Reminds me of a story I read some years ago. There’s this guy talking to his friend and the friend is complaining that his tube radio isn’t working right. The guy tells his friend that his nephew is really good at working with these things, and the friend tells the guy to bring his nephew over to take a look. So the guy shows up with his ten year old nephew, and the friend is a bit surprised but says “ok, let the kid see what he can do.” So they leave the room and the kid turns on the radio. It is silent for a bit, then starts squealing and making odd noises for a while, and then works fine. So the kid turns it off, lets it cool, and then switches the two tubes. Problem solved. The kid figured out that the second tube was putting out the crap that the first tube was generating as it warmed up more slowly. Once the tubes were swapped by the time the second tube warmed up the first tube was stable. The 10 year old was Richard Feynman. He went on to win the Nobel prize in physics for the path integral formulation of quantum mechanics. Smart kid.In a pinch (replacement tube not immediately available) you can swap the left 6SL7 with the right 6SL7 until you can get a replacement. I know it sounds silly, but it isn't. It has done the trick more than once. I could bore us all with a technical diatribe as to why this works, but life is too short.
In those 9 years, and many hundreds of Moniques, I think I have heard of only a couple of 12AU7 tubes quitting. They just keep working. Do not use a 12AX7 as a replacement. It will work, but as Ray Wylie Hubbard says in Snakefarm, "It just sounds nasty."
The rectifier tube (EZ81) is also very robust. I have never heard of one failing while in service, and if it does, we get no sound at all and it stops glowing. For the tube-rollers out there, yes, an EZ80 will work quite nicely as a replacement - but no EZ80s are being manufactured at this time.
The primary tube failure, just about the only failure you will ever experience with your Monique, will be the 6SL7GT on the far right. The Shuguang 6SL7/6N9P is my preferred tube for sonic reasons and for its metal base which keeps noise out. It also isn't crazy expensive. The Shuguang 6SL7/6N9P is a higher gain 6SL7. It is a "GT" version.
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