Power Tube Question

Nightfall

Is it normal for a 5U4G power tube to glow purple, flicker purple, etc. when it's first turned on? I don't recall ever seeing this happen. Even after they settle down there's still two small sections of "wire" (filament?) inside the tube that stay purple until the tube has been powered up for several minutes which the purple then goes away completely. I feel like this is an indication that the tube is on it's way out or is somehow leaking but thought I'd ask the forum since after it totally warms up it looks like normal. Should I pull and trash it?

Sorry for the horrible picture. I don't have a tripod and close up shots the shutter stays open so long that I inevitably move the camera.

Nightfall

Here's a video

https://www.youtube.com/watch?v=YuCrK3is85M

mhardy6647

Not normal in a 5U4 IME & to the best of my knowledge. My guess is that it's "gassy" = bad. Now, it is possible that it's a symptom of another problem (specifically, excessive current draw by the components "downstream" of the HV rectifier), but it may well also be (as an MD would put it) idiopathic. I.e., it's gone bad because... it's gone bad

Is that, by any chance a "modern" 5U4 (more to the point, a Chinese or even Russian tube)?

I'd replace it, were it mine. Do you have access to a tube tester?

https://hackaday.com/2015/01/17/vacuum-tube-repair-after-a-spectacular-failure/
http://k5jxh.com/odds-ends/gas.html

marvda1

Nightfall wrote: »

Is it normal for a 5U4G power tube to glow purple, flicker purple, etc. when it's first turned on? I don't recall ever seeing this happen. Even after they settle down there's still two small sections of "wire" (filament?) inside the tube that stay purple until the tube has been powered up for several minutes which the purple then goes away completely. I feel like this is an indication that the tube is on it's way out or is somehow leaking but thought I'd ask the forum since after it totally warms up it looks like normal. Should I pull and trash it?

Sorry for the horrible picture. I don't have a tripod and close up shots the shutter stays open so long that I inevitably move the camera.

this is a rectifier tube not a power tube.

Nightfall

It's an old RCA.

Thanks for the correction Marv.

I'm pretty sure the only other 5U4G I have is the Chinese one that came with the preamp. I'll see if I can find it.

codycatalist

I mean I don't know much about tubes but did do some research on Google. I found a thread where a gentleman described his tube looked like a Tesla coil for a second then stopped.

To me it looks like a Tesla coil but you are the better judge of that...he replaced the tube and all was well after that. Granted this was a different tube type (EL84) but here you go if you want to read and see if it matches up to what you see.

mhardy6647

True although, of course, a 5U4/5U4G/5U4GB HV rectifier is a high power tube in most audio applications (all the HV juice, at least, flows through it!) and dissipates a lot of heat, akin to a power output tube.

Nightfall

I just meant power as in involved with power delivery to the preamp, not as in amplification power. The 6V6 in there is also in that same category. It made sense in my head, if incorrect.

mhardy6647

As the RCA manual scan I posted indicates, an HV rectifier like that one is indeed considered a "power-handling tube", so you're good. Folks with hifi interests probably tend to think of power output tubes when they see "power tube", which is OK, too.

Now, if you start calling, say, EL34s tetrodes, well, then... we'll have to have a little discussion, methinks.



PS the RCA scan is from RCA Receiving Tube Manual RC-23 (from 1964).
If you don't have any (all!) of the tube literature at Pete Millett's wonderful site... you should.

http://www.tubebooks.org/tube_data.htm

Lots of other wonderful things on Pete's site, plus, like Nelson Pass, he has a comprehensive and excellent site of DIY projects, too @ www.pmillett.com

mhardy6647

oops, derp... misfire.

sowwy.