[Amps] 2m GS35B Amplifier

[Steve]

You mention adding 'the ability to adjust output loading'. What was 
added and where? The original circuit does allow the loading to be 
altered, just not in the way that you might be used to with 'tune' and 
'load' controls on a pi network.

Different combinations of Cc and C1 allow the load presented to the GS35 
to be made resistive and varied over a range but the tuning is very 
sensitive to either of them being varied. I looked at it on a Smith 
chart to get a feel of how it operates; I'm sorry but I lack the skill 
to explain it simply in any other terms. Perhaps others can help.

Getting the wanted load at the anode is also quite sensitive to the 
inductor and total capacitance to ground at the GS35 (which can be very 
much more than the Cag figure in the data). Copying the circuit without 
knowing the original layout in detail could easily throw the end result out.

 From what you describe, it seems like a thermal thing; maybe valve 
capacitance, maybe the inductor or capacitor(s) are heating up. All of 
them could result in what you describe. Can you get an infrared camera 
to peer in there?

Both anode and grid current high suggests to me that the load impedance 
is too low. Less Cc and more C1 is the combination to push it higher.

I took a guess at component values to get a realistic end result and the 
loaded Q at the L/C1/Cc junction can head up into the 30+ region. 
Increasing Cc and C1, decreasing L results in lower loaded Q for the 
same load resistance at the GS35, which might help, but you'll have to 
cut and try until it works.

73, Steve G8GSQ

> 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