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Re: [Amps] GI-7b issues

Subject: Re: [Amps] GI-7b issues
From: Paul Whatton <>
Date: Tue, 09 Sep 2008 19:58:12 +0100
List-post: <">>
Hi Bob

I haven't used them on HF but I've used a pair of GI-7bs trouble free on 
2m for 4 years.

Sorry if this sounds like a stupid question but the force air cooling is 
actually working? The fan is running OK?

Assuming you do have plenty of air through them are the coolers screwed 
on tight? Ian GM3SEK suggested to me that the machining on the underside 
of the cooler isn't great and skimming them in a lathe can improve 
things a lot. I haven't done that with mine but it's a good idea. K4POZ 
found some have cooler flaws.

They do have plenty of gain at VHF, the GI-7b works even at 1296MHz so 
they might be oscillating. But I think you'd see that on the plate 
current meter? Provided the grid rings are solidly grounded I wouldn't 
expect them to be especially prone to oscillation. There was a 
discussion about parasitic supression in the GI-7b in an earlier amps thread

I can put my foot on the key and run my 2m pair with them dissipating 
700W for hours without a murmur. Lots of cooling air though.

73 Paul G4DCV

Robert Perdue wrote:
> Looking for any info on the GLA-1000 conversion to the GI-7b tube. I have 
> done the w4emf conversion but I am having problems with the tubes self 
> destructing. has anyone else had issues with this tube? The amp runs fine and 
> then boom the tube melts down without any indications I have watched current 
> draw very close and that seemed fine but the tube just fails with a pop it 
> melts the solder out of the tube around the Anode cap. Could it go into 
> oscillation causing this? And for some background I ran the filaments for 48 
> hours before use. I have got 4 more tubes ready to go again but would like to 
> find the problem before proceeding. Anyone else having failures?
> 73, Bob kc0hfl  
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