## It takes 9 kv to jump .25 inch. And 15 kv to jump .5 inch.
His 3650 vdc is not going to jump very far, something is resonant,
or a parasitic etc. Still, 27 / 31.8 uh, IMO is not enough uh for
use on 20M band. 50 uh always works.... and 1st series resonance is
35 mhz. Something is amiss, but he did have it fully functional on
160-10m. Something has changed, somewhere, and creating issues.
Jim VE7RF
-----Original Message-----
From: MU 4CX250B
Sent: Thursday, January 04, 2018 10:05 AM
To: Jim Thomson
Cc: amps@contesting.com
Subject: Re: [Amps] Plate Choke Issue
Hard to believe a flashover arc could jump over a quarter of an inch.
Maybe there’s a VHF/UHF parasitic of some sort. Did the ceramic just
overheat and crack, or did it shatter?
73,
Jim w8zr
Sent from my iPhone
On Jan 3, 2018, at 8:26 PM, Jim Thomson <jim.thom@telus.net> wrote:
Date: Thu, 4 Jan 2018 07:21:37 +1300
From: Steve Wright <stevewrightnz@gmail.com>
To: amps@contesting.com
Subject: Re: [Amps] Plate Choke Issue
On 04/01/18 04:28, AJ wrote:
This time I was able to measure where it blew apart, ~20 uh from end to
end. I slid the wire off the broken ceramic and slid it onto a new one
mounted it in the amp to measure. I am referring to RFC-1
<Well, 20uH in series with the tubes output capacitance is resonant on
<about 6-8 MHz.? That might be worth exploring..
<S
## Nope. Tube output capacitance between anode and grounded grid is
in PARALLEL with the plate choke, NOT in series with it.
## There is no way in hell a 27 / 31.8 uh choke will have its 1st
series resonance
in any HF band. A 50 uh choke, regardless of length to diam ratio,
will always
have its 1st series resonance at 35 mhz, well above the 10m band.
## Henry radio, in its 8 K ultra, using the 3CX-3000A7 tube, used a 2
piece plate
choke setup. Large choke was 180 uh, wound with 24 gauge wire. Small
choke
was just 20 uh, and wound with 18 gauge wire. Last production units made
used
a different 2 choke setup. The original 180 + 20 uh plate choke setup
was goofy at best.
## a tube arc on any of the 4 x 3CPX-800’s would not cause the
destruction of the
ceramic form he has the smaller choke wound on.
## His larger choke really requires 2-3 x 4700 pf ceramic bypass caps.
His smaller choke should have its uh increased to 50 uh. The smaller
choke
should ideally use 1-2 x 500 pf doorknob style bypass caps, like a
HT-50
or HT-58 series, which are available in 5 kv..and also 7.5 kv.
## I would like to know what bands each choke is used on, and what type
of relay he is using to toggle between chokes.
Jim VE7RF
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