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[Amps] 6 meter SB220 plate choke

To: <>
Subject: [Amps] 6 meter SB220 plate choke
From: <>
Date: Sat, 1 Jun 2019 07:49:24 -0400
List-post: <>
I'm in the process of performing some updates to my converted SB220. While I
have the lid off I thought that I would remove some of the turns on the
plate choke. Looking for advice on how many turns to remove?


-----Original Message-----
From: Amps <> On Behalf Of Doug Renwick
Sent: Thursday, May 30, 2019 9:28 PM
To: 'Jim Thomson' <>;
Subject: Re: [Amps] Removing grid shorts

Let's see if this e-mail goes through at Amps. It didn't go through at


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.


"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

<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?


##  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
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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