Here’s the deal: I bought an old Supro amplifier, the kind that used to be sold with lap steel guitars in the 1940’s. I got it cheap because the power cord’s plug had been cut off; it’s very dirty, and the owner had no way of knowing if it worked or not. In cleaning it, the circuitry, while very old, looks largely intact. The tubes are dirty but not damaged or cracked.
I’d like to try to hook up a new power cord; I’d like even better to do so without electrocuting myself. This is obviously a very old piece of equipment, so the power cord would have been a two-prong ungrounded affair. Inside the circuitry enclosure, the power cord’s two wires hook into separate components: one wire is soldered to the on/off switch; the other is soldered to the fuse enclosure. From there, both wires find their way into a small, mysterious metal enclosure which I’ve yet to unscrew. There is no obvious ground.
First question is, given a new power cord, is it safe to assume the live lead would attach to the power switch and the neutral lead would attach to the fuse complex? If it’s not safe to assume that, is there a way to determine which point the live lead would connect to with a multimeter or continuity tester?
Second question is simple: one of the tubes was unplugged from its socket. The socket has more holes than the tube has prongs–does it matter how I plug the tube back into the socket? Do the prongs have to go into certain holes or will any orientation that seats all the prongs work?
Any other suggestions/caveats/“don’t try this you fool” comments are welcome; thanks…
For your first question, it should be a safe bet that the “hot” wire would go to the switch, so you should be OK there.
As far as the vacuum tube, every tube I have seen will only fit one way into the socket. Larger tubes usually have a center post with a “key” to make sure it goes the right way. Most of the smaller size tubes that I remember had a gap in between regularly spaced pins which corresponded to a similar gap in the socket.
Bottom line – if you can plug it in more than one way and get it to fit I’d be very surprised.
The best thing for you to do is to try and find a circuit diagram - do you know the make and model?
One Vital safety point. Even if you wire it up correctly, the whole chassis will be live when it’s plugged in (even if the mains switch is off). Take appropriate precautions. And, if you have to ask what those precautions are, don’t attempt the repair in the first place.
On the tube with too few pins, what’s probably happened is that one or more pins have broken off, and are still in the socket. On a standard base, there will be a definite gap in the holes in the socket to indicate the orientation, with a corresponding gap in the tube pins. And, yes, it is important to put the tube in the right way round.
I can plug it in any number of ways, but after reading your post, I took a closer look a the bottom of the tube, and in the center, there’s a small plastic key that would match only one position on the socket–were it not broken off. So that’s the answer to that–thank you!
What, if anything, is the wisdom in replacing the tubes, if they work? Obviously, I don’t want to ruin the “vintage” quality of the sound, but I don’t want it to be so vintage it sounds like crap…
Sequent, there are very cautious techniques for restoring old valve equipment, and the longer they’ve been gathering dust, the more cautious you have to be. Often just reapplying mains will produce a big bang and lots of acrid smoke. This will be the electrolytic capacitors giving up the ghost. When they’re stored for extended periods they tend to go leaky, and require replacing or reconditioning. The latter can be quite a complex and lengthy procedure (see the end of the page on this Mallory document), to some of the simpler methods outlined by Harvard. Slowly ramping up the mains voltage to the amp with a variac is a quick (well, still several hours, but relatively quick) and dirty way to reform all the electrolytics at once. Better would be to remove them all and reform individually by slowly charging up to the rated voltage with a constant current source, or current limiting resistor and a voltage source. Or you could replace all the electrolytics; 1940s 'leccys were shit when they were new, and only got worse with age.
It doesn’t end there. The horrible carbon rod resistors will have drifted by a large amount. Measure the value of each and every one, and replace where necessary. The smaller paper and wax capacitors might have dried out somewhat and lost a chunk of their value.
Pay attention also to the mains wiring. Often the rubber or cotton insulation will be perished to the point of deathtrap, and they didn’t design with safety foremost in those days anyway. I’ve seen all sorts of nightmares in old amps - single-strand mains wiring flapping round loose, fuseholders that expose a live terminal when unscrewed, mains voltage selectors that expose live parts when used… frightening. It’s no wonder so many musicians used to get electrocuted.
The transformers will either be OK or not, and you should get away with powering up the valves as they are, even if one or two may need replacing. Be sure to take each valve out and clean the socket contacts and valve pins before replacing, as they’ll be a bit oxidised with age.
If all this sounds a bit daunting, then don’t touch it and flog it on ebay as is. You’ll be surprised what someone will pay for it. There’s a good chance it needs fixing even without any restoring, as a chopped-off mains lead is usually the sign of a piece of it that’s been forcibly decomissioned.
Thanks, Fridgemagnet…sounds like just plugging it in isn’t a good idea, then, since there’s no way of knowing how long it’s been since it’s been used. I had anticipated the smoke and some crackling, but a loud bang followed by silence would have been most distressing. Now I’m better prepared, at least.
I think I’ll try the quick-and-dirty variac method method to begin with. Individually reforming/reconditioning the would be a bit over my head, especially since I can’t even identify all the parts I’m looking at. :rolleyes: Otherwise, it could be a fun project. The chopped-off mains was a big red flag to me, too, though I could just as easily envision someone doing it simply because the unit looked old and they were afraid to plug it in (my grandmother used to do this with radios).
Yes, everything was dirty and oxidized. Just the cleanup was quite an endeavour. I’m not holding out too much hope; if nothing else, the case is pretty cool for a speaker box.
On a side note: I’ve heard of musicians getting electrocuted, of course, but I wasn’t aware that it was more commonplace back in the day. Definitely food for thought…
I’ve found a lot more information on catastrophic electrolytic capacitor failure. Apparently apart from a really loud bang, you can expect a godawful smelly discharge and mess. Good times.
Anyway, some sources I’ve found describing the use of a variac to reform the electrolytics says the tubes/valves should be removed first, which doesn’t exactly make sense to me.
That last link gives good advice, and taking the valves out before reforming the caps with a variac is safest, and the valves usually don’t set the DC working point of the big electrolytic caps. You’ll still need the rectifier valve though, or you’ll get no DC at all onto your old caps. Wind it up good and slow for best results - take a few hours of gradual incrementing to get it up to full voltage. Good luck!
During the time period, Valco apparently made amps for Supro, Gretsch, National, and others, so finding the specific schematic is difficult. Here is a National model that is, apart from the logo, completely identical to mine. The date code on the speaker indicates that its manufacture date was a year ending with ‘1’, which would be either 1941 or 1951–I’m guessing the latter. So far, the only electrolytic capacitor I can find is orange and clearly marked as such; I count five wax capacitors.
Thanks! I count five tubes/valves; four of which are similar. One has an aluminum cap on it with a large wire going into it–I assume that’s the rectifier tube?
The valve/tube model number will be marked on the glass - look it up to see what it is. Careful when cleaning - these numbers sometimes rub off! A metal jacket over a valve is there for EMC screening purposes.
Some hollow-state links:
Watford Valves - wonderful valve vendor, and a mine of useful information.
Tone Lizard - sound valve amp theory and information, and no misleading bullshit.
Most of the engineers who knew how to design properly with valves are dead or retired, and that left a bit of a generational knowledge gap when valves fell out of favour in the 70s/80s. Consequently, there’s an awful lot of superstitious nonsense with regard to the design and construction of valve amplifiers. Don’t get me started…
A quick update to say that restoration is complete and the amp seems to be working well! I hooked up a new power cord and replaced the only electrolytic capacitor, and she fired up like a champ the first time! There’s still a lot of cleaning to do, but at least now I know it’s worth it. Thanks to everyone for the input and advice!