[Amps] AL-1500 Plate Meter Circuit

[Jim Thomson]

Date: Thu, 23 Dec 2010 17:59:06 -0800
From: "Jim W7RY" <w7ry at inbox.com>
Subject: Re: [Amps] AL-1500 Plate Meter Circuit 

The diode in this amp on the filter cap board shorted. That caused the grid 
and plate current meters to read the same. Just the plate current divided on 
the two meters.


Jim W7RY

##  If that's the safety diode... wired between B-  and chassis... Ameritron sez in their
manual to check to see if it's shorted... as it's a wimpy 1N4007.  

## On any of my HB amps..I use RVS  connected  6A10's  between  B- and chassis.  
Now if the one with the  anode facing the chassis..shorts, because of a glitch/surge.. or typ a B+  to chassis
arc/short,,, what  happens is that shorted diode now provides a 'back door' path  for grid current to flow
through the shorted diode.... instead of the grid meter.    It's easy to spot.    The amp puts out
normal power output.... but the grid meter  reads  zero ma.   The plate meter  reads way on the high side,
and is actually reading combined plate + grid current [ = cathode current]. 

##  with a XXX ohm resistor in series with the B+.... it's easy to calculate the max fault current.  I think the AL-1500
now uses a 25 ohm glitch..and with 3600v  no load.... it's  just 3600/25 =  144A  of fault current..which  will
short a 1N4007  in the blink of an eye.    If it's only 10 ohm..then fault current is 360A.    [This assumes the new style
HV lytics are low esr types]. 

##  I use  a 50 ohm glitch in series with the B+.   The safety  diodes  now consist  of 2-4 parallel  6A10's.   2-4 in parallel
6A10's with all their anodes  facing the chassis.    And  2-4  more in parallel... with all their cathodes facing the chassis.
So  4-8  x safety diodes in total.    That results in a 800-1600A  surge rating.    OK,   now you never have to replace the
damn things. They will never short.   I also insert a fast, sand filled  BUSS HV fuse... in series, and just  prior to the
50 ohm glitch R.   Any B+ to chassis  fault..or B+  to grid... and the  50 ohm  glitch  LIMIT'S  the fault current to a 'safe value'. 
The HV fuse blows asap.. <2 msecs.   The 6A10's safety diodes  merely conduct the fault current.   3500 v / 50 ohms = 70 A
of fault current.   70A  will open up a fast  1A   HV fuse...very quickly.   That amounts to a 7000%  overload.   Even with 6A10
safety diodes, rvs connected   across the plate/grid meter  [ or grid shunt].... the fault path is always the chassis, up through the
ssafety diode... then back to B-  of  HV supply  route... and nevr through the grid-plate meter to B-  route. 

later... Jim  VE7RF