[Amps] 2m GS35B Amplifier

Sat Oct 1 14:28:48 EDT 2022

Hi David,

That could be. I have two more tubes I should test to see how they 
behave in the amp.

Paul N1BUG

On 10/1/22 11:01, David G4FTC wrote:
> Hi Paul,
> 
> You could be lucky with your tube.
> 
> Googling "GS35b tuning drift" will yield many examples.
> 
> David G4FTC
> 
> 
> ------------------------------------------------------------------------
> *From:* Amps <amps-bounces at contesting.com> on behalf of Paul N1BUG 
> FN55mf <paul at n1bug.com>
> *Sent:* 01 October 2022 12:52
> *To:* amps at contesting.com <amps at contesting.com>
> *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
>> https://pa3csg.nl/?page_id=17 <https://pa3csg.nl/?page_id=17> .  Also 
> see VK3NX's comments
>> http://www.vk3nx.com/files/432_MHz_PA.pdf 
> <http://www.vk3nx.com/files/432_MHz_PA.pdf>
>> 
>> 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
>> <amps-bounces at contesting.com> on behalf of Alek Petkovic
>> <vk6apk at bigpond.com> Sent: 30 September 2022 23:34 To:
>> amps at contesting.com <amps at contesting.com> 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
>> https://www.qsl.net/yu1aw/VHF_PA/PA%20for%20lazy%20build.htm 
> <https://www.qsl.net/yu1aw/VHF_PA/PA%20for%20lazy%20build.htm>
>> 
>> 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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