[Amps] Fwd: Re: Removing grid shorts

Tue Jun 11 20:07:57 EDT 2019

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?
>
> 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
>>>>> _______________________________________________
>>>>> Amps mailing list
>>>>> Amps at contesting.com
>>>>> http://lists.contesting.com/mailman/listinfo/amps
>>>>>
>>>>>
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>>
>> -- 
>> From sunny Binningup
>>
>>
>>
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-- 
 From sunny Binningup

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