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[Amps] Ameritron Al-1500 excessived grid current & shut down

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Subject: [Amps] Ameritron Al-1500 excessived grid current & shut down
From: "Jim Thomson" <>
Date: Mon, 27 Aug 2018 09:36:53 -0700
List-post: <>
Date: Mon, 27 Aug 2018 09:40:13 -0400
From: w8hw <>
To: Herbert Schoenbohm <>, Amps group
Subject: Re: [Amps] Ameritron Al-1500 excessive grid current & shut

<90% of the time it is a diode on the hv board that is near the edge. If 
I were home, I could give you the exact one, but won't be home in 
Florida for a week (Sept 3). I repair amps and have worked on that 
problem on that exact amp many times, and only once was it the tube. 
Hope that helps. 73, Bruce, W8HW

On 8/26/2018 4:30 PM, Herbert Schoenbohm wrote:
> Today I turned on the Al-1500and immediately notice that the grid current
> went all the way over to the right and the amp grid protection circuit kick
> in.  Even with only 5 watts of drive, the grid current is about 175 ma.  I
> also notice that I don't see any HV indication.  I hope this is not a bad
> 8877 final tube. Any suggestion to confirm this would be appreciated.
> Thanks,
> Herb, KV4FZ

##  IF its a case of a power supply diode  arcing to metal, with not enough 
a thin sheet of teflon etc,  can be inserted in the problem area.   Local  
plastic store
has loads of  scrap teflon sheeting.   IF the  B+ arced to the chassis, it will 
try and return back to the
B-  of the filter caps.... via the single safety diode.... which is located 
between the chassis..and  B-. 
That  safety diode will typ  short out.  Its purpose is  to keep the  B- above 
chassis   potential.... but
keep it within .7 volt.   Then you can do normal metering.   The safety diode 
should not short out.
Ameritron used a single  1N5408..with its  200A  surge rating..which is for  
8.3 msecs.   They should have used
a  6A10, with its 400A  surge rating.  A  6A10 is the same body length as a 
1N5408, but bigger diameter.  They both
have the same  16 gauge  /  .051  diameter wire leads..and same length leads.   
A  6A10 will drop into where a 1N5408
once stood, ditto with the rectifier diodes in the  FWB. 

##  with no B+....  on any  GG triode amp,  and with drive applied,  you  will  
peg the grid meter every time.
With no  B+, the tube  now thinks the grid is the anode.  All the electrons are 
now attracted to the grid. 

##  Check all the diodes on the  diode board on the B+  supply.  If they fail, 
they will fail shorted. 

##  To stop any potential blow ups,  I install a fast..single  HV fuse  between 
 one leg of  plate  xfmr sec..and  input to
FWB.    That  HV fuse can go into either leg of the HV xfmr sec.    I typ use  
a 50 ohm glitch resistor in the  B+  output. 
A 2nd  HV fuse is located   in the B+ output, just  prior, and in series  with 
the 50 ohm glitch resistor.   In the event of a 
B+  to chassis arc, or  internal  anode to grid arc, etc, the 50 ohm glitch R  
limits the  fault current to a safe value. The 
HV fuse in the B+  interrupts the  fault current... in less than  1-2 msecs. 

##  IF the fault is on the main FWB board, or between FWB board..and  input to 
filter caps, or between   B+ of filter caps..
and   input to  2nd HV fuse.... then  HV fuse  #1  will blow open asap.   I use 
 several paralleled  6A10s  for greater surge capacity,
installed between  chassis..and B-.   At that point, anything can be shorted to 
anything else,  B+ to chassis, B+ to B-....  raw DC
to chassis, anode to grid, etc, etc and nothing bad happens. One or both  hv 
fuses opens up, thats it.  No blown tube,  no destroyed
safety diodes, no damaged diodes on FWB..or FWD.   Then its  fully protected. 

Jim   VE7RF    
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