[AMRadio] RF Amp

ARS W5OMR ars.w5omr at gmail.com
Thu Dec 7 07:00:13 EST 2006

On 12/6/06, Rick Brashear <rickbras at airmail.net> wrote:
>   I do want to be sure I pay attention to the
> comments about having ample and ten some when it comes to audio
> reserve.  I can always turn it down.

that's not the point.

The point is, to be free and clear of distortion and flat-topped audio
that causes distortion.

(John knew this was coming, so here it is.)

Read, my northern Texas friend, and see -why- I say that.

First, read this.  The theory behind it will startle you.

When that's comprehended, then read -this-...

John told me one time (when we were talking about audio) that a good
analogy would be like tossing a rock into a lake.  Sure, the waves,
the ripples go off in a rather symmetrical patter, once it's away from
the actual event.  When the rock hits the water, the wave pattern is
anything -but- symmetrical.   The same for the sound of your voice,
hitting the diaphragm of the microphone element.  If you talk far
enough away from the microphone, then your audio will be relativly
symmetrical.  But, if your like John and I, where we've 'always' been
right up close and personal to the microphone (a half-inch away or so)
then there's going to be a bombardment of asymmetrical spikes that hit
the mic element.

Now, let's follow that down the audio chain...

you've got a good microphone and a great speech-amp, and it's
faithfully reproducing your voice.  Great!  What happenes when it hits
that driving transformer?  Transformers have loss, and their
efficiency is sometimes not that great.  How much grid currnent is
there?  What's the load on the secondary of the transformer?  How well
is the audio being transferred to the grids?  Is the bias voltage as
stable as it can be?

Ok, let's say -this- has been accomplished and now we're coupling the
audio from the modulator to the final.  What's the Z of the final?
1500v @ 300mA is 5,000 ohms.  What kind of modulation transformer are
you going to use?  Does it have enough current handing capbility on
the secondary to withstand the current from the final -and- the aduio
you'll be producing? What's the P-P Z of the modulator going to be?

If he's still got one, I'd snag one of those big chokes from Brett.
70Hy at 700mA would do ya for a modulation reactor.

Except for the modulator I'm using now Rick, I think the best
modulator I had in my rig, was a pair of 811's, and one of those old
buzzard RCA 1:1, 5,550 ohm transformers.  The Z was low on the final,
and 811's will work into most -anything-.  I had butt-loads of audio
from a pair of 811's.  And, then I went and read the handbook.  BIG
mistake.  811's only produce 165w of audio in class B?  810s give 750?
  because the final Z was low, and the p-p Z of the 810s is high, it
took me 4 810's in push-pull parallel to get the same audio I had,
from a a pair of 811's.  Perhaps I had some bad 810s, but having those
4 sockets let me play a little bit... and I'd leave 1 tube in the
negative side, and place 2 in the positive side, and watch the spikes
on the scope.  a peak-baseline ratio of 4:1 on the envelope.  Audio
was plentiful, again! ;-)

I'd run the pair of 250TH's in the final, and modulate 'em with the
450's, personally.  You can get a half-gallon EASILY from a pair of
250TH's.  Using the 450's will just be icing on the cake ;-)

*I* used to think that AM was just inserting some audio in the B+
line, to obtain high-level plate modulation.  There's a -lot- more to
it, than that.

Operating your AM rig without a scope
is like driving our car at night without headlights.(~K4KYV)

73 = Best Regards

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