[AMRadio] My Amplifer Quest took a strange turn today

Bob Bruhns bbruhns at erols.com
Mon Apr 23 20:46:01 EDT 2007

Hi Bow,

Linears are tuned for peak power.  The idea is that the system is set up so that the tube conducts and pulls the plate voltage
down to minimum, a few hundred volts, on negative RF waveform peaks at full power.  This is where you get maximum efficiency and
maximum output.

When you tune, you apply the necessary drive and tune the tank to load the tube(s).  You can do it with a steady carrier, or
with a pulsed or modulated waveform, but with a steady waveform, a linear is easier to adjust with simple tools (grid and plate
meters and maybe an output wattmeter).

Basically you apply the right amount of drive for the desired peak output, and tune and load for the right plate current and
output at the peak output level.  (With a pair of 813s this will be around 1 KW, but for AM with a pair of 813s you might want
to start out at 500 watts peak because of plate dissipation.)  Usually people load a linear a little heavily, which reduces
efficiency slightly but improves linearity.

Then back off on the RF drive, and RF output falls.  Set the carrier level around 50% to 40% of the plate current compared to
the peak level you just set.  This allows for about 100% to 150% modulation respectively.

That also means carrier output is 1/4 to 1/6 of peak power, respectively.  If you tuned for 600W peak, and you backed off to
50%, then your carrier output will be about 150 watts for an input of about 450 watts, for a carrier dissipation of 150 watts
per tube - a bit beyond spec, but probably doable with a fan or two. If you tuned for 600W peak and backed off to 40%, then your
carrier output will be about 100 watts for an input of about 400 watts input, for about 150 watts dissipation per tube.  AM
linear is not too efficient.

To be within the plate dissipation spec for 813s, you would tune for 500W peak and run about 125W of carrier output at 100%

Linears with better carrier efficiency are very complex.  60% and higher carrier efficiency is possible with trick designs, but
they are difficult to tune.  Plate modulation is much simpler, and a pair of 813s can produce 500 watts or more of carrier

You can certainly set up a linear and convert it to plate modulation when you get the additional parts for that.  That's what a
lot of people do.

   Bacon, WA3WDR

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