[AMRadio] AM Amps

Gary Schafer garyschafer at comcast.net
Tue Jan 11 14:34:26 EST 2005

By "legal power out of the 813's" I assume you mean 375 watts carrier? 
If so there is no way the peaks are going to 1500 watts. A pair of 813's 
will not put out that much power in GG linear configuration. At least 
not very long.

A pair of 813's in GG are good for about the same power out as 4 811A's. 
If you run them with more than 150 watts carrier out you will be flat 
topping with modulation, unless you keep the mod percentage very low.

AT 375 watts carrier out at 33% efficiency that would require 1140 watts 
plate input power. That would be a plate dissipation of around 380 watts 
per tube! Cooked to well done in minutes!

Now if you tune them so there is higher efficiency, the plate 
dissipation will be lower, but then your mod peaks will never reach any 
where near 100%. Even if your driver is modulated 100%.

In AM linear service the efficiency of the amplifier must be 1/2 as much 
for the carrier as it is for the peaks. This is how the voltage doubles 
to obtain that peak power needed.

Without that ratio you will have distortion and splatter. As you noted 
when running 600 watts carrier out you must back the modulation down. At 
that power level you have almost no headroom at all for audio. You are 
at near maximum capability of the tubes with just carrier.

Gary  K4FMX

ronnie.hull wrote:
> This aint rocket science at my shack.
> I have a pair of trusty old 813's in grounded grid with 3200 Volts on the 
> plates. I drive them with my Ranger all the time with no problems at all.
> I always tune the ranger up to full power into a either a dummy, or my 
> dipole. THEN switch the 813's into the line and then quickly dip the ranger. 
> It usually will dip back to about 80'mils.  I leave it right there and get 
> legal power out of the 813's.
> If I want to bring the Ranger back up to full power, I can and it will drive 
> those 813's to 600 watts and sounds great.
> I have been using this combination for 6 years and many of you have heard me 
> on it. 
> If I DO bring the ranger back up to 120 mils, I have to be careful of the 
> audio drive, as it will tend to start to distort if the gain is up to high. 
> But if I leave the Ranger at 80 mils, it works fine and I work coast to coast!
> Gotta love those old 813's!
> 73's
> de W5SUM
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