GU-81 vs. GU-81M

[janno288]

I have recently bought myself another example of the GU-81 tube family and discovered that its the earlier GU81 instead of the GU81M.

The datasheet shows the tubes internal connection, including 2 capacitors from the filament ends to grid 2 / screen grid.
Mesuing the total capacitance from the Cathode centre tap to Grid 2 my meter shows 900pF of capacitance, the datasheet gives no mention of these capacitors, only that they exist. No value or voltage rating outside the maximum 600V on the second grid.

Is a general VTTC circuit suitable for such a tube, I havent found a reason against it, which also is made confusing with the interchangable use of "GU-81" and "GU-81M".

If you have any experience with the GU81 and GU81M please share it!

Thank you!
janno288 14.11.2023 22:42

[Mads Barnkob]

I would guess it is just decoupling capacitors, to fix some kind of early issues with induced currents or noise. Properly nothing to worry about

[janno288]

I would guess it is just decoupling capacitors, to fix some kind of early issues with induced currents or noise. Properly nothing to worry about

that was its intendant task, to decouple grid 2, thus keeping RF from the screen voltage.
In VTTC operation, especially in Higher frequencies i feel like it could pose a small issue, especially if one of those capacitors gets damaged it would short out the feedback high voltage directly into the filament, tho I think this would be a very rare at best occurance.
I have tested the tube in my VTTC setup with a frequency of 160kHz and it has the same performance as my GU81M (which is slightly damaged, yet still performs NOS according to the datasheet).

I have since spoken to my VTTC expert friend and even in HFVTTC situations he hasnt found any issue with it.
So I suppose its fine.

Thank you for the reply!

[thehay95]

Recently, I added another GU-81 tube to my collection, and to my surprise, it turned out to be the earlier GU81 rather than the GU81M. Upon a closer look at the datasheet, I noticed intriguing details about the tube's internal connections, specifically the presence of 2 capacitors from the filament ends to grid 2/screen grid.