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Re: [Amps] Removing grid shorts

Subject: Re: [Amps] Removing grid shorts
From: Alek Petkovic <>
Date: Sun, 02 Jun 2019 15:29:40 +0800
List-post: <>
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.

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.

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


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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