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Re: [Amps] Amp Supply LK500ZB - strange failure mode

To: Amplifier Mailing List <>
Subject: Re: [Amps] Amp Supply LK500ZB - strange failure mode
From: Alek Petkovic <>
Date: Fri, 03 Oct 2014 11:25:09 +0800
List-post: <">>

They appear to be the best deal around.

73, Alek

On 3/10/2014 10:05 AM, k0zx Rich wrote:
Right you were Colin. After pulling and swapping the result was one of them had the symptom and the other didn't. Pretty clear cut. So I guess I'm on the hunt for another, first price i've seen was 189, hoping can do better than that. Thanx for the suggestion, I got focused in on possible semiconductor failure, replaced the zener in the cathode path.
Are u and owner of one of these amps?

On 10/2/2014 9:07 AM, Colin Lamb wrote:

Hi Rich:

Pull one tube and try it - then the other.  I suspect one tube.

Tuning will change and the input swr will increase - but you are looking for an obviously bad tube.

Assuming the filaments are not in series and the cooling is not solely through the socket, it will fire up fine. Watch out for hv bite.

73,  Colin  K7FM

On 10/2/2014 7:59 AM, k0zx Rich wrote:
Wonder if anyone has experienced this failure mode with this or a similar amp:

This amp is 80s vintage and since then has been a solid performer, grounded grid with two 3-500z. But it has a developed a problem where at power up (no rf drive, ant relay input not active for tx), the plate current rises to about 3x normal (about 300ma, 100 is normal), stays there for 5 to 10 seconds and then drops to zero. If enabled to transmit with antenna relay input on, plate idle current is normal, going from 0 to 100ma. If apply rf in, it will output but at only about .6 or .7 its normal output. After inspecting the schematic am not seeing an obvious source for the problem.

Any suggestions appreciated
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