Tubes glow unevenly whent starting up?

fredjones

Active Member
My little Elekit TU-879S amp uses EH 6L6GC tubes. Used them for little over one year. I moved my amp beneath a shelf and now have noticed that when starting up, one gets bright on bottom, the other gets bright on the top. After they warm up the glow evens out. Switch sockets and they maintain this behavior. Is this normal? How long can I expect these tubes to last? I have used them nearly daily for several hours a day for about the last year and half.

Thanks,

P.S. Tried search but really could not find anything.
 
All of the glow from a tube should be coming from the filaments (heaters). Their only job is to get hot enough to heat up the cathodes adjacent to them. In doing that job, they may take a while to get up to their nominal temperature, and may glow unevenly as the heater heats up.

Just make sure you don't have red plating, which will characterize itself as a reddish glow directly on the plate, which is the largest piece inside the tube. Otherwise it's OK for the heater to glow, and for the glow to poke through any cracks or holes in its surrounding structure, based on how the tube is built.

BTW, does the amp sound good? Have you taken voltage readings on these tubes? If the answer to both the above questions is "yes", I see no reason to panic, especially if there's no red plating.
 
The amp still sounds good. I do not know how to take voltage readings on the tubes. I do have a voltmeter so I guess that will be my next undertaking.
 
Ok. If you have used a voltmeter before, take readings on each tube pin. Clamp negative to ground, and positive to each individual tube pin, measuring DC volts. As long as your tube sockets are solid and not loose, you can turn the amp sideways and take measurements while powering it on.

B+ is very hot though, could be between 250 and 400 volts depending on the amp, so use caution with the probe, and don't short B+ to other tube pins. Even if the amp is turned off, there's a chance that the power supply filters retain high voltage, so keep your bare fingers away, and keep your non-dominant hand behind your back or in you pocket.

If you post DC voltage readings on each pin, there are others on this forum who may be more familiar with this particular amplifier than can give guidance as to whether or not your readings are expected.
 
My little Elekit TU-879S amp uses EH 6L6GC tubes. Used them for little over one year. I moved my amp beneath a shelf and now have noticed that when starting up, one gets bright on bottom, the other gets bright on the top. After they warm up the glow evens out. Switch sockets and they maintain this behavior. Is this normal? How long can I expect these tubes to last? I have used them nearly daily for several hours a day for about the last year and half.

Thanks,

P.S. Tried search but really could not find anything.

If it only occurs at cold start up, you have nothing to worry about. Remember, tubes are hand made, and sometimes the builder will push the heater (filament) assembly further into the tubes than other times. Mullard small 9 pin signal tubes will actually flash a white light at cold start up due the filament wires hanging down out of the internal structure. Enjoy the music. :music:
 
Wanted to update this thread. The tubes work despite the uneven glow on startup. I recently purchased some new EH 6L6GC tubes just like these. The new tubes don't have this behavior on startup. They also sound very different, even after 20-30 hours break in. I think the old tubes are just near their end-of-life. Don't think I'll tempt fate and use them til they blow. They are now officially retired.
 
Wanted to update this thread. The tubes work despite the uneven glow on startup. I recently purchased some new EH 6L6GC tubes just like these. The new tubes don't have this behavior on startup. They also sound very different, even after 20-30 hours break in. I think the old tubes are just near their end-of-life. Don't think I'll tempt fate and use them til they blow. They are now officially retired.

I'm working on that myself.:yes:
 
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