• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

What's the average high-voltage supply voltage in tube amps, in general?

Stupid questions come from complete ignorance of the subject. Replies here have been polite. I went through growing pains trying to learn about railroads/trains and there were a few there who let me know their intense impatience with questions from a person ignorant of the subject.

This is for a warning that I was writing for someone who might try to repair a tube amplifier. I was under the impression that 450v was fairly common. I have no plans on using/building/repairing anything with tubes. The question was simply to confirm that I was in the ballpark with the high voltage present in a tube amplifier.
The speaker ohms load has little to do with it, but the primary impedance of the output transformer does. For a 6.6k load and 50 watts it would be between 450 and 500 volts. Use the primary impedance divided by 4 as the effective load, and assume a 60 to 100 volt drop across plate-cathode when the tube(s) are fully “on”. That will give a rough idea of achievable watts vs. plate voltage - if calculating the way you would a transistor amp.
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Hi Perry,

The answer is, "it depends".

Class of operation / operating point (Class A tend to be lower B+ than say AB for a given tube type);
Tube type (max plate / screen voltages and dissipations are specified for each tube type);
Designer choices!

In the tube guitar amp world, typically for a 50-100W amp with EL34 / 6550 / KT88 / 6L6 etc you'd see around 420 to 550V. Naturally, there are exceptions to this "typically": some Musicmans which run in a fairly cold class AB can have 700V plates (screens are half that, mind...),

A 30-ish Watt EL84 / 6V6 amp will run lower voltages - 320-350V. Which is a bit hot for them if they are biased close to Class A.

Cheers, and regards,

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wg_ski is dead on. Push tubes and they will get you. The best case is shorter service life. Worst case comes with cheap, marginal tubes where you can actually lose the output transformer, and sometimes the power transformer.

Want a good design? Be boring, do what others before you have with good brands. Look at Heathkit, Eico and those designs. Yes, the CJ Premier 1 ran ~600 VDC on 6 KT-88's. Special high voltage fuses and everything becomes much trickier. Even resistors matter.

Perry, 300+ volts. 450 VDC will get you into the 55 watt range with 6L6GC (better 7581A). Tube car amps are not a good idea, and as my tag line says "just because you can, doesn't mean you should". Humidity and temperature change means conductive surfaces, and at higher voltages that means service problems. The high power needed for the amp just to sit running is a poor match for a car where you want efficiency. Vibration as the car moves will shake the tube elements - not good. Cathode coating may flake off. This would be classified as severe service.
I've seen several car tube amps and the tubes were in the preamp section. They did very little. The output stage was all SS.

I've watched a couple of videos and read for about 10 minutes and I think I'll go ahead and try to improve on (get more power from) an old design. The VTL (Vacuum Tube Logic) MB-1250 Wotan Monobock Tube Amplifiers. I'd guess that it wouldn't take more than a couple of hours to do that. I'll do a tri-amp for each channel and get some quadraphonic equipment out so I can have 4 discrete channels. I'll check the supply voltage for the tubes like we used to check 9v batteries.

On a more serious note, thanks for the replies. As always, nothing is as simple as it may seem.
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Hi Perry,
As you know, common sense goes a long way.

VTL used very high voltage and low current. You probably don't want to do that, seriously. I would run standard voltages, allowing stock audio transformers. Parts are less expensive that way, performance is higher and you aren't open to increased leakage currents. Heathkit or Eico would be your best models. I update those all the time for higher performance.

I get you're joking about that amp ...

I would run mids / highs together and go SS for bass / sub if anyone is serious about a car tube amp. Otherwise, this is pretty simple and straightforward once you get the basics.
1. US Navy Sailors:
More have died from 110VAC/120VAC than all other voltages combined, AC and DC.
Watch out for the amplifier's 120VAC 220VAC circuitry where it is . . . power cord and/or IEC socket, fuses, power switch, power transformer primary, etc.

2. Always check that [your/their] amplifier has B+ Bleeder resistors (that are working).

3. Always turn the amplifier off, and remove the power cord.
Now remove the bottom plate, and Wait!
Then use a DMM to check for how much voltage is
on the B+ capacitors.
Only after that are you free to inspect, poke around, move wires to see if there is a loose connection, etc.

A complete and accurate schematic is required.
Check all connections. Check the resistors, check for shorted capacitors, burned or discolored parts, etc.
Familiarize yourself with the parts locations, connections, inputs, outputs, AC power circuit loop, B+ parts, etc. (Before you start any powered amplifier testing).

4. Connect a load on the speaker output.
Securely prop up the open bottom upside-down amplifier.
Plug in the power cord, and turn the amplifier on.
Very carefully use a DMM to check the voltages of the B+, tube plates and self biased cathodes, fixed bias voltages, etc.
Check for leaky RC coupling Caps (by reading the grid voltages of self biased tubes, should be zero).

5. (really number 1) Put this advice at the TOP.
Safety First!
Prevent the "Surviving Spouse Syndrome".

Just my opinions
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There is no excuse, every vacuum tube amplifier that has high voltage B+ should have bleeder resistor(s).

Well, there is one exception:
12V car radios that used Space Charge Vacuum tubes (not exactly high voltage in my book; just do not get that screwdriver across the 12V input and ground, it will weld in place).
(Oh, for those guitar effects boxes that only use 12V, and no switcher, you are safe there too).
But, if the car radio used a vibrator for B+, there should be a bleeder resistor.

Safety First . . .

Addendum: One hand in the pocket when probing HV.
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