[AMRadio] Amplifier to use with my DX-60


Donald Chester k4kyv at hotmail.com
Thu Mar 16 16:48:26 EST 2006




>From: Alan Beck <abeck at xplornet.com>

>I would like to use a cheap am with my DX-60.
>
>An SB-200-230 seems to be a Class B amp. There for it only conducts on the 
>positive going cycle. I don't mean to sound silly, but someone told me I 
>could run this in SSB Mode using AM input from my DX-60, I run 100 Watts 
>carrier for 400 Watts peak, now that makes sense.
>
>What does not make sense is how do I get the other side of the wave 
>form???? The Tank??? I guess the tank.
>

The linear will work on AM as long as you don't exceed the peak power output 
rating.  Exceeding the peak output rating will cause the signal to flat-top, 
distort and splatter.

Another thing to watch for is the plate dissipation of the tubes.  If I 
recall correctly, the SB-200 series uses a pair of 3-500Z tubes in the 
final.  That means you have 1000 watts of plate dissipation available.  
Running AM linear @ 100% modulation will give carrier output efficiency of 
about 30%.  So you could run maximum 1500 watts DC input to those tubes, 
with 500 watts carrier output, and 1000 watts dissipated by the tubes.  With 
modulation, the tubes will actually cool down slightly, since the DC input 
will not vary, but the amplifier will deliver sideband power in addition to 
carrier power output.  So some of the input power will be converted to rf in 
the sidebands instead of heat in the tube plates.

But you also have to be careful with the power supply.  AM runs at 100% duty 
cycle, so the power supply in the amplifier may not be rated to run 1500 
watts continuous duty.  After a few minutes, the power transformer may 
overheat.  In that case you will have to run it at reduced power.  But be 
careful that the plate efficiency does not exceed about 33%.  If you run it 
at too high plate efficiency, it will not leave you enough headroom to 
accomodate the positive peaks, and flat-topping/distortion/splatter will 
result.

Don't worry about the missing half of the rf cycle.  It works with AM 
exactly the same way as it does with SSB.  Since the amplifier is single 
ended and not pushpull, the missing half of the rf cycle is filled in by the 
"flywheel effect" of the rf tank circuit.

In summary, with class-B linear AM operation, the final will run about 33% 
carrier efficiency.  The peak efficiency on modulation peaks will be about 
double that, 67%.  Two-thirds of the DC input to the final will be 
dissipated as heat in the plates of the tubes under carrier-only, no 
modulation conditions.  That means the carrier output will be one half the 
plate dissipation of the tubes.  The peak power output should be about 4 
times the resting carrier output at 100% modulation, if flat-topping is to 
be avoided.

Linear amplifier AM operation dates back to the very earliest days of radio. 
  The earliest high power broadcast stations used it.  It was used for years 
before anyone figured out how to run audio amplifiers in class-B.  Before 
then, the only kind of high level plate modulation that was used employed 
class A audio amplifiers, usually the "Heising" circuit but sometimes series 
modulation was used.  Both those systems ran at lower ovarall efficiency 
than linear rf amplification.  Therefore, AM linears were used long before 
high level plate modulation for high powered AM transmitters.
_____________________________________________________________

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