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Re: [Amps] 2m GS35B Amplifier

To: "" <>
Subject: Re: [Amps] 2m GS35B Amplifier
From: David G4FTC <>
Date: Sat, 1 Oct 2022 08:54:03 +0000
List-post: <>
Hi Alek,

I have a 70cm GS35b amp which exhibits most of the characteristics which you 

My amp is based upon the design of PA3CSG .  Also 
see VK3NX's comments

It looks as if your 2m design also uses a half wave anode line - I found with 
my amp that the length of this half-wave anode line was fairly critical in 
achieving optimum efficiency - with mine the anode line is copper tube which I 
can slide it up and down the anode cooler give the best length. I found I 
needed a longer anode line than that of PA3CSG's design (as per VK3NX's notes). 
My guess is that you may need to experiment with different lengths of anode 
line to achieve best efficiency.

Re: tuning drift. I get a similar power drift as the tube heats up. This seems 
to be fairly well documented with this type of tube (disk seal triode??) which 
I assume is due to the various structures within the tube expanding as they 
heat up. I found that if I mistune the anode slightly, it provides a good 
compromise so that when I first apply drive, the power comes up, then after a 
few seconds it will drop back and then after about a second climb to full 
output.  Once it's got over this initial change, it's OK for contest style 
QSOs, i.e., a few seconds TX followed by a few seconds RX.

As an aside, I have a 23cm amplifier which uses a ring of six 2C39A tubes (as 
per OZ9CR) - this also exhibits tuning drift although very much more pronounced 
than with the GS35b amp. Most users seem to have resorted to water cooling the 
2C39As to overcome this but I don't really want to go down that route.

BTW I'm using about 40 watts of drive to the GS35b with a 3.2kV supply and 
getting about 650 watts out. I can tune for a higher output (~750 watts) but 
the warm up drift is more pronounced. Grid current is minimal (~25mA).

Apart from that, I find the GS35b amplifier very docile, no signs of it taking 
off or anything like that.

Hope this helps

David G4FTC
From: Amps <> on behalf of Alek Petkovic 
Sent: 30 September 2022 23:34
To: <>
Subject: [Amps] 2m GS35B Amplifier

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

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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