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Re: [Amps] Plate Choke Issue

To: "Fuqua, Bill L" <wlfuqu00@uky.edu>, "MU 4CX250B" <4cx250b@miamioh.edu>
Subject: Re: [Amps] Plate Choke Issue
From: "Jim Thomson" <jim.thom@telus.net>
Date: Fri, 5 Jan 2018 09:14:56 -0800
List-post: <mailto:amps@contesting.com>
##  on his original post, its the ceramic form that bit the dust. 
On his last blow up, he slid the intact windings off..and onto a
new form, which lasted one week.   20-25 uh is what would be used
on a 6M,  50 mhz amp, not a  20-10m amp.   He needs a bigger plate
choke  for  20-10m,  50 uh would be ideal.   He has a total of 2 plate chokes,
with B+  being fed to the cold ends of each plate choke.  His bigger choke
is 255 uh, small one is  now 27 uh.  Base of both chokes is bypassed  with 
a pair of 2500 pf  at 15 kv caps.   A spdt relay switches between the top
of each choke.   http://oi67.tinypic.com/2lwmb5u.jpg     What he didnt say
was the spacing between the large and small choke, etc.   It all worked at one
time, on 160-10m.   The idea behind the 2 x choke setup is the bigger one is 
used on
160-80-40-30m.    Smaller one is used on 20-17-15-12-10m.   It should work, and 
there
should be no issues.   But his smaller choke was only 31.8 uh...now down to 27 
uh. 
31.8 /27 uh is too low a value to be used on 20m band.

I have seen whereby just one strand of cat hair can cause an arc.  I have also 
experienced on hb
HV fuses, where the plasma vapour deposited itself to the ceramic stand off 
insulator..right from top to
bottom. 

Jim   VE7RF


From: Fuqua, Bill L 
Sent: Friday, January 05, 2018 12:39 AM
To: Jim Thomson ; MU 4CX250B 
Cc: amps@contesting.com 
Subject: Re: [Amps] Plate Choke Issue

   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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