First things first: change the filter caps. Even if the original filter caps were not the problem, they are sixty years old, and J bought this thing in order to use it live, so reliability is paramount, and filter caps are cheap so long as you use the small plastic single-units as I did above (as opposed to paying $40 for one of those identical-fit newly-manufactured multi-caps). I disconnected all wires going to the original multi-cap and left it in place.
Next: that crackling sound. In my experience, this is generally caused by aged pre-amp plate-load resistors. These resistors drop a LOT of voltage (which they do by turning it into heat) and they don’t last forever. Changed all of those out and the crackling went away.
Now about that low B+ reading and the constant distortion: I had noticed that the volume pot had a slight crackle to it. This can be caused by one of two things: dust and/or mechanical failure in the pot itself, or there could be DC voltage present at the pot. The coupling capacitor ahead of the pot is supposed to block this DC, but these are 60 years old ain’t they. After a quick spray of contact cleaner the crackle was still there, exactly the same. I put the DC meter on the input of the pot and sure enough: 2 volts were present. This is not a lot (I’ve measured as much as 25 volts here in amps that are actually in-service) but it’s enough to cause some noise. I replaced the coupling cap with a Sprague Orange Drop (the big orange thing in the photo above) of the same spec and done. Noise was gone. The pot actually worked fine and did not require replacement.
Anyhow, the fact that this coupling cap had failed lead me to believe that the other similar caps in the amp were also suspect. And as it turns out, the two .02 coupling caps feeding the grids of the 6V6 power tubes were VERY leaky: I measured 22 volts+ present at the grids of those tubes. Since this is a cathode-biased amp, I would expect to see 0 volts DC at this point and around 20 volts present at the cathode. Instead what we had was an amp with a 100% bias error. It’s amazing that it worked at all. The 22 volts present at the grids of the 6V6s meant that they were drawing a TON of current, which explained the low B+ reading.
So what else did this lil Supro get? New matched JJ 6V6s, to start with. I auditioned several NOS 6SC7s in the phase inverter position and none of them made any difference, so I left that tube and the rectifier tube alone. The 6J7 preamp tube sounded fine, but I was curious how much improvement could be gotten there, so I put in a 1620 (special selected low-noise 6J7 variant) and there was a marked reduction in white noise and less hum too (hum level had been low after the re-cap but now it was nearly gone). I added a grounded, 3-wire AC cable, removed the ‘death cap’ from the AC primary, and bolted it all back together. The amp sounds fantastic. Really, really great. Much better, in all honesty, to my DIY’d version, although how much of that is due to the speaker I’ll never know. I’ll end on this note: people seem to be under the impression that these old Valcos and Supros are somehow low-budget, cheaply made, etc., but I found quite the opposite to be true. This piece was extremely well-constructed, with very high-quality components throughout, and the soldering work was nearly flawless. For a very small amp that only weighs about 15 lbs, it’s loud, clear, and dynamic, and overall just a very cool alternative to a tweed fender deluxe for about 1/4 the price.
It’s very easy to service a previously-working amplifier that has ‘suddenly stopped working.’ I plug it in and start measuring the voltages, starting from the power transformer primary, on to the secondaries, then to the B+ supply, etc, the tube plates, the cathodes, etc., until I find a voltage that looks off. The component failure will likely be somewhere at that point. On the other hand, an old amp that ‘works’ but which has numerous deficiencies (crackles, hums, noise, distortion) can seem a little more daunting, but if one employs a similarly systematic approach, all issues will eventually reveal themselves. Even if a technician doesn’t have the service data on a particular amp, they have become familiar (through studying many amp schematics that do have test voltages indicated) with the kind of readings that one would expect to see on the grids, plates, and cathodes of various tube amp circuits. That’s a good place to start. The rest of it – learning which components are likely to fail, and what the symptoms of these failures are – learning all that stuff just takes time.
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I just shotgun them. By this, for the uninitiated, I do not mean blasting it with a 12-gauge, I mean summarily replacing certain parts which are known to go bad , many of which you have identified, such as preamp load resistors and coupling caps, and cathode resistors.
The sad thing is a lot of guys would run it that way until the transformers gave out.
I don't summarily replace coupling caps but I use a rig to test for leakage. If the old cap has survived this long there is no reason to put it down. You should have a copy of Alan Douglas' book which has the design for the leak tester, it can be built out of old junk for a few dollars.
Old caps can be fine one minute and go bad in a few days in old unused electronics, so it's safer to not take chances. Especially if untechnical people are going to use it. If I'm using it myself I know what to look for, so will take a little more leniency.
How much would this kind of work cost? I'm looking at purchasing one or two similar amps but I'm curious how much I might have to put into it. Thank you so much for your time
This cost the customer approx. $300 plus shipping. c.