[AMPS] Smallest practical 1.5KW 20uH coil
Radio WC6W
wc6w@juno.com
Wed, 26 Jan 2000 23:27:39 EST
On Wed, 26 Jan 2000 22:27:16 +0100 "Harald Garberg Gule"
<harald.gule@mimer.no> writes:
>
>Hi all
>
>I have been following this tread with great interest. I have also
beenthinking
>abouth using toroids in the tank cirkuit, as they seem to offer at least
space
>saving whenever that is a problem.
>For me it would be more expensive to use , since I have to buy them,
>but have enough surplus of both nice L's and C's.
>
>I must admit I was becoming afraid at using toroids after a few initial
>comments to this tread. I was getting really confused since successful
producers
>have been using them a long time both for commercial and
>amateur service and suddenly it almost appeared a no-no.
>If the producers save money thus giving them a better profit,
>and hopefully giving their customers good value it's all right with me.
>
>Thus I appreciate the the helpful ideas put forward recently.
>
>For helpful hints on making toroids you can find a practical approach
>article in Ham Radio, August 85 by Robert E Bloom, W6YUY
>
> Design a toroidal tank circuit for your vacuum tube amplifier.
>
>
>Harald, LA6LHA
>
Hi Harald,
I met Bob (W6YUY) when we both worked at the same company in the early
'80's.
He installed that tank described in the article in an amp that used a
pair of 4CX250F's. I don't think he was as impressed with the
performance as the article might imply. He didn't attempt to reprise
this method with his 8877.
Bob passed away about 8 years ago and I "inherited" one of his coils
constructed as per the article based on a T400-2D core. He wound it with
20 turns of #14 wire which I would consider rather smallish for the
higher (HF) bands. There were taps at every turn for experimental
purposes. The full coil measures 15 uH. I think that the casing method
would exacerbate any core heating problems.
There was an earlier article, "The Whole of the Doughnut" by E.L. Klein
in the June '67, 73 Magazine, that claimed 1KW capability with just a
pair of T-200's.
The other references I've seen for this method all dissuade from using
one of these as a full (hi-impedance) tank coil. The Radio Handbook,
22nd & 23rd edition, mention the limited capability of these cores in
this application. And there is an article, "Iron Powder Cores for RF
Power Applications" by Jim Cox (of Micrometals) in RF Design, February
'87 which confirms these limits with lots of math and graphs.
The problem is the (high) tank circulating current. This makes the
toroid core a poor component for a traditional Pi-net. The worst
condition would occur, in a tune situation, with the loading cap fully
meshed where all of the power is wandering around in the tank.
However, if the tank Q is low enough, which is possible with
(relatively) low impedance tubes and/or a Pi-L network then the approach
becomes... marginal.
The cores do appear to be useable for a portion of the Pi network or in
the L section which is relatively low Q.
Some of the proponents of this approach may have overlooked the fact
that the cores have considerable thermal mass and that, plus typical low
duty cycle SSB operation, may make them appear to be more efficient than
they are in reality. A one hour key down CW test would tell for sure!
Of course, not many amplifiers can survive that nowadays. I'm certain
that, shortly, a (short) list will be posted to the reflector in reply to
this last comment. :-)
The short answer: Remain afraid! ... or at least cautious and...
calculating.
73,
Marv WC6W
*
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