[Amps] Question about grid current in 4CX or 3CX

[Jim Thomson]

Date: Fri, 15 Mar 2019 10:37:37 -0500
From: donroden at hiwaay.net
To: amps at contesting.com
Subject: [Amps] Question about grid current in 4CX or 3CX

<Question

<For the larger tubes with handles  ( commercial service )

<Does grid current increase or decrease as plate voltage and screen  
<voltage is removed. ?

<If it takes 250 watts to produce 100ma of grid current in a 4CX10,000.
<is the grid just a big dummy load and is it doing the tube harm to  
<dump 250 watts into it when both plate and screen voltages are removed ?


<Don W4DNR

##  I am assuming in the case of the  3CX  series,   that the grid is bonded to chassis
via the  GG socket..and tube is cathode  driven.... just like any GG triode. 

##    I am assuming the  4CX  tetrode is being grid driven,  with the usual  screen  and  B+
voltages applied. 

##  In the case of the  GG triode, and no B+... but drive still applied, you will send the  grid
current through the roof !   Without any B+, the  tube now thinks the control grid is the anode.
All the  electrons  will flow from directly heated cathode to the  control grid.   I have verified this
several times on  several different triodes, including the  larger metal types.   Thats why I  install
a  rear panel  3AGC  fuse holder, and wire it between  chassis....and  the negative terminal of the
dc grid meter.  Then use a FAST fuse, thats  barely rated for normal average  dc grid current. 

## If you lose the B+,  and dc grid current skyrockets,   grid fuse blows open asap.  With  NO  path
for dc grid current, triode can not be driven.  Power  output drops to ZERO.   Input  swr, between 
xcvr and  input to the GG triode amp  rises to infinity.   Xcvr then shuts itself  down asap.   Event over. 

##  On GG triodes, with delicate 4 watt grids, like the  3CX-800A7,  and loss of B+,  whats then required
is high speed electronic grid over current protection.  Another method  is if  B+   falls below a certain threshold,say 
less than  70%  of normal,  amp kicks off line asap.   For  GG triodes like  3-500Z,  3x3, 3x6,  3x10, 3x15   etc, the
grid fuse is ample. 

##  IF the tetrode is run like a high mu  triode, with both screen grid and control grid  bonded to  chassis,
use the grid fuse method.    This config requires a lot of drive...like  625  watts.   Back in the day, a SB-220,
run in the  CW position was used for  the ipa. 

##  IF the tetrode is grid driven, with the usual  screen and  B+ applied, and  the typ  450 ohm globar used to 
derive grid voltage..when driven  through the  typ 1: 9  un-un......and screen and  B+  are simultaneously 
removed... I THINK the  globar just dissipates the normal drive power.   But  dunno if the  dc grid current increases
or not.  On the  grid driven tetrode, run in passive grid with the 1:9  un-un  and 450 ohm globar, the  dc grid  current
is typ 0 ma... if tube run in  class  AB1.     Typ the  4CX-10,000A7, when grid driven In the passive grid config, 
only requires  130 watts  for 14 kw pep output.   You would never require  250 watts. 

##  On tetrodes like the  4CX-1000  variety, complete  with their  ZERO watt grid  diss rating, grid over current is 
required, and will kick amp offline with  just  100  microamps of  grid current. 

##  Then u can also get into goofy configs with the same tetrode, like just loss of  screen voltage.... or   just loss
of  B+.   On the commercial tetrode amps, like collins  208-U  series and similar  from others like aerocom, etc
they all had  plate over current, screen overcurrent,and grid over current relays and protection built in..and all 3
have adjustable thresholds.

Later... Jim   VE7RF