[Amps] Plate Choke Issue
jim.thom at telus.net
Thu Jan 4 14:33:02 EST 2018
## 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.
From: MU 4CX250B
Sent: Thursday, January 04, 2018 10:05 AM
To: Jim Thomson
Cc: amps at 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?
Sent from my iPhone
> On Jan 3, 2018, at 8:26 PM, Jim Thomson <jim.thom at telus.net> wrote:
> Date: Thu, 4 Jan 2018 07:21:37 +1300
> From: Steve Wright <stevewrightnz at gmail.com>
> To: amps at 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..
> ## 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
> was just 20 uh, and wound with 18 gauge wire. Last production units made
> 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
> should ideally use 1-2 x 500 pf doorknob style bypass caps, like a
> 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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