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Apart from very specialist audio amplifiers?

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closed as too broad by PeterJ, Daniel Grillo, uint128_t, Andy aka, Tom Carpenter Mar 13 at 6:12

There are either too many possible answers, or good answers would be too long for this format. Please add details to narrow the answer set or to isolate an issue that can be answered in a few paragraphs.If this question can be reworded to fit the rules in the help center, please edit the question.

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High power RF amplifiers. – Chu Mar 11 at 13:42
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... including microwave ovens. – Dave Tweed Mar 11 at 13:43
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@MaximGi: They all use magnetrons. – Dave Tweed Mar 11 at 14:24
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@MaximGi: Magnetrons are vacuum tubes. – Dave Tweed Mar 11 at 14:32
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Most guitar amplifiers made by Marshall, Fender, etc still use tubes. For Rock, Heavy Metal, Blues, you need a tube amplifier to get the right sound. Jimmy Hendrix just would not sound the same through a transistor amp. In the UK they are called "valves". – Steve G Mar 11 at 14:33
up vote 16 down vote accepted

Maybe still some EMP-resistant radio front ends for military purposes. Magnetrons, TWTs and Klystrons for RF, including microwave ovens and industrial microwave sources.

Also ignitrons and hydrogen thyratrons, and, of course photomultipliers are widely used.

As Dave Tweed says below, solar-blind flame detectors (such as UVtron) are a current application.

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+1 for photomultipliers. Also UV flame detectors. – Dave Tweed Mar 11 at 14:37
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russian bombers (the bear definitely) still uses valves for EMP immunity – JonRB Mar 11 at 15:31
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The MiG-25 had them in order to resist EMPs and to withstand variable temperatures. – harley_woop Mar 12 at 19:14
    
@harley_woop And the Americans were shocked when they learned that at the time; they hadn't realised there was a reason – cat Mar 13 at 2:59
    
@tac Yup, interesting how different the approaches were. – harley_woop Mar 13 at 20:10

X-ray equipment and radar sites as they, tubes, can handle the high power demands. As well as, of course, vintage ham equipment, radios, and TVs.

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Not just vintage. As mentioned in the comments to the question, plenty of very modern high-power RF equipment use vacuum tubes. In amateur radio, take for example (just to mention one specific example) the Acom 1000, which uses a 4CX800/GU74B tube for power amplification. – Michael Kjörling Mar 11 at 23:01
    
and some high end stereos as well. Thanks . – Tim Spriggs Mar 11 at 23:04
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More than magnetrons/klystrons, X-ray tubes really are a category to themselves. You couldn't possibly replace them with solid-state elements, since they need electron energies an order of magnitude above the valence band. – leftaroundabout Mar 12 at 11:41

The are also used in guitar amplifiers. Most audio amplifiers work under the assumption that they do not color the original source audio in any way. The amplification should be transparent. Guitar amplifiers however, are built specifically to color the sound and tone in their own unique ways. Different types of tubes can achieve different sounds.

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but aren't those same guitar amps vintage? i.e. 1970's – Tim Spriggs Mar 11 at 23:06
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@TimSpriggs, nope. These are new hybrid designs that mix solid-state and tubes. For example, many Fender amps have two completely separate power supplies that use solid-state for switching and control, and the pre- and power-amplification is done 100% through new (not new old-stock) tubes. – jdv Mar 12 at 2:19
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@TimSpriggs: No no, tube amps are still being built. Not only do some manufacturers still build some of their original '70s designs, they also build "re-issues" (i.e. amps using modern manufacturing methods based on the original designs), and even completely new designs. There are even completely new companies formed by people who weren't even alive in the '70s for the sole purpose of designing completely new tube amps. And it's not just guitar amps, also microphone preamps. – Jörg W Mittag Mar 12 at 2:20
    
There are even devices made that offer "valve sound" to the audio recording hobby by passing the signal through a tube, lacking a high voltage supply, whose main function seems to be to glow prettily behind an observation port! Here's a device that seems to use tubes properly - note the "no starved-anode frippery here!" comment. soundonsound.com/sos/dec12/articles/preamp-results.htm – Laurence Payne Mar 12 at 13:02
    
"Starved anode" preamp designs are sold to the audio recording hobby. The main purpose of the tube seems to be to glow prettily behind an observation window. – Laurence Payne Mar 12 at 13:12

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