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Re: [Amps] 2m GS35B Amplifier

Subject: Re: [Amps] 2m GS35B Amplifier
From: Alek Petkovic <>
Date: Mon, 3 Oct 2022 06:57:16 +0800
List-post: <>
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

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
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

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