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Questions about tubes and gear that glows. FAQ

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Original Message

Re: Gone in a Flash!

Posted by Chris O on March 24, 2002 at 19:04:36:

Can you measure the tubes operating parameters? Check the voltage drop over the cathode bias resistor. If the drop is, say, 2 volts and you have a 1K cathode resistor, the tube is pulling 2mA (2v/1kohms)=2mA. Check the plate voltage. If upon checking you find that the tube is operating within its spec, well then, the tubes must be gassy. If the tube was just ran too hard the plates might glow and they could sound terrible (or great depending on your application).

Someone testing them, or using them before you, might have damaged them with too much voltage or current for a short period of time. I know from experience that this is a sure way to wreck a tube and still have it look OK and work for a limited period of time.