The AM Forum

Happy Holidays!

I have a homebrew exciter running a
beam power output tube and playing
well over the month. Recently, the output
power dropped suddenly 4-6 dB. Re tune
the output tank, the output power will
snap back to the proper level. And then
moments latter will snap back down. Sometimes
the output power will remain proper for
quite some time, even display proper tuning
as I can peak power and load to the 50 ohm
power meter... and then ZIP!

There is blue flash color between inner
elements of the tube, grid 1,2, and plate.

No blue flash to the glass envelope. Only
between the inner electrodes.

Time to replace the tube?

Are these classic gassy symptoms of a tube going gone?

Thanks!

Sounds like both gas and the possibility of tube geometries changing.

Yes.  New tube time.

--Shane
WP2ASS / ex KD6VXI

Thanks Shane for the feedback.

Will do.

At this stage hard to chase down
any other reason for such behavior.

Through the little window in an EL34 anode of a push pull audio amp, there was a glow maybe like what you are describing.

Through a magnifying glass it appeared to be between or on the grids but not really at the inside of the anode. The screen grid was red hot. Those tubes were well-worn and running in an old ultralinear amp.

It was a brighter and lighter shade of blue, like a sky blue. It was continuous and began once the tubes were hot. The voltages were about right on the thing.

This was not the deeper blue which sometimes appears on the inside of the glass on some tubes.

The tubes worked, that is the amp sounded OK with a few Watts but I didn't try to measure anything.

Thanks Patrick for the info.

I changed out the "gassy" tube and replaced.
This issue of some unusual instability still persisted.
An instability that was addressed prior, Tx worked fine and then re appeared.

So, with the 2nd tube inserted, still a tinge of gas.
These tubes are WWII NOS and about 50+ years old.

I have been addressing the instability with
parasitic suppressors and have made some headway. 

So at this point the issue is a fundamental circuit issue, TBD.

You may have a mechanical issue with the tuning cap usually known as C-1 variable type. It could be in the ball bearings if it has ball bearing or the bushing if there are no bearings or a wiper mechanism that rides on part of the shaft or in groves in the cap. Clean it with a good electronic control cleaner that has some sort of lubricant in it. After you treat the cap with the spray work the cap though its range for about 5 minutes if you agree that this may be the problem? You don't need to spray the plates, just the moving parts. It may take 24 hours for the spray to work and for excess to evaporate.

Terry

HNY Terry.

Thanks for the feedback.
I had the Tx/Exciter running today with no issue.

There was/is a couple of issues, however, most appear to track back to the
original post of tube gas! Tubes with gas are like people... They are irregular!

That said, I have been "healing" the tube via filament and the more I run it the more it approaches stability. As I approach a condition where I see no gas, I will attempt to alter the output circuit back to my original design... which worked quite well in the starting phase.

A bit of info on gassy tubes.

73DG

Continued.

73DG

The last pages.

A bit tricky to implement today, but it does work.

73DG

Dennis, I copied those papers when you posted them on my thread on QRZ. I still have to build the test jig because I want to find a suitable transformer. I also referenced this article on another thread where the OP was trying to de-gas a Russian 4CX2500 for an amp. I don't know if this method could be used for what I call 'seasoning' other Rusky tubes that need de-gassing.