[Amps] Removing grid shorts

[Jim Thomson]

Date: Fri, 24 May 2019 10:38:25 -0600
From: "Doug Renwick" <ve5ra at sasktel.net>
To: <amps at contesting.com>
Subject: [Amps] Removing grid shorts

<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