I repair tube guitar amplifiers from time to time, and I cringe when I see the 50L6 power tube and the 35Z4 rectifier. These tubes light up like lightbulbs, and run so hot that the amplifier case is often charred near them. The chassis shows staining from the heat, and the plastic tube socket can even melt.
These amps always hum because the same reasoning that goes for the cheap on the circuit design also uses cheap and low value capacitors.
I hate these circuits. Several times I've gutted the chassis, screwed a power transformer in place and wired the amp as a tweed champ, Gibson Skylark or Premier 50, so I have a chassis with the right number of tube sockets and pots, and I get a brand-new vintage amp for the cost of a used transformer.
I have a Gretsch amp that had two 50L6 tubes, that made a beautiful 1957 Fender Pro amp and sounds wonderful.
The hot chassis design is mind-blowingly dangerous by contemporary standards. These things weren't designed to modern creepage distances either. I would worry about creepage to controls on these things. THe article mentions screws potentially being hot as well as other shock hazards finger-accessible through vent hols so no joke be careful to unplug a radio before moving it or otherwise handling it in a way beyond turning it on and tuning it. All of these safety standards are written in blood.
The safety interlock cover was mandated at one point by law (or by the UL), because of this - because there are user replaceable components inside (tubes), they mandated a pressboard cover that contained the power lead, so you couldn't hook it to power while also having the case open.
I've modified and disassembled several of these radios. There's a second connection behind the cover. This will be disconnected as you remove the cover, but you can easily reconnect it afterwards. I've done so myself to probe the various components (I only manipulated it while the power was off, though).
Yes they are terrible. Should be consigned to museums and never celebrated.
My grandmother had a tube TV with a hot chassis. The Bakelite controls were on chassis connected metal shafts. The only protection you had from being killed was beeswax you had to rely on the TV repair guy to apply to the grub screw holes on the shafts.
I think you misunderstand. "Mind-blowing" is not about the courage or skill it takes to work on them, and it's in relative terms. They didn't have contemporary safety standards back then, so I don't think the manufacturers were being completely feckless. All that had to be learned.
But today, would you build something that puts one side of an unpolarized plug onto finger-accessible screws and then give it to a neighbor?
No, but I would gladly repair it for them and explain the danger, if it were a family heirloom.
For example, I recently helped a friend get his grandfather's 1949 Airline Television working. There were some interesting aspects of that circuit.
My late father had a mint condition Sunbeam Automatic Toaster he loved to show off. It has a hot chassis.
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Back in high school(1980), my chemistry teacher, Mr Tchalo had an old conductivity demonstration apparatus that was essentially an incandescent light bulb in series with two copper prongs that would dip into a beaker.
After explaining how it worked, he would slowly pinch them together, then challenge us to do the same. I never tried it. As I said, anything more than 12 volts or 15 amperes, and I get scared.
Mr Tchalo was from the old school of teaching, he had been brought back from retirement to fill out the staffing levels required for certification. He spoke with a heavy accent and had some seriously thick glasses, his eyes had been injured in a gas attack in his childhood in Macedonia during The Great War. He was one of my favorite teachers ever.
Even wiring the chassis to the neutral line with a polarized plug would be a bit archaic today, though it greatly improves things (assuming the power-switch isn't on the same line as the chassis, which on many of these radios it is !!)
Early guitar amplifiers were expected to be used that way. There was a polarity switch, and you flipped it to the position where the neutral was "connected" to the chassis via a capacitor. This arrangement is now known as the "suicide cap."
There's a general agreement among players, that having a tech install a proper grounded cord doesn't spoil the "vintage value" of an old amp.
I had one of these when I was a teen! Granted, it wasn't originally mine, but I found it in my grandfather's shed in the early 1990's and I kept it on my nightstand for years. It worked too (I might have replaced the vacuum tubes originally...I don't remember now) and the thing got uncomfortably hot.
It had an "alarm clock" feature which would turn on the radio at the time the connection for the alarm would happen. But I was always too nervous to use it because I was afraid the house would burn down if I left it unattended.
Edit: Also, I don't know if it was the tubes or the internal speaker, but the damn thing was LOUD.
The old Zenith stuff of this era is nice, because they used almost exclusively disc caps, which means if it doesnt work, all it needs is the power caps (and one more in the tone section usually). What that translates to, is it usually works out of the box.
Yes! It needed a new speaker and the clock motor needed lubrication, but I also added the polarized cord and moved the switch to the hot side. It has the floating chassis, but it's not very isolated from the external metal case (just some cardboard).
Like many of these it uses a "couplet" (early IC) for the audio inter-stage coupling.
All of mine are plastic or wood cased, so I've not bothered with the polarized cord. I also use vintage coffee makers (percolators) that have significant leakage out the chassis.
Even when the local department store was selling home computers based on microprocessors, they still devoted a chunk of retail floor space to a self-service vacuum tube tester.
Well, Halted had a tube tester in the corner until the day they closed a couple years ago. There are NOS audio tubes still available. But if the random stuff showing up in my Youtube feed is any indication, the cool kids these days are learning glass blowing and making vacuum tubes from scratch.
I remember them, and it would have been the late 80s or early 90s. I had to ask my dad what it was, but I remember thinking maybe I would need to use that service someday.
If you want to go down a rabbit hole, here's a classic video where a guy makes a brand new tube by hand: https://youtu.be/EzyXMEpq4qw
A whole lot of those home computers got used with old vacuum tube sets. I used a B&W GE portable tube set with my VIC-20.
There are a lot of NOS television and radio tubes out there, and they are mostly less expensive than the ones used in guitar amplifiers and hi-fi equipment.
She is the absolute best! Though the stuff she works on is typically much older than an AA5, shango066 is also great (though, his politics are somewhere between wacky-wacky-wacky and crazy town express), JordanPier too.
Glassslinger is probably the best bench tech I've ever seen, she has an encyclopedic knowledge of vintage electronics.
Yeah, they are dangerous, but their safety can be improved: add polarized cord (probably have to move the power switch to the hot side for this) and replace the chassis ground capacitor.
Table top FM radios and TVs also tend to be hot chassis.. (that mains transformer is just too expensive). Even today you can find capacitor dropper powered devices which are hot.
AA5s are nice in that you can almost always get them to work for cheap. I mean if you have an older radio with a bad mains transformer, it's probably not worth fixing.
Another advantage: they will work on 120V DC.
I don't think low end hot chassis radios were popular in Europe, they all seem to have a transformer. Maybe 220V is just too dangerous... (but you can certainly have a primary to secondary fault in a transformer..)
Polarised cord only works if your house sockets are wired correctly which is not necessarily the case and your earth bond might not be as good as you think it is. Always assume that the lowest competence is applied when it comes to safety evaluation.
At this point I wouldn’t run one of these things at all. If you drop dead you have no idea if some relative is going to plug it in and kill themselves with it. They were designed to a cost target when safety wasn’t a business concern. Steal the tubes and variable caps out and dispose of the rest. At best, stick it in a museum, preferably of dangerous things that should never be plugged in.
There’s plenty of tube kit out there you can buy which isn’t even remotely this dangerous.
Please do not willy-nilly dispose of beautiful pieces of interior design which likely won't ever be produced again with similar aesthetics. [1]
People constantly throwing away cultural heritage without any second thoughts is the reason why we barely have any left from only even more than 100 or 200 years ago - thousands of years of furniture, decoration, etc. are gone.
If you don't want them to be dangerous, make them safe.
If thats too complex for you, remove the cable and clearly label the backside with "DANGER! [...]" to explain that this shouldn't be used as is, and ideally put a letter inside which explains the technical details.
[1] Yes, I know there are things which are newly produced to "look" old. But they only do to people who aren't into that taste. If you are, all of the modern reproductions look like cheap, insufferable garbage.
I wonder why faux vintage stuff with modern electronics isn't more realistic and higher quality. It's not like high end woodworking isn't still a thing.
At the very least, if the alternative is full disposal, the old ones could be upgraded with modern electronics.
Okay but assuming there's no exposed metal parts, is one of these radios with a polarized cord really more dangerous than a regular old two pronged lamp with a polarized cord?
If you plug that lamp into an outlet with positive and neutral wired wrong you will have mains voltage on the light bulb threads and socket threads instead of ground, and that's gonna be pretty unsafe for anyone changing the bulb.
But lamps are so common, we don't even think about them as safety hazards.
240 was too high, and most of the non-110v sets use a transformer.
In reality, the case is meant to isolate the set, so the thing is only relevant when performing service work, and honestly, I just measure voltage between chassis and earth and make sure there is zero volts of potential.
Well, in most cases I'm sitting on a chair isolated from earth, several feet in the air (wearing rubber soled shoes), so long as my test equipment shows 0v of potential to the chassis, its pretty safe. It could be that the earth bond has failed, but provided I have both pieces of gear sharing an earth, its all good.
I wanna note, I worked as an FSE for a telecom, I've been 'bit' more than once by 130v, and by 160v B+ in shit, the sparks from crossed neutral are fine.
Anyone who is deeply concerned can buy an isolation transformer.
110v can fuck you up. Don’t take your own advice please. At least no one else should. Isolation transformer will only stop you getting killed one of three ways.
Worth backing my experience up as well. Ex EE covering RF and power in defence sector.
There is no room for arrogance or assumptions in dealing with safety.
My dad used to repair TVs; I asked him what the easiest way to discharge a tube was and he said "grab the anode lead and the chassis" then after I gave him a look he said "well you didn't ask for the easiest safe way to discharge one"
Not necessarily. The main filter caps can retain a high voltage depending on how it’s wired and if the designers installed a bleeder resistor and if the production guys didn’t remove it to save a few cents.
The CRT acts like a capacitor, storing charge for quite considerable lengths of time. Consider that a B&W would typically run 10kV to 15kV, and color sets easily 25kV, and that should give you some idea of how much energy one of those things can hold. And with those kinds of voltages, you don't even need to make contact to find you've become a discharge path, that stuff will reach out and grab you across considerable air gaps.
oh, boy. the temptations of telling how to find out on your own are strong!
the capacitors in CRTs could store a charge for a long time. i've seen people that assumed the caps were discharged find out the hard way when they shorted the leads with a tool. the noise alone is enough to make one need clean shorts. the only good thing is that they discharge quickly, but they give you "both barrels" worth to borrow a phrase.
CRTs hold a deadly charge for minutes to hours after being unplugged. CRTs can also emit x-rays if you bypass safety circuits. And touching a live flyback would be shaking hands with death. They were also manufactured before RoHS, so don't lick the PCBs or throw them in the ocean. Other than that, they're perfectly safe.
They're perfectly safe to work on, just measure voltage between chassis and earth and make sure there is zero volts of potential - or be fancy and use an isolation transformer.
You can still kill yourself with an isolation transformer fairly easily if you don’t know what you’re doing. See my other comments about earth assumptions.
Many consumer appliances are also still unisolated inside, it saves cost and you are unlikely to be touching anything inside if you don't know what you're doing (hence the usual "do not open" warnings on them.)
Yeah, do not attempt to use. High voltage inside. Better to buy any new low voltage piece of shit. Throw this all away. Be very afraid of this old stuff. Hmmm....your Tesla, runs on 3.7V LiIon...safe.
It is better to say that you should not go inside of an old television unless you know what you are doing, and don't assume that you know what you are doing unless you have guidance from someone who is experienced. Reading about or watching someone work inside of high voltage devices should not be considered sufficient since it is easy for either the author or reader to miss important details.
It is also worth noting that electrical safety depends upon far more than voltage. We think of 3.7 V battery operated devices as safe because our bodies are not very good conductors (when dry) and some types of batteries have an internal resistance that will limit the current draw. If I recall correctly, the internal resistance of LiIon batteries is sufficiently low that they are dangerous if not handled correctly (e.g. if you short them).
When I was a little kid one of my favorites was staring at the back of the huge tube based radio. It had boatloads of glowing tubes and the whole thing looked to me like a small city.
That sentiment resonates with me. I first got my ham license back in the days of vacuum tubes. There is nothing like the smell of a warm, dusty vacuum tube. I can’t describe the emotions that evokes.
I was recalling the same thing. For me it was our little black and white TV we had when I was young. I loved going around to the back and side and looking in at the little city inside.
I've made my share of tube amps from scratch off and on for a few decades now. Must be due to nostalgia of some sort.
This high voltage hot chassis was clearly a dangerous design but at least there has been little use of it in the last fifty years. I find dangerous designs in current products fairly frequently.
As an example I was debugging my electric clothes dryer that stopped drying and found the wire to the nichrome heating unit was disattached. It had been attached to the heater with a push-on, pull-off spade connector at the factory. I maintain it should have been attached with a bolt-on connector with a lock washer. Having what is probably a 230 volt wire slip-off its connection due to normal dryer vibration and bounce around the steel chassis is concerning. If the dryer ground had been faulty (it was good) that could have placed a 230 volt potential on the entire metal cabinet of the dryer in a laundry room that may have a little water on the floor and usually has other ground paths like turn-off faucets for the washer as well - a super dangerous situation. This danger is increased because dryers are heavily used by kids, women, and non-technical men, most of whom have no idea of the risks involved.
As a kid, I acquired a few of these from my grandparents' farm. At some level it struck me as odd that they shared so many common features and details. It was many years before finally hearing about the "All American 5" and then everything made sense.
If you read down you'll see mention of A batteries and B batteries, used in "farm radios". That's why the batteries you buy now go from C and D, with AA and AAA coming later.
Another case of A and B becoming obsolete, like floppy drives did.
I used to buy and do light repairs to shortwave and AM radios. A relic of that time is that I have an "isolation transformer", a 1:1 transformer to isolate the radio from ground. 110 in, 110 out.
