On Mon, 2006-05-01 at 08:56 -0500, Grant Youngman wrote:
> > There's this mythical idea hams seem to have, that decent AM
> > can only happen
> > with a big plate modulated tube rig.
>
> It is mythical. While a big transmitter will keep your room warmer in the
> winter and give you more exercise when you move it around, low power DSP
> generated AM or AM from something as pedestrian as a little Heath DX-60
> (modified), boosted in an ordinary amp can be indistinguishable from a
> power hungry, 1000 lb, room warmer to the guy at the other end.
NO! NEVER!
The overall efficiency of the high level plate modulated rig is better
than any linear so there's less heat in the room for the same power
output.
It takes lots of modifications to the DX-60 to produce AM that can't be
distinguished from the high level modulated rig because the screen
modulation used controlled carrier and so shifted the carrier level to
match the modulation level and that shows on the receiver S-meter. At
least it did 30 to 50 years ago...
>
> Of course you'll miss the warm red glow of the finals and modulator toobs,
> but there are other ways to stay all warm and cozy :-)
>
> Grant/NQ5T
>
You still get the warmer glow from the linear tubes running
inefficiently to make the same carrier and peak output as the high level
modulated rig did and probably have as much iron in the power supply
unless its modern grain oriented steel.
A linear that IS running linearly rarely does better than 55 or 60%
efficiency at output peaks, poorer at AM carrier level while a Class C
stage can do 75% with decent tubes (more with 304TL or third harmonic
peaking of the drive and output tank). These efficiencies you can only
improve on with lots of distortion from the AM linear.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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