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Re: [Amps] 2m GS35B Amplifier

Subject: Re: [Amps] 2m GS35B Amplifier
From: Paul N1BUG FN55mf <>
Date: Sat, 1 Oct 2022 08:52:33 -0400
List-post: <>
For whatever it may be worth, my 2m GS35b amp doesn't show any tuning drift at all running with 75 watts drive and 1300 watts out. It uses a quarter wave tubular plate line. Efficiency is about 55% with 3.8 kV on the anode.

What I do have is high grid current, which I have never been able to understand. But that seems unrelated to the issues John is having.

Paul N1BUG

On 10/1/22 04:54, David G4FTC wrote:
Hi Alek,

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

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

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

Thanks and 73, Alek, VK6APK

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