[AMRadio] 575A's

D. Chester k4kyv at charter.net
Mon Oct 5 11:20:52 EDT 2009

Some of the older, "built like a brick Scheiße-Haus" kw broadcast 
transmitters, used 575A's to run 2500-3000 volts to 833A's.  But the later 
"Lite" versions of the 833A kilowatt rigs by the same manufacturers used 
872A's, which seemed to work OK up to 3 kv or so.  IIRC, the PIV of 575A's 
is 15 kv, while that of 872A's is 10kv.  10kv piv should be ok up to 3100 
volts DC using the full wave midtap circuit.  Apparently when the 
manufacturers cheapened their rigs, they decided the extra 50% safety margin 
was unnecessary, and with good 872A's, rectifier flash-over was never a 
problem if they were operated properly.  I believe 575's and 872's run at 
the same filament voltage, so they should be directy interchangeable.  Four 
872A's (or 4B32's) in a full wave bridge should be good for about 6 kv at 1 

I haven't tried to build my own solid state  rectifier stacks for many 
years, but back in the 60's and 70's I never could get them to work very 
long before having a firework display, no matter what equalising resistors 
and by-pass capacitors I used, or how many extra diodes I included in the 
string for safety margin.  So, with several bushels of mostly used 866A's, 
3B28's, 872A's and 4B32's on hand and plenty of filament transformers to run 
them, I have always stuck with hollow state rectifiers.  I use commercially 
built, direct replacement solid state 872A's and 866A's in my Gates BC1-T 
and after 5 years they have never given me any trouble.  I replaced the 5R4 
bias  rectifier with diodes purchased brand new (don't recall if I used two 
in series for each leg or just one in each leg) but I used the highest piv 
diodes I could find and so far, no problem with those, either.  I solid 
stated the bias rectifier to free up the octal  socket, to hold the 6AS7G I 
use as a DC regulator to isolate the protective bias tap that allows me to 
run the Gates on CW.

Don k4kyv

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