[Amps] Amp Supply LK500ZB - strange failure mode

[Alek Petkovic]

See http://www.rfconcepts.com/PRODUCTS/tubes

They appear to be the best deal around.

73, Alek
VK6APK

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?
> rich
>
>
> 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
>>> Rich
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>>>
>>
>
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