Blue glow on power tube as it warms up?

[bdr529]

Hello, since installing my speedball (maybe before) I noticed that my power tube will give off a blue haze around the cathode once the filament begins to glow and before the cathode heats up during start up. The blue light kind of expands and gets sucked in as it warms up. It looks neat but I haven't really seen anyone mention this so I wanted to check on the forum.

Is this a sign of anything bad? The amp sounds normal once it warms up.

Also on one 5998 tubes I have if I look closely between the plates at the cathode one side appears a little splotchy and not a completely even tone of orange.

Thanks

[Zimmer64]

AFAIK a bit of a blue cloud is no problem. That is just a bit of gas that remained in the tube and should not be a problem. Regarding the other spots, did you check all your voltages again? Spots on the plates might be the result of bias being too high. I hope others will chime in.

Michael

[bdr529]

I just checked the voltages around the sockets and everything appears to be within 10-15%, although they are on the high side of the range.

Is there a specific voltage I should check during power on to see if things are turning on normally? I see the blue in more than one power tube, so I am not sure it is gas or too much current before the tube warms up... Can this damage the tube?

If i plug in my headphones I can usually hear buzzing until the blue flash goes away, the buzzing is independent of volume

[bdr529]

I am seeing around 220V on B+ during startup until it falls back down to 175ish. Is this normal?

[galyons]

I am seeing around 220V on B+ during startup until it falls back down to 175ish. Is this normal?

Yes, there is an initial power up surge until the tubes start drawing current.

Cheers,
Geary

[bdr529]

Isn't it not great for the tube to apply high voltage before the cathode heats up?

[galyons]

Yep!  Tradeoffs! The cost to do it "right" is higher than the cost of the tubes.  Most tube amps and the power tubes survive the surge well.  Doesn't make sense to spend time & $$$$ on a soft start circuit for tubes as inexpensive as the 6080.  It is a tough tube. 

Enjoy listening and stop worrying!! 

Cheers,
Geary

[bdr529]

Yeah I suppose, but while the 6080 is cheap, the 5998 isn't.  Maybe I will just add a switch or something...

[Doc B.]

With indirectly heated tubes the startup is not an issue because the heater warms up slowly. Don't worry, be happy.

[bdr529]

Isn't that the problem though, the heater warms up slowly but the plate voltage is already high. That is why there are timing circuits to delay the plate voltage until the tube is warm.

I wouldn't really be worried if I wasn't seeing a blue haze around the cathode or a buzzing in my headphones as it warmed up.

Edit: Like i said in the rolling thread i have some new findings. The buzzing is only present if the volume is at 0. As soon as I move it away from zero the amp goes silent. Some kind of grounding issue in the pot?

[Paul Birkeland]

Isn't that the problem though, the heater warms up slowly but the plate voltage is already high. That is why there are timing circuits to delay the plate voltage until the tube is warm.

These tubes were designed to operate in circuits before this type of circuitry really existed.  Most of the problems associated with warmup in indirectly heated tubes would have to do with the tube never actually getting all the way hot (low heater voltage), which can result in cathode poisoning.

[Doc B.]

In theory there may be cathode stripping, but we have found that in practice there aren't really any concerns unless the direct coupled circuit contains a directly heated tube in the second stage and an indirectly heated tube in the first. However I have only been doing this for 23 years and I can only base my judgements upon that limited experience.

I suggest that you look very closely at that blue glow. It is probably in the glass, not in the vacuum around the tube. If it is actually in the space between the plate and the glass you have gas in the tube and it is shot. Typically the glow of a gassy tube is pinkish, not blue.

[bdr529]

Thanks for the information. I didn't mean to doubt the wisdom of the Doc, but I was going off some general tube best practices I found on the net while investigating this issue. 

The glow is faint, but it certainly is there. I can only see it in a dim room and it forms in the space between the cathode and the plate as it it warms up and fades just before the cathode begins to glow orange. It happens on both power tubes that I own.


Also, any idea why I might get a buzzing @ 0 on the volume pot during this warm-up period. If i move the pot the slightest amount past zero the buzzing goes away. The buzzing is completely gone after warm-up at any volume though.

Thanks again, I don't want to be a bother tonight, go enjoy the holiday 

[Grainger49]

One of my stock Sovtek 2A3s that came with my Paramours had a blue glow right at the top.  It didn't harm anything and they lasted for years.

[galyons]

The blue glow is relatively common in power tubes. Just part of the experience....enjoy!

Cheers,
Geary