Re: [Amps] Removing grid shorts

[Alek Petkovic <vk6apk@bigpond.com>]

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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