[Amps] Fwd: Re: Removing grid shorts

[Boye Christensen]

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 <vk6apk at bigpond.com>
To: 	n2ic at arrl.net
CC: 	amps at contesting.com, Steve London <n2icarrl at gmail.com>, 
ve5ra at sasktel.net



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.
VK6APK

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.
>
> 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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>>
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-- 
 From sunny Binningup



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