[Amps] Removing grid shorts

[Steve London]

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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