|[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
> 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.
> This message was typed using the DVORAK keyboard layout. Try it - you'll
> like it.
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