Footswitch for Sunn amp

[gutsofgold]

I recently found a Sunn Concert Bass amp. I'm told the distortion on it is definitely useable and I'd like to find out. The distortion is activated by a footswitch which plugs into the back of the amp via two RCA jacks. The Left jack controls the distortion and the Right controls treble boost. How easy would it be to build my own? All it needs to do is ground each RCA jack separately. Is it as easy as running RCA cables from two switches?

[soulsonic]

I had to make one of these a few months back for exactly that model of amp....

Here's what you need to do: First, you must absolutely use shielded cable, otherwise the amp will hum very badly. You need a cable with two conductors and a shield. Microphone cable works well. Have one conductor going to the center pin of one RCA plug, and the other to the center pin of the second RCA plug. Connect the shield to the ground connection of both plugs.
Next, you use a DPDT footswitch to do the actual switching. Connect one wire to one pole and the other to the second pole. Connect the shield/ground to both poles. Wire it so that both connect to the shield/ground when the switch is activated. It's important that the two lines be kept on separate poles even though they connect together when grounded. If they are accidentally connected to each other otherwise it will cause a feedback loop that makes the amp not sound right, so the separate poles are absolutely essential.
The distortion actually sounds pretty decent once it's all working. The Concert series amps have a nice crunchy JFET preamp that overdrives really nice.

[soulsonic]

I wanted to one thing more to clarify..... you said one jack is for distortion and one is for treble boost? The one I worked on wasn't labeled that way, it just said both were distortion, so I have them wired to both switch simultaneously with one footswitch. I can say for sure, that the footswitch DOES NOT affect the setting of the Treble Boost switch that's on the front panel - that Treble Boost circuit is a separate thing entirely and not footswitchable. Both those jacks connect to the Distortion control, so I believe both need to be switched together for the Distortion circuit to be fully activated.

[gutsofgold]

Hmm, well what year (generation) was the Concert Bass? Was it black with red lettering on it OR a silver face with black lettering?

And thank for the building tips.

[soulsonic]

Both the ones I've worked on have been early/mid 70's Silverface w/black lettering. They conform to the schematic I found at Schematic Heaven.

[gutsofgold]

Ok, well I made up some RCA connections and found out just what each jack does. The left jack simply turns the distortion on/off. The right jack is a bit more interesting. When the distortion is off and the right jack shorted, the volume is real low and there is very little low end. When the distortion is on and there is nothing plugged into the right jack, the distortion causes for some SERIOUS volume. I'm talking speaker killer at 3 or 4. When you plug both jacks in, you can get distortion at moderate, regular volumes. It's also more of a crunchy "overdrive" this way, as opposed to the actual distortion sound when just the left jack is shorted.

In my opinion, both jacks would need to be shorted to get a useable distortion and I'll take up your instructions to make a single-switch pedal.
Thanks

[soulsonic]

Yup, that's how it works - both jacks have to go at once.