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Old 14th December 2009, 11:54 PM   #11
john65b is offline john65b  United States
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Just go around and make sure the bias is good on all tubes before calling it good. You will find out once you set one tube, all the others will drift off. Its an iterative process that should be complete when all the tubes are biased.

You may notice that the bias drifts too much and you cannot keep the bias stable for any period of time. At this point I would wonder about a bad tube or circuit issue. But checking the tubes would be a good place to start.
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Old 15th December 2009, 01:02 AM   #12
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Don't let a good opportunity go to waste!

Turn off the lights, and get a good picture!
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Old 15th December 2009, 03:04 PM   #13
jrenkin is offline jrenkin  United States
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Quote:
Originally Posted by Kashmire View Post
Don't let a good opportunity go to waste!

Turn off the lights, and get a good picture!

I know! I have been following the glowing tube picture thread and thought of this too late! I t hasn't gone haywire a second time, so no picture. I was too freaked the first time and turned it off right away...

It was pretty spectacular. I am not sure I have the cajones to force it to do it again. Maybe I will try with a lesser quality/older set of tubes. Boom....
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Old 19th December 2009, 06:19 PM   #14
dhaen is offline dhaen  Europe
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I'm a bit late on this but here is my take:

The vacuum of many valves/tubes on sale today is not as "hard" as some of the originals. Sometimes it's from new, sometimes it's due to gas emission from the electrodes after some use. Whatever the reason, when run hard, the device can run-away. This is especially true in fixed bias arrangements. Cathode bias or automatic bias arrangements suffer less. You will notice that on spec sheets there are 2 different values given for grid-leak resistors (Rg1). The fixed bias one is lower, indicating that the manufacturer takes the run-away issue more seriously in this mode. Often the fixed bias value is less than half the auto-bias value.
So check the value of the grid-leak resistors (with an ohm meter) and compare them with the max value on the manufacturers data sheet (the actual manufacturer of your valves). Choose a value that is at least one value lower than the max. Caution: If too large a downward change is made this can affect the distortion of the driver stage as well as the frequency response. This can be a problem on older amps designed around original valve types.

This is just one element of ensuring that you are running within the design values for the device. If you can ensure that all are within ratings (remembering that the mains might by a percentage fluctuate too) then assuming no component failures you will get the best out of the valves/tubes and can expect a long and trouble free life.

Other reasons I have found for this behaviour are leaky coupling caps and poor contact between valve g1 pin to socket.

I repair quite a number of valve amps for people and always check the above points before fitting new output valves.

Hope this helps.
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Old 20th December 2009, 02:06 PM   #15
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Might be a good idea to change the suspect tube over to a position in the other channel. That way should you again see a problem, it would help isolate whether it follows the tube or stays fixed to a particular socket and associated supporting parts.
Do reset bias if you move it. It would be interesting if bias changes much between the positions.
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Old 20th December 2009, 05:06 PM   #16
jrenkin is offline jrenkin  United States
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Thanks! tons of great advice. Of course, just like a car, it hasn't done it again since i started this thread. Go figure. I must have just screwed something up.
Just to check, KT88 with quiescent plate voltage 445V, 10R cathode resistor, trying to bias class A, 42W dissipation. I should be measuring somewhere around 95 - 100mA?
So i measure 0.95 - 1.0 A on my meter (10R). Right now I am at 90mA, because it seems I shouldn't push that hard.
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