[Amps] 2m GS35B Amplifier

[Alek Petkovic]

G'day all.

I've been helping my friend John, VK6JX, who is not a subscriber to this 
group, with the building af a "1 x GS35B 2m Lazy Builders PA" as 
described at https://www.qsl.net/yu1aw/VHF_PA/PA%20for%20lazy%20build.htm

The intended use is for digital EME.

The build and the testing has been quite successful but for one problem. 
It is as per the specs in the link above, except he has added the 
ability to adjust the output loading, which the original design does not 
have.

The very robust power supply gives 3.2kV DC. The cooling blower supplies 
heaps of CFM to keep everything cool as a cucumber. Filament volts is 
steady at 12.6V at all times.

The amplifier tunes smoothly and perfectly and puts out around 1000 to 
1100 Watts but only at around 45% efficiency. We were hoping for better 
than that but maybe that's how life is.

Now here's the problem.

With 100W of drive and 1000W plus out. Plate current is around 800mA and 
grid current at around 300mA.

The amp holds up for around 20 seconds and then the power spirals down 
quite rapidly and it is accompainied by a corresponding rapid increase 
in plate current.

Let go of the key and let her cool for a minute and try again. Once 
again, after 20 seconds or so, the symptoms occur.

As the power output goes down, we can follow it with the tune control 
and raise it a bit for a second or two but then she crashes as before.

We have tried a different NOS tube with exactly the same results. 
Something is taking off.

We are at a bit of a loss as to what is actually happening so we don't 
know what the fix might be.

All suggestions will be gretaly appreciated and vigorously investigated.

Thanks and 73, Alek, VK6APK