You don't necessarily need a very high voltage to get a long arc.
If the choke became resonant and burnt the
vaporized copper wire or even a flame can produce a plasma path for the arc and
things get even worse.
The conditions for arcs suddenly change once you toss in some ionized
molecules.
I have had some spectacular arcs and electrical flames in some amps I built.
Mostly
when I was much younger.
I once built, just for fun a 4x 837 grounded grid amplifier that I drove
with my Viking Challenger.
I did not have a case, just chassis and front panel. I had set my over current
relay shunt resistor to zero.
And as a kid I took some short cuts. My power cord was a zip cord and no fuse.
I used a surplus
multi section pi wound choke. The power supply was capable of several amps.
Made from some surplus
power transformers from a old tube digital computer.
As I was tuning it up I noticed the output was fluctuating and suddenly a
bang and ZOONK!!! and a flame
shot more than a foot into the air and did not stop. I reached for the power
cord which was hot by this time
and yanked it out of the outlet.
I had hit a resonances in one of the pi wound sections of the RF choke and
all that was left was a vertical
ceramic form and a pile of hot stuff on the chassis. I scraped the stuff off
the chassis with a putty knife and wound my own choke and got it working
properly. But the flame was spectacular. I made a few contacts and set is aside
and went on to some other project. I used to do a lot of tinkering as a
teenager.
73
Bill wa4lav
________________________________
From: Amps <amps-bounces@contesting.com> on behalf of Jim Thomson
<jim.thom@telus.net>
Sent: Thursday, January 4, 2018 2:33 PM
To: MU 4CX250B
Cc: amps@contesting.com
Subject: Re: [Amps] Plate Choke Issue
## 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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> Amps@contesting.com
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