[Amps] Don't do this

[Jim Thomson]

Date: Sat, 18 Dec 2010 19:16:59 -0800
From: "Bill, W6WRT" <dezrat1242 at yahoo.com>
Subject: Re: [Amps] Don't do this
ORIGINAL MESSAGE:

On Sat, 18 Dec 2010 14:26:29 -0800, Vic K2VCO <vic at rakefet.com> wrote:

>
>Some of my happier mods were the addition of a glitch resistor and back-to-back 1N5408 
>diodes across the meters,

REPLY:

Just a word of advice - back to back diodes across the meters adds zero
protection. Diodes don't conduct until the voltage across them reaches
about .6 volts and by then the meters will be toast. You need to add a
resistor in series with the meter of such value that the voltage across
the meter + resistor equals about .6 volts when the current is just
above full scale. Then place the diodes across the combination of meter
+ resistor. Use heavy diodes such as 6A10 and your meters will be nearly
bulletproof. 

73, Bill W6WRT

###  Partially agreed.  Bill's idea will  work very well...and is a good
fix  for standard meter's.   I have not tried it...yet...except in a test
jig on the bench. As is, the amps, including my hb ones.. just have
back to back 6A10's  across the grid and plate meters [ and multi meter's].
However, I also installed  back to back diodes  between the B-  at the cold
end of the string of HV lytics....and the chassis. 

##  In normal operation, everything is fine.  Now is you slowly over drive the
amp.. and slowly bury the  0-1A  plate meter to the peg, yes, Bill is correct that the
back to back diodes will not protect the grid/plate meter's.  However, if you short the
B+  to the chassis, the diodes will conduct asap, and save the meter's  every time.
It also  depends on how much  leeway you have on the grid/plate meter.  IE: is the meter
when  full scale,  just a bit over what normal grid/plate current is... or a whole bunch over.

##  On my hb GG triode amps,  I wire a FAST 3agc fuse between  chassis and POS terminal  of the
grid meter. [ nothing wired across the fuse].   Blow the grid fuse, and no path for DC grid current. 
PO  drops to zero.  Input swr rises to infinity, xcvr shuts down.   I also wired a fast 3agc cathode
fuse in the CT of the fil xfmr...[with a 100 k resistor across it].  If cathode fuse blows, the 100 k
resistor is now in the circuit, and amp is biased to cut off.  I use this scheme on both the 3CX-3000A7
and also the YU-148 tubes.   Both have 225 W grids in em.   If the 3agc fuses  are sized correctly,
they will blow 1st b4 the plate/grid meter's are pegged....with a CXR.     To pull that off on ssb/cw..
the 3agc fuses have to be sized smaller, so they are on the ragged edge.  

## Bill's  idea works good on the test bench, and is easy to implement into any amplifier.   IE: to 
protect a 0-1A  plate meter,  .6V /1A =  .6 ohm.   [ R will diss .6W with 1A  CCS].  Don't forget the
part  where the safety diodes are wired across the COMBO of meter + resistor.   Next time I have
the amps apart,the  resistor's will be added.  OK, at that point, there is no way you can destroy a 
grid/plate meter.... the 6A10  will conduct before the meter is pegged. 

later... Jim  VE7RF