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Re: [Amps] 2m GS35B Amplifier

Subject: Re: [Amps] 2m GS35B Amplifier
From: Steve <>
Date: Mon, 3 Oct 2022 08:46:37 +0100
List-post: <>
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 

    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 

    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

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