[Amps] Removing grid shorts

[Alek Petkovic]

Yeah, my local club has a GU84B with the same condition. No short when 
she's cold but a couple of minutes into the warmup, the belly ache 
starts and stays.

The tube is a display piece.

Cheers, Alek.
VK6APK

On 31/05/2019 8:46 PM, Steve London wrote:
> I am the "proud" owner of two Alpha 3CX800's with grid-cathode shorts 
> that start 2 minutes into warm up. I would be thrilled if they cleared 
> themselves after 6 minutes of warm up.
>
> 73,
> Steve, N2IC
>
> On 05/30/2019 07:27 PM, Doug Renwick wrote:
>> Let's see if this e-mail goes through at Amps. It didn't go through at
>> Ham_Amplifiers.
>>
>> Jim,
>>
>> I am not sure what to try next. Others have said not to use a 12v car
>> battery for this tube. So I am at a point of scraping the tube.
>>
>> It works fine if it is allowed to warm up for over 6 minutes. The 
>> short now
>> lasts from about 2 min to 5 1/2 min after turn on.
>>
>> Doug
>>
>> "The difference between stupidity and genius is that genius has its 
>> limits."
>> Albert Einstein
>>
>> -----Original Message-----
>>
>> <I have a GU-78B that has developed a grid short. I have determined 
>> the grid
>> <short initiates about 2 minutes after power on and ends after about 5
>> <minutes.
>>
>> <I have tried unsuccessfully to 'burn' the short by hooking up a 560?f
>> <capacitor fed with 300vdc.
>>
>> <What other methods that can be tried to 'burn' this short?
>>
>> <Doug
>>
>> ##  I assume you mean  the  control  grid....and not the screen  grid ?
>>
>> ##  If it is the  control grid......  Im assuming  the  control grid  is
>> shorting to the cathode ?    Or is the  control grid  shorted to the  
>> screen
>> grid ?
>>
>> ##  560 uf cap charged up to 300 vdc  =   25.2  joules.   Thats not  
>> a lot
>> of energy.
>>
>> ##  What happens after  5 minutes..and the short  vanishes?   Is the  
>> tube
>> now useable ??
>>
>> ##  watch out, the  control  grid  dissipation  is no doubt  very 
>> little, as
>> whats  indicated on the spec sheet.  In  your quest to burn the short 
>> out,
>> you could destroy the control  grid in the process.
>>
>> ##  folks who have had  control  grid to cathode shorts  on a 3-500Z, 
>> have
>> typ  a  50% chance of burning the  short open... using a 12 vdc car 
>> battery,
>> which will  supply a LOT of  current.   Depending on the  ESR of 
>> your  560
>> uf cap, the  ESR  will  limit the  current   somewhat.   Typ  560 uf  
>> @  450
>> vdc
>> lytics  have a ESR of  .203 ohms..... which is low enough to deliver 
>> a lot
>> of current... albeit for a split second.
>>
>> Jim   VE7RF
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