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Re: [Amps] Fwd: Re: Removing grid shorts

To: Carl <>
Subject: Re: [Amps] Fwd: Re: Removing grid shorts
From: Alek Petkovic <>
Date: Wed, 12 Jun 2019 08:07:57 +0800
List-post: <>
Ok. I see that you want me to think big.

I'll do it again and again until there are no more grid wires left. :-)

73, Alek.

On 6/06/2019 6:34 AM, Carl wrote:
You give up after one try?


----- Original Message ----- From: "Alek Petkovic" <>
To: "Boye Christensen" <>
Cc: <>
Sent: Tuesday, June 04, 2019 7:47 PM
Subject: Re: [Amps] Fwd: Re: Removing grid shorts

I had reason to test some unknown 572B valves yesterday. I tested them in an old Yaesu amp.

While I was at it, I thought I would try to fix a brand new 572B that has a G-F short. My supplier had already replaced the valve and didn't need the faulty one sent back. With an Ohmmeter, I determined which end of the filament the short was closest to. I then charged a 10,000uF capacitor with a 12V battery and discharged it across the relevant grid and filament pins. The fault cleared immediately. When I touched the leads of the capacitor together, after that, there was a good spark, indicating that there was still a lot of the charge left in it.

Well, I plugged the valve into the 2100 amp and switched on. The short immediately returned, as I could see that the plate current meter showed current draw without keying up.

So, with that particular 572B valve, the procedure did not work very well at all. Another display piece.

Cheers, Alek.

On 3/06/2019 11:08 PM, Boye Christensen wrote:
Hi to all

This is the normal life out time of tubes

If the tubes is open after some time of warm up there might bee

a change to repair !

How to repair will bee  different of tube type (life time extended)

But mainly it has to burn g1 - catode connection away !

73 Boye OZ7C

-------- Forwarded Message --------
Subject:     Re: [Amps] Removing grid shorts
Date:     Sun, 02 Jun 2019 15:29:40 +0800
From:     Alek Petkovic <>
CC:, Steve London <>,

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.

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