[Amps] problem with homebuilt GS35B 70cm amp

[Benedikt Sveinsson]

And just a bit of an update

I put the capacitors on the grid - and that raises few more questions

Now I get a slighlty elevated (50mA) plate without amp being keyed - but all the same when keying and driving it - no loading possible.

Using pot for the BIAS adjustment when keyed with no signal - I can elevate the plate current up to around 150mA - so the BIAS control seems to be functioning.

I'm reluctant changing tubes - I have more GS35's  ;)

73, Benni TF3CY




-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Benedikt Sveinsson
Sent: 7. október 2014 16:27
To: amps at contesting.com
Subject: [Amps] problem with homebuilt GS35B 70cm amp

Hi all

I have been building a 70cm GS35B amplifier and it's not working

Here are pictures of the amplifier
http://www.tf3cy.is/70cm-gs35b-amplifier

So - I have done and noticed:

* turned on the amplifier with HV
*When I started the amp for the first time the anode trip on the GM3SEK triggered.
 (video: https://www.youtube.com/watch?v=cU4wXqeDOjo )  <- first time I turned him on with HV Perhaps I did a mistake there of clearing the alarm - bit anxious.


* When keyed - it draws around 200mA current on Plate.
* When keyed and and driven (10-20W) no changes on the plate/grid meter (grid meter never moves)
* input tuning works - I can tune to low SWR
* Grid meter never moves (all calibrated using methods in the Triode manual by GM3SEK)
* 12V AC (bit low) filament measured on the feedthrough capacitors into the input compartment.
* When I was driving the amp - the PSU tripped a 16A breaker few times - but after letting the amp sit with Filament + HV for some time - (1hr) it stopped doing that - this must have been a arc inside the tube - I really cant find what would have caused this.

When writing this I notice that I don't have any capacitor on the filament transformer. - is that perhaps key? 

I notice in my design there is no RFC on the output - I would assume that is a major safety issue.

Or could this be a faulty tube ? It went through some knocking when getting the cooler off (it was supposed to go into a 2m amp)


And a bonus question
* if you look closely at the picture of the power supply - I put a 4 way ceramic switch on the AC output of the Peter Dahl transformer so I can switch from approx. 2kv-4kv
* CAN I move this switch with HV on ? (have not tried it!)


73, Benni TF3CY

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