[AMRadio] AM Transmitter Advice??


Jim Candela jcandela at prodigy.net
Fri Feb 17 07:29:29 EST 2006



Hi All,

   I recently had a conversation with a ham in San
Antonio (forgot his call), and he told me that he
built a unique linear amplifier for his central
electronics 10a, and for tubes he chose 3 type 12AX7
in parallel grounded grid. He said he could run 30
watts input (300v @ 100ma) with no problems, and about
20 watts out. I find it odd though that a 10a can do
10 watts with a single 6AG7, and 20 watts is only a
3db boost. 

To my way of thinking, a linear amp needs to boost
your power at least 6 db (~1 'S' unit) to be worth the
trouble. For us AM'ers, going from 100 watts to 375
watts carrier does not meet the 6 db boost criteria,
and that explains why a good antenna on a DX-100 is
better than a average antenna on a Globe King 500.

Still, as I once posted last year, a dual 304TL
grounded grid linear seems to fit the bill as a 6 db
'brick' capable of 400 watts AM carrier output with
100 watts AM input, or said another way it takes 400
watts PEP and boosts it to 1600 watts PEP. There was
an old W6SAI construction project about this (single
304tl GG amp), and I recall that the setup in class C
could run 1 kw dc input with over 1 kw rf output
because of the low gain, and massive amount of
feedthrough power from the exciter that finds it's way
to the output. This was a way around the FCC power
rules of the day.

Regards,
Jim
WD5JKO

--- Donald Chester <k4kyv at hotmail.com> wrote:

> >Assuming one is going to build a linear, and so
> putting aside other issues
> >such as linear vs plate modulation, why do you
> think it makes a difference
> >what tube is used? Are you referring to running a
> linear at greater than
> >legal limit?.
> 
> Well, go ahead and try building a legal limit linear
> that runs a pair of 
> 807's in the final.
> 
>
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