A while back I went through the calcs in the Amidon reference book
trying to find a way to stack a bunch of Mix 2 powdered-iron cores for
the 160m "helper" inductor in a legal-limit project.
I don't remember the specifics of the tank offhand, but I do remember
that no matter how I juggled things I couldn't come up with a stack that
would hold up without overheating under at least some of the conditions
I set up. Those conditions include 50% overload for tune-up purposes, so
I suppose you could say that it was a self-fulfilling expectation that
it would fail. So be it - in a conservative design where size, weight
and noise (and WAF) aren't the primary considerations I believe 50% for
tuning and occasional mis-tuning is reasonable.
My .02,
Jim, N7CXI
On 2/3/2011 4:02 AM, Jim Thomson wrote:
> Date: Mon, 31 Jan 2011 10:52:57 -0500
> From: "Carl"<km1h@jeremy.mv.com>
> Subject: Re: [Amps]To: What to buy?
>
> Hmmm, ferrite has never been recommended for RF tank circuits that Im aware
> of. Ferrite is fine in receiver and other low level applications mostly at
> low frequencies.
>
> Good amplifier engineering has used powdered iron in various mixes,
> generally from Micrometals in North America.The #2 mix is the usual choice
> in amps for 160-80M and has been in use since the late 70's.
>
> There are huge differences between the 2 materials in regards to losses and
> temperature stability.
>
> Carl
> KM1H
>
>
> ----- Original Message -----
> From: "Jim Brown"<jim@audiosystemsgroup.com>
>
>>> How are things different in a transmitter's resonant circuits? First,
>> we want to minimize loss, so we (should) use low loss cores (like
>> Fair-Rite #61 or #67). These are NiZn cores, and have much lower mu than
>> those designed for RFI suppression (like #31 and #43).
>>
>> Inductors wound on #61 or #67 can have Qs on the order of 10 or more,
>> and are typically self-resonant a bit above HF.
>> Note also that my work has focused on ferrite materials, NOT powdered
>> iron.
>>
>> 73, Jim Brown K9YC
> ### Ferrite would not last more than 30 x seconds in any tank circuit, in
> any HF amplifier. Type #2 mix [ red cores] powdered iron cores are what is
> normally used. Beware that there are TWO types of T-225 cores... lableled
> 225-2A and 225-2B. The A version is only 1/2" thick. The B version is
> 1" thick.
>
> ## For 160m use.. typ 3 x 225-2B's are stacked... so the entire mess ends up
> being
> 2.25" OD x 3" long..and very heavy. That's before you add the wire. I
> modified one
> of my hb amps..and added 160m to it. It was a lot easier to just use a small
> piece of
> air-dux with 12 ga wire.. vs 3 x torroids. Interesting enough, I found
> some air dux
> that was 12 ga... but with slightly closer spaced turns vs the 'normal 12 ga
> airdux'.
> Now this stuff worked ideal for 160m...since it was used on 160m only.
>
> ## The original 80-10m rf deck now is 160-10m. The new close spaced 12 ga
> air-dux
> coil was the ideal ticket. Weighs virtually nothing..and ZERO heat. The 3 x
> torroid
> approach weighs a ton..and dissipates aprx 57 watts. 2 x torroids will also
> dissipate 57 watts.
>
> ## I have given up on torroids for tank coils on the low bands. I have found
> some small diam, edge
> wound ribbon coil stock that works the best for low band work. 3/8" edge
> wound ribbon [.072" thick]
> is the same as .28" tubing for current handling. 1/4" ribbon [ also .072"
> thick] is ample in most cases
> and is the same as .2" OD tubing. A friend also gave me a bunch of air
> dux.. that uses 8 ga wire.. and the
> wire was copper..and not the usual tinned material. These are things of
> beauty. I didn't know you could
> get coil stock in 8 ga wire.
>
> Jim VE7RF
>
>
>
>
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