[Amps] Plate Choke Issue (AJ)

Chris Hays chris at chrishays.com
Wed Jan 3 13:26:38 EST 2018

First thing to do is a careful inspection of all components: parasitic
suppressor (Z1-Z4) resistors for signs of overheating (cracks and
discoloration), coupling capacitors, and band switch contacts for signs of
arcing (and any components associated with the 20 m position. Basically we
are looking for anything that might cause a parasitic oscillation or arcing.
Also verify the 1000pf coupling caps are in spec.

If everything looks good, I would suspect a gas arc in one of the tubes.
since there are four of them it is hard to test unless you have a set of
known good tubes.  You could try running with 3, pulling them one at a time
to try to figure out which one it is. 

Our other experts here may have other ideas.

Chris AB6QK

-----Original Message-----

Message: 1
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
Message-ID: <20180103102854.1954a5e3 at linux-tiep.site>
Content-Type: text/plain; charset=US-ASCII

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?



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