Monday, January 27, 2020

Valco Supro Rebuild Completed

And it was a nice little side project that I've been meaning to get completed since I found it in the garage in Long Beach.

On the bench making the final voltage measurements.

The first thing I did was to bring it up on my Variac with the tubes pulled, measuring the input voltage and current draw. Since this had been stored for many years under less than ideal conditions, I had some concerns that the power transformer might be bad. It came up fine, and the filament voltage and high voltage measurements went as expected.

One interesting observation I made was that this was designed for 110VAC voltage. The nominal filament voltage for the rectifier tube is 5 Volts, provided by a separate secondary winding, and it measured 5 Volts with 110VAC applied to the primary. At today's nominal 120VAC, the filament voltage measured 5.65 Volts. The other filament winding for the 6 Volt tubes measured 6.7 Volts. The increased filament voltage will definitely have a negative effect on the life of the tubes, but there's not much you can easily do about it, so you just have to accept it. I've read about this in "Electric Radio" magazine, a magazine devoted to "Boat Anchor" (tube type) Amateur Radio gear, and read about it on other equipment restoration websites, so I was well aware of it.

Top of the chassis after testing completed. I didn't check to see what size tape I had in my "P-Touch" label printer so the tube type labels are a bit bigger than I would prefer.

And back in the cabinet.

So how does it sound? About what I expected from an amplifier of this design and construction. Has a bit of hum when the volume is wide open, and it's not as loud as I expected, but overall it has a nice "warm" mellow sound. All I had to plug into it was an Audio-Technica ATR30 500 Ohm dynamic microphone, which is probably not optimum for this amp, but ya' gotta' work with what ya' got sometimes.

I have no idea what I'm going to do with this little amp. I don't play, and I don't think any of The Clan does, either. Oh, well, back of the closet, I  guess.

Next up will be the Arcuino-based lightning detector, something I need to get finished before Spring.


  1. A lightning detector in our part of the world is wise.

    1. Oh, yeah! Especially when you have big aluminum tubes sticking up 25', and 90' of wire at 25' running across the yard.....

  2. Too bad you're a couple thousand miles away. I'd come over with an electric guitar. I'd be interested in hearing it myself.

    I have a couple of vintage radios, including a Collins KMW-2 (power supply is the Heathkit equivalent). Everyone talks about the higher line voltage these days, but I don't know a single person who has set up another transformer to buck the one in the PS and subtract some voltage off the output.

    1. Yeah, and it's not too difficult or expensive to do.

      The main effect is to run the filament hotter, which shortens the tube life. The other problem is that with fixed bias (cathode resistor), raising the plate voltage without changing the bias resistors will change the operating point of the tube. I've this alluded to, but as with bucking down the high line voltage, nobody deems to do anything about it.


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