Supro/Valco Spectator Amp - Preamp Mods

I had put the Valco amp aside when I went and hung that Bigsby on the Gibby.  But now I'm back to it.

One of the items I had to address were the washers on the input jacks.  As you can see, they were rusty.

I may need to replace them with new ones.

They don't look good.  But before I replace them, I decided to dunk them in Naval Jelly overnight.

Well, golly.

Lookit how clean they came out.  I'm going to reuse them.  I'd rather reuse old fasteners when I can.

I took the three input jacks out as an assembly so I can rewire them.

The jacks from left are marked 'Instrument,' 'Instrument,' and 'Mic.'  That's the way they line up on the panel as well.

I'm going to rewire them to make the leftmost 'Instrument' input the hottest, then the next 'Instrument' have a little less gain, and then the 'Mic' input the least gain.

You can see how they were originally wired.  The two 'Inst' inputs go though 100K resistors (blue arrows) and connect to the hot side of the 'Mic' jack.  The 'Mic' jack is wired straight to the input (grid) of the 6SL7 input tube (blue arrow, yellow wire).  The green arrow points to the common ground.  More on that in a minute.

So the instrument inputs have the same resistance, and the mic input is the hottest.  I'm going to change this so the first instrument input has no resistor, for max gain, then the next instrument jack will have like a 68K or 47K, and the mic input will wind up with a 100K or so.  I'll have to experiment, but I want to wind up with three different levels of gain on the input.  Cheap thrills!

The way the amp was wired originally had the preamp ground for the cathode resistor and capacitor going to a terminal strip, and then to the red wire you see above wired to the center input jack.  So the jack served as the ground.  Not the best practice for reducing noise.

I'm going to wire the 3 jack grounds to a common buss wire, then run that to chassis ground.

Not difficult to do.  See the picture on the right.  That left tab is where the cathode resistor and cap go (still do).  Instead of that then connecting up to the jack, I just ran a bare wire jumper to the center tab, which is riveted to ground.

Poof.  One ground point.

I'll run the input jack buss ground to the same point as the cathode resistor and cap connection.  I try to run all my preamp grounds to one point for the least noise.

Next, we'll rewire the inputs, run some new filament wiring, and change out some caps.