[Amps] L4/L7 Power supply B+.....'The fix.'

Jim Thomson Jim.thom at telus.net
Fri Feb 19 03:53:43 PST 2010


From: Michael Tope <W4EF at dellroy.com>
Subject: Re: [Amps] L-4/L-7 Power Supply

Hi Ed,

What you refer to as bleeder resistors are really part of a voltage 
divider that provides cutoff bias for the 3-500Zs. There are two 50 kohm 
x 50 watt resistors in series on the top side of the supply which are in 
turn in series with a 5K resistor on the bottom side. Each of the 50k 
resistors dissipates around 30 watts when the mode switch on the amp is 
in the SSB position.

###  Drakes ancient idea of using the V drop across the 5 k resistor
was not a smart move.  2650v /105k ohms = 25.2 ma .
.025 x 5k = 126 vdc   [ used to cut off tubes on RX]   On the lower
V  CW position, it's now  1900V / 105 k ohms = 18 ma. 
.018 x 5 k =  90vdc   [which also cuts the tubes off on RX]  

##  the 2 x big 50k bleeders diss 62.5 watts between em [31 w each]
on ssb pos... and only 32.4 watts  between em [16.2 watts each],
when on CW  position. 

##  I tossed ALL  3 x resistors, you don't need em!   OK, now you lost
the 130/90 v  cut off bias.   Better method is to just wire a 100 k
[or 2 x parallel 100k, for redundancy]   3 watt, mof resistor's in the
CT  of the fil xfmr.  You will see the yellow wire, that's the CT. 

## OK, now use the center pole of the stock 3pdt T/R relay,  and wire it
over to the newly installed pair of 100k 3 watt mof's. [COM  + NO] 
2 x wires required.  Ok, now when on RX, idle current flows through
the pair of 100k 3 w mof's.. and resulting V drop, CUTS THE TUBES OFF.

## Total DISS  in the parallel 100 k 3 w  mof's is  virtually zero watts. 
## the drake method is silly imo. When T/R relay is in ..'mid air' , the
Cathode is momentarily open for a split second.  Any time you open
the cathode, the cathode will try and rise to full B+  potential. 

##  U also end up with the center  pole of the T/R  relay, burnt
to a crisp, from hot switching the 130/90 V.. 'cut-off bias' 

##  Pole #1 used for 100w input.  Pole #3 used for kw output. 

## presto, you just eliminated 62 watts  of heat in the L4PS,
and now the B+  supply  runs stone cold, instead of blazing hot. 

##  I installed a  50 ohm 50 watt  WW  for a glitch R in the L4B itself,
one end mounted  directly to the red Millen, using SS internal tooth lock
washer, flat washer, etc.  Top end of same Glitch R is unsupported, does
not budge. New  5 kv rated red belden wire run from top of new glitch.. to
base of plate choke.  In this case, the HV interlock was tossed as well.
[ to make room]. All 4 x L4B's  were modified like this, zero problems.
glitch diss  with 800ma cxr = 32 W.  On ssb,it's 8W [400 ma average]
V drop across glitch = .8 x 50 ohms = 40 v   

##  In retrospect, the 50 ohm ,50W  WW glitch  could have been located
in the L4PS, outboard power supply. [ since both 100 k's were tossed]

My No load B+  is still exactly 2650 vdc.. with the 2 x 100k bleeder's
removed !   I was expecting it to rise, but it doesn't budge. [RX] 
The 8 x 100 k  3 watt MOF  EQ resistors, across the 8x lytics is plenty,
and does the EQ  and the bleeder function. 

Later... Jim  VE7RF  






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