than a standard 1/4" plug.

I have good soldering skills and, though I'm not extremely well-versed in wiring schematics, I design computer networks for a living, so I'm reasonably good at figuring out technical things and the abstract thinking that's sometimes required to do so. I'd like to build myself a pedal for this amp. I have enough dead stomp boxes and other assorted junk (had punk rock shows in my basement for 10 years - touring bands leave stuff) that I think I have what I need to make a pedal, physically-speaking, I just need your assistance with the electronic part. Before I build the pedal, however, I'd like to make sure that the vibrato is still working and strong - from some of the other posts my search returned, it looks like this goes out sometimes. I'd also like to test it in this band before building it, as the other guitar player is a bit of pedal whore with a pedal box half the size of his guitar case, so I wanna make sure we're not using the same effects.

Question 1 - What the heck kind of plug is it?

Question 2 - Can I get one at Radio Shack/Home Depot/Lowe's or do I have to break out the Parts Express/McMaster-Carr catalogs?

Question 3 - Are the two pedal controls basically setup so the two wires being connected (completing the circuit) turns the vibrato or reverb on and them being disconnected turns them off?

Question 4 - Or is there some kind of resistor in there that presents a different ohm load to turn on the effect?

Question 5 - If the assumption in my Question 3 is correct, can I then just jumper them to make sure the vibrato works before I expend time and effort building a pedal?

Question 6 - IF the answer to Question 4 is correct, is 22g wire sufficient or is there actually a decent amount of power running through the pedal?

Question 7, 8, and 9 - Where can I find schematics and whatnot for this amp? Fender's site has a few, but they're not year-specific and I'm not sure which one I should use for my amp. What year and model amps are similar to mine? What years and models use this type of pedal and pedal plug?


I look forward to your replies and have band practice tomorrow night. I'd love to hook up some vibrato for practice.


Thanks!

[Enzo]

Well, most old Fender footswitch jacks are plain old RCA jacks, like on the back of any stereo receiver. SO it is an RCA plug, also referred to as a "phono" plug. COntrast to the 1/4" plug which is a "phone" plug. Phono, as in record player, and phone as in telephone.

You could make an individual switch just for the vibrato, though the amp came with a two-button switch with two plugs on the cable end.

You will see both terms vibrato and tremolo. Musically they mean different things, but in terms of guitar amps, either one means the warbling variation in volume. SO any discussion of tremolo or just "trem" will apply to vibrato discussions.

Yes, the reverb footswitch closes to kill the reverb, and the trem footswitch closes to turn ON the tremolo. SO they work opposite.

Footswitch is often abbreviated FS, and I will be doing that.

SO to turn the feature on, you can temporarily just ground the center of the vibrato footswitch jack with a clip wire or even a soldered on jumper. Also many stereo receivers and amps came with shorting RCA plugs to be put into unused phono inputs to reduce noise. If you plug one of those into the FS jack, it should activate the circuit.

Radio Shack should have RCA plugs. Another alternative is to get an RCA patch cord - interconnect as the hifi crowd calls them - and cut off one end and put the FS there.

There are various suppliers we use. One with a good selection of amp relayed parts is Antique Electronics Supply at:

Antique Electronic Supply

They will have entire FS with cable pre made, or you can but the parts for them and make your own. They also will have the trem "Bug" that Fender uses in case you need one. The bug is the optocoupler - a neon lamp and a photocell in one package. And tubes and speakers, and so on.

A good general parts supplier is Mouser.

The stock Fender FS has a shielded wire for the reverb FS. The way it works is the FS just grounds across the reverb unit signal, so it must be shielded cable. With that shield being ground, they then used just one plain wire for the trem switch, the reverb switch shield serving as the ground return for both switches.

The FS is just two simple switches wired on/off. NO decoding or level sensing or anything fancy - just two switches. Much later amps have decoding, but not yours. There is no current, the reverb wire is signal path, and the trem wire is grounding a timing cap in a feedback loop.

The Twin Reverb did go through an entire lifetime of changes, Shematic Heaven has most of them:
Schematic Heaven. Where All Good Amp & Effect Designs Await Resurrection...

Go to the Fender section, then Twin Reverb. The 100 watt is probably a good place to start for yours.