[AMRadio] Amplifier to use with my DX-60

crawfish crawfish at surfmore.net
Thu Mar 16 20:52:36 EST 2006

The SB-200 uses two 572B's. The SB-220 is the amp that uses two 3-500Z's.
                                Joe W4AAB
----- Original Message -----
From: "Donald Chester" <k4kyv at hotmail.com>
To: <amradio at mailman.qth.net>
Sent: Thursday, March 16, 2006 3:48 PM
Subject: RE: [AMRadio] Amplifier to use with my DX-60

> >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
> >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
> rating.  Exceeding the peak output rating will cause the signal to
> 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.
> modulation, the tubes will actually cool down slightly, since the DC input
> will not vary, but the amplifier will deliver sideband power in addition
> carrier power output.  So some of the input power will be converted to rf
> the sidebands instead of heat in the tube plates.
> But you also have to be careful with the power supply.  AM runs at 100%
> 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
> 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
> "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
>   The earliest high power broadcast stations used it.  It was used for
> 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
> 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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