I use those tubes in guitar amps- the newest one I have was made in '59. I also have a friend who builds and repairs them and he's always looking for good tube sources. Many of the new production tubes aren't good and the old ones were far more consistent & reliable. I was talking with someone else who repairs & builds guitar amps and effects- he said the 12AX7 and the others used in preamps (AT, AU, AY) are so frequently microphonic that he started buying them in case lots for testing before use. He even built a jig for testing them, to cut down on wasted time and money when someone would ship a chassis for repair and then call back PO'd because it started ringing after using it for a short time. It worked before it left his shop, but it just took more time to be a problem and he had a failure rate of around 30%.

You think this is bad? You ought to see/hear what the real cork-sniffers talk about when it comes to tone caps, tone stacks and phase inverters, etc. The terminology used when discussing the sound is worse than the geeks who are absorbed in high-end audio gear. I do it because a lot of the old guitar amps were bery durable, good-sounding and if they're only a service call away from being fully operational, the cheap ones can be donated or sold for a low price to someone who needs one but can't really afford it, whether they're kids, teens or adults. Same for inexpensive guitars- one organization is Guitars For Vets, who gives instruments to US service vets who need help/therapy re-entering non-military life when they have serious problems due to their service injuries, mental and physical.

Some amps are collectable and need tubes, but some of the new production are so far out of the original range of specs they can't be used safely- I had a Sovtek 5AR4, in a very collectable amp, that was supposed to provide B+ in the mid-high 400VDC range and it was actually over 500VDC. That could have caused component failures which might have damaged the transformers, resulting in a drastic decrease in its value. This isn't a rare problem with that particular tube, but I hadn't heard anything about the others until I inquired after I measured the B+ in my amp. When good, new tubes can't be used as a sub for the old ones, the old ones are a good choice because the supply is still good. People are still finding case lots of new, old stock and some are military surplus.

Re: magic eye- green, or orange?