[Amps] Plate Choke Issue

[Jim Thomson]

Date: Wed, 3 Jan 2018 10:28:54 -0500
From: AJ <iamfromcanadaalso at gmail.com>
To: amps at contesting.com
Subject: [Amps] Plate Choke Issue

<Hi people.

<I have some sort of issue I have not been able to figure out or do not
<understand the issue. I checked the bypass values the relay the wires
<etc.

Ok here's the skinny, multi-band amp. Works fine on 160,80,40.
It was fine on all bands at one time; do not know what happens on 15 or
10 at this time. Tune the amp for ~2200 on 20 meters all is fine; get up
to 2600 obviously into the load. And kerbang! ! ! Blows the ceramic and
high voltage arc to some part of the chassis and a corona burn off of
the wire. 

Get a new ceramic insulator ( .75 x 4") as large as there is room. 
Wind 25 awg wire and ~65 cs turns on it. Fasten the ends to machine 
cir-clips. Measure inductance in the amp mounted 31.8 uh. Looked for
any series, checked for heating with 2 amps of current etc. Checked the
relay for function.

Well it lasted about 3 months, kerbang ! ! wife was in the room when it
happen. muwahahaha actually it really was not funny, 3650 vdc at the
choke, ~1 inch to any part.

So I dropped the number of turns now 27 uh, well that lasted about a
week, not sure how many tx's; again on 20 meters.

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

pic is here http://oi67.tinypic.com/2lwmb5u.jpg

What am I missing?


AJ___ VE3HJ

##  That is a VERY slick plate choke  switching arrangement...dunno why I didnt even think of the
unique scheme u used.... simple and effective..hats off to you.    And a helluva lot better than
what is normally used for a 2 piece plate choke switching setup.  IE:   big and small  choke in series,
at right angles, with a bypass cap from junction of caps to one side of a relay, with other side of same 
relay to chassis.   Then  3 x 1 meg resistors in series, from cold side of  bypass cap..to chassis.  
Then another bypass cap from  base of big choke..to chassis.   Even on that setup, both chokes in series
can only be used on 160m.   Small choke used on 80-10m.   Relay  RF grounds the cold side
of the bypass cap hanging off the junction.   Resistors are there to   DC  ground the cold side of the caps.
If the resistors are not there, the relay will arc badly. 

##  On ur scheme, u have the cold ends of the big and small chokes bonded in parallel..... and ur  spst vac relay
just toggles the top ends of the chokes.... to the  anodes.  

##  I would suggest using a 50 uh choke for your small choke.   That will series resonate at 35 mhz every time..and provide
loads of Z on 20m.  Since its ur ceramic that is exploding, I suspect the mounting scheme at each end of the small choke. 
IE:  IF set screw is tapped into the ends of the ceramic used for the small choke, the  3650 vdc  could possibly be arcing
through the ceranic..into the grounded set screw ?  

##  I typ use  1 inch solid teflon  rod for both the big and small  plate chokes, then tap  chassis  end with a 1/4 –20   or  
1/4-28  SS  machine screw.    Then a smaller brass screw, like a 8-32,  or 6-32, transverse mounted, side ways through
the teflon, to terminate the windings.  

##  what are u using for the relay, a vac relay , like a spst,  15-25 kv type ?   It might be cuz of the typ 1-3 pf between
contacts of the relay that is creating issues.   IE:  when small choke is used, 1-3 pf is there, between top of small choke,
and top of big choke, creating a resonance.  But u previously had this all working on all bands.  

##  Ok, where does the bigger  255 uh choke..series resonate ???    What  bands is the big choke used on ?    What bands
is the small choke used on ?? 

Jim    VE7RF