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How to tell when a tube's going bad?


fini

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I've recently picked up several used American made 6550's (RCA, GE, etc.). My general understanding is that 6550s are pretty rugged, and every one I've tried has sounded great. I've easily biased them in my Pcats (in which biassing is a no-brainer). My question is, what signs should tell me the tubes have pooped out (besides the obvious glowing, sounding bad, and exploding)? Has anyone here "worn out" 6550s?

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if they test good use them. if you are worried about tube flashing scale back on the fuses. odds are you won't use the tube amp at max power anyway and scaling back on the fuses gives you a buffer to help prevent blowing resistors.

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I'm testing them in the circuit. So far they sound good. I'm not worried, just curious. No, I will be using far less than full power. I've got my headroom up the wazoo (I think that's how it's phrased).

So, I guess there's no way other than testing to know if a tube is going bad, and no way at all to tell what is it's remaining useful life.

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Low Gain, No output, High Intermodmulation distortion, High noise level, excessive hiss, microphonic, glowing excessive red, blowing fuses, poor frequency response, too much tube inductive reactance, too much tube capacitive reactance, excessive distortion, excessive hum, poor transient response, amplifier driven into oscillation, and I think there were more but I can't remember them.

JJK

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I hear ya OT,,, no resemblance intended. Until Space Ghost comes back, I make do with Robot Chicken, Squidbillies, Metalocalypse and Frisky Dingo. Shows like Assy McGee and 12oz Mouse aren't particularly entertaining but they are so in your face off the wall its hard not to watch - I feel like a nihilist watching a train wreck.

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Regarding tubes nearing the end of their useful life. I don't change tubes very often but if I start hearing noise or a 'whooshing' sound, I usually can track it down to a driver tube and if there's a hum or the highs seem to be cut off, its often a power tube. If the overall volume is lower than it should be, either tube could be the culprit. Some of the time, you can get more life out of your tubes by simply switching sockets - left channel to right channel. This only seems to work with twin-triodes like 12AX7s, 6SN7s and such but doesn't help with pentodes, tetrodes and triodes. Of course, even the simplest of tube testers can be very useful and may save an expensive piece of audio gear.

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I hear ya OT,,, no resemblance intended. Until Space Ghost comes back, I make do with Robot Chicken, Squidbillies, Metalocalypse and Frisky Dingo. Shows like Assy McGee and 12oz Mouse aren't particularly entertaining but they are so in your face off the wall its hard not to watch - I feel like a nihilist watching a train wreck.

One of my new favorite bands does the misic for Squidbillies "Unknown Hinson". If you can see them live do it.

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A glowing red tube is a definite indicator that something is wrong but often, its not the tube. Try another tube in its place, if the new tube doesn't glow red, the 1st tube is bad and you're done. If the replacement tube glows red as well, then check the bias. If its still red after biasing, chances are the socket isn't contacting all the tube pins. If there's any dirt or corrosion, apply contact cleaner or you may have to use a precision screwdriver (flathead) to ever so slightly push the socket tabs out. If the tube is slow glowing red, its time to take it to the shop.

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A glowing red tube is a definite indicator that something is wrong but often, its not the tube. Try another tube in its place, if the new tube doesn't glow red, the 1st tube is bad and you're done. If the replacement tube glows red as well, then check the bias. If its still red after biasing, chances are the socket isn't contacting all the tube pins. If there's any dirt or corrosion, apply contact cleaner or you may have to use a precision screwdriver (flathead) to ever so slightly push the socket tabs out. If the tube is slow glowing red, its time to take it to the shop.

Is there any chance that this glowing tube (fairly dim, not bright, glow) damaged the amp in some way? I'd hate to blow more than I possibly have by trying that tube in a different socket. I use Pcats, and was not able to get that tube to bias. I tried a known good tube in that socket, and was almost able to bias it with the bias knob cranked all the way. Perhaps the computerized bias circuit is damaged?

Link to Pcat specs

I'd best give Mark a call.

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