[AMRadio] Amplifier to use with my DX-60

abeck at xplornet.com abeck at xplornet.com
Thu Mar 16 21:12:13 EST 2006

Thanks so much for taking the time to remind me about all this stuff!!!

When I was 25 I worked on a Gates 50B that was as old as I was. I mostly just 
monitored voltages and looked for trouble.

I was facsinated back then as I find myself being now while I foray into the 
world of AM again.

I spent too much time in Broadcast as a studio tech.



Quoting Donald Chester <k4kyv at hotmail.com>:

> >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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