Date: Wed, 1 Mar 2023 16:01:05 -0500
From: Steve Bookout <steve@nr4m.com>
To: amps@contesting.com
Subject: [Amps] rectifier diode strings
<What is the current thinking of using an equalized resistor, a cap for
<transients, and the diode.? I seem to remember reading that because of
<modern manufacturing, either the resistor, or the cap, or both, were no
<longer needed.
<Can someone elaborate, or send me to the right place.
<73 de Steve, NR4M
## I just went through all this with W8JI, The resistors are not
required. In fact, they will make it worse in most cases, unless you are
using 1% Tolerance resistors..which are stupid expensive. . Modern diodes
don't require the resistors. I was buying 1N5408 diodes back in 1977....
and they didn't require resistors. Tolerance of modern diodes is very
tight. The trick is not to mix diodes from one lot, or type, or brand,
with any other...in the SAME string.
You could use 1N5408's for one leg..and 6A10's for the next leg etc.
W8JI sez they used caps across each diode, because the cost was dirt
cheap. Without the caps installed, they could hear weak junk on 160m, on a
quiet ant..and sometimes 80m...or a DL. Per Ji The ameritron amps had to
have the caps installed, in order to pass the stringent European CE
specs. And all that testing was done in the UK and Europe. He said they
are talking abt ultra weak crud. And he sez u won't see it with a scope
across a diode, nor a spectrum analyzer.
JI sez the other place to put a .01 uf cap is at the cold end of the plate
choke . (bypass cap at base of plate choke). Typ value at base of a plate
choke is only 1000 pf. (001uf). A 1000 pf bypass cap is no where big
enough in value as a bypass cap. JI sez only 1/2 the residual RF coming
down the plate choke is bypassed to chassis..the rest goes into the HV
filter caps. The issue was cost. Any ,01uf bypass cap at the cold end of
a plate choke has to handle full B+, so a 10/15/20 kv rated disc would have
to be used...and they cost $2.20
Whereas the .01uf @ 1 kv disc were like a penny a piece. (bulk buying 100K
at a time)..and only 20 cents worth of caps required. ( 5 x diodes per leg
on a FWB, on an ameritron). .
KM1H came out with another idea, put the .01uf @ 5 / 7.5 / 10 /15 kv disc
across an entire leg on the FWB / FWD. 2 required for the doubler..and 4
required for a bridge. Again, double / quaduple the cost of just one
.01uf @ 5 / 7.5 / 10 / 15 kv cap at the cold end of the plate choke.
$10.00 vs 20 cents.
Short of it is, you don't need resistors....and myself, I would install
1-2 x .01 uf caps at the base of the plate choke vs a cap across each
diode / entire string. I just got 3 doz .01uf @ 30 kv disc caps from
china..via e bay. Huge things, 7/16" thick x 1.25" diameter. Cheap.
Then at least residual RF coming down the plate choke will be properly
bypassed, vs 1/2 of it flowing into the HV caps. That plus kill any
residual low band hash / crud.... moot point since my noise level is too
high to hear it.
.001uf = 88 ohms of XC @ 1.8 mhz.
.01 uf = 8.8 ohms of XC @ 1.8 mhz.
.02uf = 4.4 ohms of XC @ 1.8 mhz.
.03uf = 2.9 ohms of XC @ 1.8 mhz.
.04uf = 2.2 ohms of XC @ 1.8 mhz.
Jim VE7RF
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