Uneven Plate Glow

[payloadde]

One of my little Amps uses two sweep tubes.
In total darkness I see a faint red glow on one of the tubes, not on the other.

Aiming a CCD video camera w/nightshot at them - which is sensitive in the near infrared - reveals why only one tube seems to be affected:
The heat on this tube concentrates on one side of the plate structure (?!).

Whereas on the other tube the heat is dissipated evenly from both sides.

Both tubes are same brand and equally biased at 125mA / 180V = 22.5W.
I know this is heavy for a PL504 w/ spec 17W center max / 24W abs max (plate+screen).

Manufacturing flaw, Cathode off center, or just overdriven ?

[artosalo]

If both tubes dissipate same amount of power, then the glowing tube is faulty.
But I can not say how.

[tubelab.com]

It is some type of manufacturing imperfection. I see it a lot in tubes that I push a bit to hard. Probably grid rods that arent perfectly straight, off center screen grid maybe, or even a warped cathode or plate.

You can ignore it or turn the bias down a bit. I have seen sweep tubes go for years this way, then I have also seen them die in a few months.

[DavesNotHere]

hotspots on plates is indication of excessive grid emission and/or plate conduction bias voltage applied to the grid is different, but cathode currents are equal.

I heard some people call this grid leakage, some say its "gassy in conduction" (but that is usually associated for the cathode or the whole tube itself).

either way, the one that has the hotspot is worn and it has to conduct more than the other to keep up.

this does not apply ti 6as7 as 70% of their operating range the plates glow red.

It might last you 1000 hrs maybe more, you'll definately hear it when it totally goes out.

[20to20]

I know this is heavy for a PL504 w/ spec 17W center max / 24W abs max (plate+screen).

Manufacturing flaw, Cathode off center, or just overdriven ?

The manufacturers don't expect perfect uniformity from tube to tube and so they set tolerances and limits. What's the mystery?

If you don't drive it over the design max and it red plates, then it has a problem. Perfect tubes could run over max limits and not show the strain, but that is just the luck of the draw.

No mystery.

20

[payloadde]

Reduced the current to 110mA.
Still uneven on the one tube, but overall healthier I guess.
Sound's still good. So I'll leave it that way.
I just didn't expect thermal behavior to be so extremely one-sided.
After all, tube parameters like transconductance seem to be almost identical.
From how both tubes behave electrically I wouldn't have suspected any unsymmetry.

[v4lve lover]

if the tube is fused against bringing down your HT tranny itl be fine . haha nice to see you using them PL504'S i should be to .. as im sitting on a stack of about 50 don't need to worry about availability

[20to20]

Once you've eliminate the possibility of a circuit defect, I'd toss it. It's not worth the trouble to have it run away on you while your back is turned.

20

[v4lve lover]

fuse as a current limiter