GU-81M VTTC does not work in Triode Mode [+ other Problems]

[janno288]

Hello, I am new to this forum, but not as new to Tesla Coils.

So I got a GU-81M Tube to replace 3 PL519 Tubes I ran the coil with previously since they were at the edge of arcing over internally (they have even done that a few times at 3kV).
The GU-81M being a more powerful tube I have not gained the output I had with the PL519s.
The Tube looked brand new, despite some damage to it (an anode support clip wasnt spotwelded on properly and is sitting at the tube base [small enough not to short anything]) and also a surpressor to anode short due to shipping damage (thanks DHL) I melted open.

I am running the tube as a Pentode because I cant get it to function as a Triode (all grids together) or (g1 and g2 together to the Feedback and g3 ground).
When it is run in triode mode, even at 230V input into the microwave transformer and ~5kV on the B+ the tube does not conduct any reasonable amount of current, no oscillation at all.

The tubes output as a pentode is around 15cm, (with the Voltage on the doubler averaging out to a bit over 1kV)
The 1.5kOhm Resistor into the screen grid is way too low and causes the screen grid to melt itself, but it was the only way of getting any kind of output. Using a 1MOhm resistor gets the output to be 1cm, ( with the Voltage on the douber being ~2-3kV)

The Other weird thing is, my Grid leak feedback is using a 100kOhm resistor, since anything under 15kOhm simply does not make the tube oscillate, with 100kOhm I got the largest PP voltage on the control grid.

When the autotransformer is at 150Vac I get the maximum and anything over that does not increase the arc size, infact the tube begins to redplate.

I have tested if my tube could be at fault. I put 500V on the anode and 500V on g2, g3 and g1 to ground.
I mesured the Plate Current (~750mA), g2 current (~240mA). Compared it to the datasheet and according to that it should be good, right?

I know my setup is messy and dangerous, this is only temporary, I just wanted to put it back together quickly today to do some testing after a month of having it dismanteled.

I am looking for answeres for these questions:
- Why is Triode Mode not working?
- Why is my Tube not conducting any current even with 5kV Plate Voltage?
- Why is my Output so low?
- Why does cranking my Autotransformer over 150Vac not increase the output size?
- Is my tube weak? [Its second grid is already a bit molten yet that should not effect it in triode mode]?

Schematic and Setup are attached below.

Thank you for reading!

- janno288, 20.10.2023


 

[Twospoons]

Perhaps your chosen triode connection is not the best. Maybe try Screen (g2) to anode, and supressor (g3)  to cathode. As suggested here https://www.audiodesignguide.com/New2A3/ETF06TS.pdf
The fact that you know you have shipping damage does not inspire confidence. If the tube got whacked hard enough to short a grid to the plate then lots of other things may be bent out of shape and affect tube performance.

[janno288]

Perhaps your chosen triode connection is not the best. Maybe try Screen (g2) to anode, and supressor (g3)  to cathode. As suggested here https://www.audiodesignguide.com/New2A3/ETF06TS.pdf
The fact that you know you have shipping damage does not inspire confidence. If the tube got whacked hard enough to short a grid to the plate then lots of other things may be bent out of shape and affect tube performance.

Well, the thing is this does not seem to work, yes anode current is being drawn but this results in excessive current into the g2 and It does not seem to want to oscillate in a stable manner.
Most designs run the GU-81M tube in a g1 + g2 to Feedback and g3 to ground way.

While I do think that shipping damage might be the cause, there was no other damage to it, it was just one bent out wire, which i was able to melt away completely.
Everything else looks new and the emission test showed the datasheet expected values.

[janno288]

I found out the reason.
Space charge always kept the grid negative thus needing way more g2 voltage to compensate for that and wasting a lot of power.

I had never tried using triode mode together with a light bulb as the grid leak since i never got the light bulb to work in pentode mode and the triode mode never worked with the 100kOhm gridleak.
Well I feel pretty stupid but it runs really well now.

The Current setup is all grids together and using a 60W 230V Bulb as the gridleak resistor.

I still thank you for the suggestion, if anyone here knows a good way to improve a GU-81M VTTC please write a message here

Here it is working