Grateful Dead Music Forum

tigerstrat wrote:To me a microphonic tube sounds like a jagged tin can softly scratching a chalkboard with every note, or perhaps just the hardest-picked notes.

TS - check your PM

I disagree, there is absolutly nothing wrong with tapping a tube lightly when it is running. 2ndly, the plates of a tube turning red is more an indication of a problem in the biasing circuit than in the tube. DO NOT replace a tube that is glowing red until you have checked out the bias circuit because there is a good chance the new ones will glow also. If the bias is fixed the tube may not need to be replaced, it depends on how much damage has been done. Setting the bias incorrectly will also cause the tube to conduct too much current and it will turn red. Tubes that don't need to have their bias adjusted manually still have bias circuits which can go bad.

So the tin can scratching a chalkboard is not my playing? Whew!

Hee Hee.

betteroffdeadnc wrote:I disagree, there is absolutly nothing wrong with tapping a tube lightly when it is running. 2ndly, the plates of a tube turning red is more an indication of a problem in the biasing circuit than in the tube. DO NOT replace a tube that is glowing red until you have checked out the bias circuit because there is a good chance the new ones will glow also. If the bias is fixed the tube may not need to be replaced, it depends on how much damage has been done. Setting the bias incorrectly will also cause the tube to conduct too much current and it will turn red. Tubes that don't need to have their bias adjusted manually still have bias circuits which can go bad.

tapping a tube does not prove it's microponic. Every tube makes noise when you tap it. Even good ones. Just as every tube is already microponic. (some are just simply much more microponic hen others) Damaging or not - there is no reason to tap a tube.

Bais circuits can indeed go bad; but now we are talking about amp failure and not tube failure.

A red plated tube may indeed be a symptom of a hot bias; but there is a big difference between a red cathode and a melting tube. you could perhaps save a tube that is red plating by adjusting the bias if the problem is indeed a symptom of mis adjusted bais(if you caught it quick enough) More likely it is a symptom of tube failure.

An easy way to be prepared for tube failure is to go to a store like tubedepot.com or tubestore.com and ask for a few matched sets; they will send you tube of similar bias, so a bias adjustment isn't neccesary after a tube failure. Your backup set just needs to match the set you are using. You could foreseeably retube mid show with a setup like this. Note - i would still recommend re-biasing later as no set of tubes are truly equal, but it should be close enough that you can play the show with good sound still.

To the poster that asked what red plate/ microponic sounds like - someone already answered microponic; but a red plating tube will typically have much more distortion then usual, also your amp will be running extremly hot. Remember - an orange glow is completly normal, as well as blue glows - but red is bad.