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I was wondering if it's possible for me to use the tubes in an old CRT TV to build an audio amp (specifically a guitar amp). If not, would I be able to salvage something else (like an old radio) to get reasonably decent results?

Daniel Oren-Ibarra
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  • Yes, but you’re probably 40 years too late asking this question. Back in the day, tubes such as the 6CM5 were used for audio amps. Other tubes could be used for the preamp like ECC83. – Kartman Apr 18 '22 at 16:11
  • @Kartman There are still very many tube (valve) guitar amps. They sound different when pushed into distortion so there's still a big market for them. Sovtek in Russia and some manufacturers in China still make tubes for audio applications, though they are now pretty difficult to get. – John D Apr 18 '22 at 16:51
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    The old television set (tube type) has an audio amplifier built in. So, yes, some of the tubes could be used to build an amplifier. Using an old radio would be the same answer, the radio already has built in audio amplifier. – Marla Apr 18 '22 at 17:31
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    Are you asking about the CRT (picture tube) itself, or the other tubes in a very old television? (If it's the other tubes, visit [the late Fred Nachbaur](http://www.dogstar.dantimax.dk/tubestuf/index.htm). – Theodore Apr 18 '22 at 19:32
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    @Theodore I would like to see if I could use the picture tube, (if it's possible) otherwise, I should be able to use the other tubes. – Daniel Oren-Ibarra Apr 18 '22 at 20:56
  • Somewhat related, I heard that it IS possible to use VFDs for amplification - someting I also wanted to try. If you want a tube amp, build that. If you want a _funky_ tube amp w/ a CRT, I suggest looking for a small CRT with reasonable acceleration voltage (<10kV, I have one with 500V) to avoid unnecessary risks and problems. – Uncle Dino Apr 18 '22 at 21:41
  • There is an audio store in my city that sells an expensive amplifier with tubes glowing on top of the transistors amplifier circuit. – Audioguru Apr 19 '22 at 01:48
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    @Sasszem I've seen 12.5kV, 3250 HP (2.5 MW) VFDs with (cascaded) H-bridges at their output, so they're not so different from a Class D amplifier even at that capacity range. – Theodore Apr 19 '22 at 15:56

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Technically, a CRT could be used to perform amplification as it possesses the three things you need to do so: A cathode (in the form of an electron gun), a control grid (grids, actually) and an anode.

There are a bunch of other parts to a CRT that you would ignore or disable.

The thing you won't be able to ignore is the operating voltage. A typical tube intended for audio amplification might operate somewhere in the neighborhood of 200 volts (for a preamp tube like the ubiquitous 12AX7/ECC83) to 400 volts (for a beam power tetrode like 6L6). A typical CRT might operate in the range of 15,000 volts for a little monochrome unit to 30,000 volts for a larger color one. This brings several complications (and safety risks) that are probably not worth the trouble. Most notably, to use the tube for voltage gain, where will you get a load (e.g. output transformer) designed to operate at this voltage?

If you did operate a CRT as an amplifier, another side effect would be that the electrons not striking the anodes would continue on to strike the screen. If you've disabled the deflection coils (which direct the electron beam to different areas of the screen) you will illuminate a bright spot at the center and eventually burn it out.

Some of the tubes used in the audio circuits of radios will be similar to the common ones in commercial guitar amplifiers. Maybe with different heater voltages used in series connections to avoid the need for a heater transformer, e.g. in the so-called AA5 types.

The late Fred Nachbaur ran a website (still maintained in posthumous operation) that included some audio projects using surplus tubes. He had a "thing" for using tubes with odd heater voltages.

Theodore
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    It is not the [CRT](https://en.wikipedia.org/wiki/Cathode-ray_tube) itself, but the associated [vacuum tubes](https://en.wikipedia.org/wiki/Vacuum_tube): *"use the tubes in an old CRT TV"* – Peter Mortensen Apr 19 '22 at 04:13
  • @PeterMortensen See the OP's reply to my comment on the question: "I would like to see if I could use the picture tube, (if it's possible)..." – Theodore Apr 19 '22 at 13:38