Amplifying like it's 1960, in 2020. Or 2070?
I remember looking through Popular Electronics magazines as a youngster. There would be tube projects that you could build almost completely with items in your 'junk box'. Fifty years later, when I finally got around to building them, my 'junk box' no longer contains the necessary items. In that time high voltage power transformers and audio output transformers are no longer common, even in scrapped electronics. Everything is using switching supplies and direct coupled solid state audio amplifiers. It's not all bad news: some tubes are still manufactured, courtesy of the audiophile and electric guitar communities, and basic components such as resistors and capacitors haven't changed.
This being the case I've taken it on myself to re-design a single-tube audio amplifier to use modern (circa 2020) components. I've also tried to anticipate changes that might take place in the next 50 years so that this circuit will still be build-able at that time. I figure if I find it interesting to build an amplifier with a 50 year obsolete technology such as a vacuum tube, fifty years from now someone else might want to do the same. Except it will be 100 years obsolete! Maybe I'll try and build it again when I'm 110 years old?
DISCLAIMER: YOU REALLY SHOULDN'T BUILD THIS! IT'S ALL KINDS OF DANGEROUS SINCE IT'S HIGH VOLTAGE AND NOT ISOLATED. AND THE TUBE CAN BURN YOU. I WON'T BE HELD RESPONSIBLE FOR ANY ACCIDENTS. THIS IS REALLY JUST A DESIGN EXERCISE.
Here are some tips for finding the components in the future and likely alternatives. It's mostly accumulated wisdom from 50 years of experimenting:
POWER SOURCE: I'm assuming 120 or 240V AC is still used. If it's been replaced with a smart DC distribution scheme, there are probably converters. You may even be able to get the voltages you need from this system in this case. (See the diagrams below for what voltages you need in that case.) The dangerous part if this circuit is that the signal input ground is connected to one side of the AC line. It could easily be the 'hot' side of the line if the outlet is mis-wired. An isolation transformer is strongly recommended.
RESISTORS: Probably still available, although the leaded components might be hard to get. A surface mount (chip) resistor can be converted to one by soldering wires to it. Or salvage them from electronic scrap. If you can't find the values you need you can construct new values from parallel and series combinations.
CAPACITORS: Electrolytic capacitors are what's used as of this writing. They don't age well, so if you're salvaging from old electronics try to find the newest you can. If they're bulged at the ends they're no good. They lose capacity over time, but this comes back after a bit of use, unless they've dried out. Hopefully they will be replaced in the future with a polymer of multi-layer or some as-yet-to-be-invented type which won't have the aging problem. Make sure the voltage ratings are at least as great as called for. And electrolytic capacitors are polarized: if you use them don't connect them backwards! As of this writing the size of capacitors you will need are used in power supplies of around 200 watts, in video displays, electronic fluorescent lighting ballasts, and computers. If you can only find lower wattage supplies, the typically smaller values of capacitor can be paralleled and the values will add.
TUBES AND SOCKETS: The venerable 6V6 tube which I'm using has been manufactured continuously since 1939. Even if production stops today (2020), you'll still be able to find old tubes 50 years in the future. I've tried several 50 year old tubes in the one I built, and they work fine. If the tube is available, the socket should also be from the same source. In a pinch, an 8 pin round relay socket is the same as a tube socket, and they may still be available. Industrial electronics changes very slowly!
OUTPUT TRANSFORMER: Use a standard 70 or 100 volt audio distribution transformer. It needs a 1 Watt tap (or 2 Watt if 100 volt), and should be designed to handle 30 watts or more since we're applying quite a bit of DC bias to it. I'm assuming 70/100 volt audio distribution will still be used in the future. If not, it should still be salvageable from somewhere. They're attached to speakers mounted in ceilings, or inside small speaker enclosures on walls. Currently they're used in stores and offices to distribute music and alerts. Are there still physical stores and offices anymore? Dig around in some old ceilings, such as schools of government buildings. These are usually built to last 50 years. The transformers can be recognized because they have several connections (or maybe a switch) labeled with various wattage settings: for instance 1W 2W 4W 10W 20W 40W.
SPEAKER: Probably attached to the above transformer. If not, find one that matches the output impedance of the transformer. It should be a moving coil type PM (permanent magnet) speaker. Will these still be used in the future? Maybe they'll be replaced with an array of integrated MEMS speakers for most uses. Those are just coming to market as of this writing. Speakers can deteriorate with age, especially if the have Polyurethane components. They're still used in cars as of this writing, although they might not survive 50 years in that environment gracefully. The moving part should move smoothly when pushed gently, with no scratching sound. The cone should not have any tears. Check the continuity of the winding: for an 8 ohm speaker it should measure about 6 ohms. Connecting 1 volt to it briefly should produce a satisfying thump.
WIRE: You need wire that can withstand high voltage. If series connected LED Christmas lights are still around, the wire from a broken set works well. Otherwise, something around 24 gauge stranded and rated 300 volts will work.
FILAMENT SUPPLY:The tube is rated for 6.3 volts +/- 10% on the filament. I've designed my circuit to run at 6 volts, a more common power supply value. An added benefit is this lower voltage will extend the tube life with only a small decrease in volume.
PROJECT ENCLOSURE: I don't expect this to change much in the future. You can attach everything to a board. Or use a cake pan like so many of the projects from the 1960's. Cakes won't change in 50 years, right? Although it probably won't be an aluminum pan but some kind of high heat plastic. Or use what I call blob construction: pick the largest/heaviest part and glue everything else to it. There's high voltage and several watts of heat involved, so design accordingly. Get creative!
AUDIO SOURCE: I really can't predict this. In 1960 it would have been a phonograph record or radio tuner. Currently, in 2020, it would be a Bluetooth wireless receiver, receiving audio streamed from a smart phone. I'm assuming these won't exist in 50 years: in fact people may not even use their ears to listen to music anymore. You'll have to come up with something, probably something quite old. The signal needs to be around 2V peak-to-peak. Something that used to drive a small PM speaker will probably suffice. I'm using a Raspberry Pi computer to drive mine, but I don't expect these to exist 50 years in the future. Just remember that the input may be directly connected to the AC supply, so it's best if the source is battery powered, and you don't have to actually touch the source to get it playing. Some kind of rf/infrared remote control would be best.
Now on to the actual circuit. First here is a simplified diagram:
Here's how it would have been built in 1960:
Here's how I built it now (2020) considering how things might change in the next 50 years: