I was a tube amp tech for several years, have built multiple guitar amps from scratch, and still dabble in it.

What may not be obvious is that modern tube amp designs are an evolutionary branch from 1930's technology, with only a little coming across from the transistor->digital tech tree. The amps of the 40s and 50s were pretty closely based on reference designs that came from RCA and other tube manufacturers.

Modern passive components (resistors, diodes and caps) are made to a far higher tolerance and are better understood, but tubes and transformers are a mixed bag. The older designs were somewhat overbuilt and can be more reliable or have tonal characteristics that are not available in modern parts.

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mickael-kerjean 6 hours ago | parent [-]

I always found fascinating the power section of valve amp for guitar will always be made of a very basic rectifier circuit to convert AC to DC that requires a expensive transformer and produce power with a terrible efficiency compared to more modern SMPS. Why is it nobody interested in valve amp never go the SMPS path? Is it all because sag is a desirable sound distortion?

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theamk 3 hours ago | parent | next [-]

You mean a modern construction with semiconductor-based SMPS, but tube for output? If so, plenty of those, here is hit #2 in my google search for "guitar tube amplifier": "Orange Micro Terror" [0]. This one take 15V DC input - no way you can get this to tubes without some sort of SMPS.

Or do you mean why people who do period-authentic tube amps don't use SMPS? That's because tube-based SMPS is very complex, often as complex as amplifier itself, and needs unusual parts [1].

[0] https://www.sweetwater.com/store/detail/MicroTerror--orange-...

[1] https://www.righto.com/2018/09/glowing-mercury-thyratrons-in...

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dreamcompiler 2 hours ago | parent | prev [-]

I noticed the same thing. Like, why not at least use a fullwave rectifier with semiconductor diodes? Surely nobody believes that a tube diode in the power supply makes any audible difference.

	▲	cartoonworld an hour ago \| parent \| next [-]
		Sometimes it does, for example, just like gp mentions, a tube rectifier in a single ended amp can have a voltage “sag” that interacts with the rest of the system and causes an interactive “color” in the output, especially when amplifying larger voltage swings of bass notes and chords. There are quite a few effects like this. In a modern design this would be eliminated, but sometimes “bad” is good :)
	▲	shermantanktop an hour ago \| parent \| prev [-]
		What, like a 5Y3? Listen to Neil Young’s tone on Cinnamon Girl. That’s the sag and compression of a cranked tube rectifier.