[AMRadio] Amplifier to use with my DX-60


Mike Dorworth, K4XM k4xm at arczip.com
Thu Mar 16 17:09:59 EST 2006


Short answer. The SB200 uses a pair of 572B/T160L rated at 160 watts each.
AM Linear  output can not be more than one half of total dissipation. The
power supplies are usually rated for continous service (AM) at 25 percent of
the peak. For SB220 a 400 watt transformer is used for 2000 watts pep input.
The SB200 is rated at 1200 watts pep input and would have about a 300 watt
transformer. If the supply were strong enough the SB200 could run 160 watts
carrier. Best to stick with one half of that..80 watts. For the SB220, L4B,
TL922A best to stick with 250 watts carrier, or less. The answer below is
exactly correct except for the tubes in the SB200.. The SB230 uses a
conduction cooled 8873 with about 300 watts dissipation, 80 watts carrier
would probably cook it in short order because of the non-cooling system heat
load. 73 Mike
----- Original Message -----
From: "Donald Chester" <k4kyv at hotmail.com>
To: <amradio at mailman.qth.net>
Sent: Thursday, March 16, 2006 4:48 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
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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>
>
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