As already reported by someone else , your plate choke was resonating in the
near of the frequency used. This is not a rare situation in large chokes
used to cover several octaves and it happens because of the existing
capacitance between turns, or because of a critical choke positioning inside
the amp cabinet.
It's not always easy to solve this type of problem and empirical
modifications often lead to bad compromises or symply shift a problem from a
band to another.
Winding an optimum plate choke from 1.8 to 30 Mhz is a quite complex job
expecially when the required current is high, the wire not so thin and the
choke becomes dimensionally large in size (expecially in diameter).
Some commercial amateur amplifiers use undersized chokes penalyzing
performances on some band (160) but acquiring safety on the remaining ones,
other designers "moved" self resonances in dead parts of the spectrum and
accepted a reduced efficiency when choke resonance occurs in the proximity
of a particular band.
A number of HF professional (military) amplifiers built for no compromises
switch automatically 2 or more plate chokes to cover properly the 1.5-30 Mhz
specrtum.
Since yours is an home made amplifier you can decide what to do, but unless
you drop to compromises (i.e no 160m) it will be not an easy and quick deal
to have a single and good plate choke good for 160m and without resonances
on higher frequencies.
73,
Mauri I4JMY
-----Messaggio originale-----
Da: Christophe Bajulaz <cbajulaz@yahoo.fr>
A: amps@contesting.com <amps@contesting.com>
Data: martedì 29 giugno 1999 13.03
Oggetto: [AMPS] Problem with Plate Choke
>
>Hello,
>
>I have built an All-band (excepted warc-band), 2000-Watt-output 8877
>linear amplifier.
>
>However, I have a problem with the Plate Choke, which is connected
>between the tube and the PI-L ciruit.
>I was testing the amplifier on 10M when the ouptut power (around 1KW)
>suddenly drop down. I immediatly switch off the amplifier and I could
>see that the Plate Choke was extremly hot, at such a point that the
>resin was running down the Choke !
>There's no need to say that the choke has burned out !
>
>I used a High power manufactured Plate Choke (B&W BBC-5K):
>Power Rating 5KW
>Maximum DC voltage 5KV
>Maximum DC current 2A
>
>
>None of these characteristics was exceeded (1KW output, 4KV DC under
>800mA).
>
>I can't understand the reason of this incident.
>
>Best 73's de F5AVK, Christophe
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