[Amps] Fwd: Re: Removing grid shorts

Wed Jun 5 18:34:59 EDT 2019

You give up after one try?

Carl

----- Original Message ----- 
From: "Alek Petkovic" <vk6apk at bigpond.com>
To: "Boye Christensen" <oz7c at cool.dk>
Cc: <amps at contesting.com>
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.
> VK6APK
>
> 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 <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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>>>> Amps at contesting.com
>>>> http://lists.contesting.com/mailman/listinfo/amps
>>>>
>>>>
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>>
>
> -- 
> From sunny Binningup
>
>
>
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