For a guitar amplifier, in terms of price and quality. I need a 6L6 tube.
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All About vintage pro audio

Your are about 30 years too late to worry about quality.
After the transistor came out. The tubes were put out to pasture.
None of the major manufacturers produce any tubes anymore. If there is one still producing tubes, the quality will be low.
RCA sold out to GE who then sold out to Thomson, I have no idea who owns it now as I have been out of the business for 8 years now.
I found this site who offers new old stock.
JJ (formerly Tesla) and Sovtek are the only “quality” tubes still produced. Marshall and Hi-Watt sound good with Sovteks. Fender and Gibson combo’s do well with JJ’s or Electro-Harmonix. If you have big bucks, get vintage tubes (tested of course) from a variety of reputable distributors. Here is a great source:
Nothing says “guitar” like a real tube amplifier.
Dang, what happened to RCA? Haven’t heard anyone talk about LET’s (light emitting transistors) in years.