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Re: [Amps] 2m GS35B Amplifier

To: Paul N1BUG FN55mf <>, "" <>
Subject: Re: [Amps] 2m GS35B Amplifier
From: David G4FTC <>
Date: Sat, 1 Oct 2022 15:01:49 +0000
List-post: <>
Hi Paul,

You could be lucky with your tube.

Googling "GS35b tuning drift" will yield many examples.

David G4FTC

From: Amps <> on behalf of Paul N1BUG FN55mf 
Sent: 01 October 2022 12:52
To: <>
Subject: Re: [Amps] 2m GS35B Amplifier

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
> 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 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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