[Amps] 2m GS35B Amplifier

[Steve]

Cc and C1 is what the two variable caps are labelled in the original 
circuit, there's no special meaning beyond that. Cc is the series 
coupling from the output, C1 the shunt capacitor.

I'd say it's misleading to call them 'tune' and 'load'. In a typical hf 
pi network the load cap predominantly sets the wanted load resistance 
and the tune cap adjusts the load to be purely resistive and (to a very 
loose first approximation) each does their job independently of the 
other. In the amp you're working on both caps affect both functions.

Think of the wanted load at the anode as a point on graph paper. In a pi 
network 'load' cap moves your position left and right, 'tune' cap moves 
you up and down. In your amp, one cap moves you on a slant / and the 
other on a slant \ so it's a much more iterative process to find the 
best tuning spot.

The original amp gives figures for running at 2.5kV and 1.2A - that's a 
much lower load at the GS35 than you would target for 3.3kV and 0.8A.

Don't forget that connections around the blocking caps contribute to the 
total series L in the circuit. Can you can patch in a short leaded 
resistor of the desired load from anode to ground and look into the 
output with a VNA to help preset the tuning on the amp when cold?

Good luck with the experiments, play safely.

73, Steve G8GSQ

> OK Steve. Thanks for your valuable input.
> 
> Firstly, the loading adjust is very simple. Both the tune and load capacitors are simply shhets of copper. The tune plate is wound in and out by a threaded rod. The load plate is fixed at a 1/2" spacing. John added another threaded rod to enable that plate to be to be pushed in, to give some adjustment capability. As it happens, we actually ended up bending that plate further away, to increase the gap and loading and then brought it back in to the best position with the threaded rod. So, we didn't actually change anything. The loading plate is now probably effectively 1/8" further away.
> 
> 
> I am hoping to get to John's place today or tomorrow, so I'll bring my laser dot temperature thingy and we'll see what gives.
> 
> 
> He did remove the plate bocking cap yesterday and showed me the picture. It has sustained damage and I suspect it is toast. It could be our problem. It was 2200pF at 10kV. RF rating unknown but obviously not good.
> 
> 
> I'm bringing 2 x 68pF 10kV 7kVAr doorknob caps and we'll put them in and hope for the best.
> 
> 
> I'm sorry but I'm not familiar with the meanings or C1, Cc etc but I guess they may be Tune and Loading in my language. John is an engineer. He'll probably slap me for not knowing. Hi hi.
> 
> 
> Anyway, we have some more investigating and experimentation to do. I'll report back with any successes oor continuing failures.
> 
> 
> 73, Alek, VK6APK
> 
> On 2/10/2022 9:12 pm, Steve wrote:
> 
>     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
>