Monday, January 13, 2020

Valco Supro Rebuild Progress

Besides bread making and playing Grandpa, I've actually been doing some other stuff, like continue the work on getting the little Supro amplifier back in working condition.

The input section has been completely rebuilt with all new parts. The resistors had all drifted high in value, well out of their loose 10% tolerance, and the tubular "paper" capacitors were all leaky. I also wicked all the old solder off the 1953-era connections, cleaned them, and resoldered them with a bit of rosin flux.

And the output section has been similarly rebuilt, adding (replacing, actually) a new fuse holder, new AC Line bypass capacitor, and a new three-wire grounded power cord, seen exiting the chassis out-of-focus.

Now I have to rebuild the power supply filter section. The big aluminum "Can" capacitor is leaky, and as usual, the resistors have drifted high and outside of their tolerance. I ordered a replacement filter capacitor for it, but, uh......I ordered the wrong one. I generally increase the capacitance a bit when I replace the filter since it gives better filtering, and the value I picked was based on a schematic for a slightly different amplifier. In short, this replacement is too big physically, and has too high a capacitance value to use. The original value(s) in the three sections of the can were 20 microfarads, 10 micofarads, and 10 microfarads. and I wanted to go to 40, 20, and 20.

The correct schematic (and the original can itself) shows all three sections being 10 microfarads, resulting in a smaller can.

The "too high a value" problem just appeared tonight when I was reading the "RCA Receiving Tube Manual", and found that for the 5Y3 rectifier tube used in this amp, the maximum recommended capacitance for a capacitor-input filter is 20 microfarads.


What I'm going to do is leave the old can capacitor on the chassis "for looks", disconnect it electrically, and use three new separate electrolytics under the chassis for the actual filters.

BTDT, and it works fine. So this is the next area of the chassis to be rebuilt.

I'll add a terminal strip under the chassis for the new tie-points I need.

And the new speaker grill material came in, and it's a joke. It looks like it was cut from a cheap suit, and would look totally out-of-place.

Hmmmmm.....this is what I had to start with. Very dirty, faded, and discolored. The "starburst" pattern around the dirty part was up against the inside of the amp, and you're seeing the imprint of where the amp's covering material was follded through the speaker opening, and glued to the inside of the amp.

But I have a plan.....I ran my shop vac over it with the bristle brush on it to get the crud off, and both sides got much cleaner. Then I realized if I just flipped it over, it would be reusable.

This is the side that was inside the amp, and had the speaker clamped down over it.

Looks presentable to me! And it fits in with "kinda keep it stock-appearing" theme, vs having a piece of cloth in there that looks like it came from a $20 suit.

I cleaned off the outside of the cabinet, and it's a mess. I don't want to recover this thing, so I'm just going to trim off any dangling pieces that are too far gone to reglue, and reglue any loose covering.

More to come.....


  1. I like your notion to leave the original can cap for aesthetic purposes. :)

    1. I've also pulled the old can, carefully sliced it open, restuffed it with new caps, and glued it back together, but that's a lot more work than I want to do for this.


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