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Hey, can anyone tell me why my brand new output tubes have brown burn like marks developing on the inside of the glass?

I just bought them about a month or two ago and now they look sort of ugly.

Could I make an adjustment to the Bais that would prevent this from happening? It's set a 500mv where the manufacturer claims the amp performs the best.

Any thoughts?

Chris

this probably normal but I sure some else will chime in
What tube type are they
Also When I get new output tubes I start off with a low bias setting 250ma for a couple of hours then 300ma, 350 and so on
It interesting that I found out that the manufacturers suggestion does not necessarily sound best on my system
Om my kt88 450ma to 500ma is often suggested and some say push to 600ma for better sound. of course the higher the bias setting the shorter the tube life. A good and very knowledgeable friend suggested a much lower bias and now my midrange is just right
Every system is different and one has to try different settings and listen carefully

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Walt

Note carefully, he said 500 mv, or 0.5 v across I assume a current measuring resistor of 10 ohms to give 50 ma of current. It make more sense to talk about it in current, as that is the TUBE current draw, NOT the voltage drop across the resistor which we really don't know the value of. At any rate, if it is blackening up so quickly, something doesn't seem right, check with your tube seller and/or figure out what is going in the circuit because it sounds like you may be over-drawing the current and thus the plate dissipation of the tubes...

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Walt

So just to refresh my memory, or confirm what I have heard: Lowering the Bais extends the tube life, and puts more of the signal into a class A weighting, but decreases some of the power or gain? It also makes the tubes run a little bit cooler?

my mistake you are right The indicator on my amps show 500 which is 500mv or 50 ma. I always forget this

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Walt

I believe you have that backwards, the lower the cathode-to-grid voltage difference, the more positive (less negative) the grid is and therefore will cause the tube to conduct harder, increasing the current thru the tube and thus the plate dissipation. Some will say this gives the tubes a "hotter" sound, but as always, it depends on the circuitry, parts, etc. Help me out of I am incorrect on this...

Have you been listening to James Brown? Or Jackson Brown? Or Brownsville Station. Chuck Brown? Maxine Brown? Charles Brown?

I have only seen that brown residue develop in Russian tubes, never in any US or Euro tube.

I think it must be from impurities inside the envelope. I'll ask my bud jc who was chief engineer at Sovtek what he thinks.