[AMRadio] Ongoing Quest: Legal Limit AM Amp

[W2XJ]

I really think AM is best done by modulating the final in some fashion. 
Even the older rigs that had high level modulation did not have the best 
iron for solid AM. Later it became the fashion to reinsert the carrier 
in an otherwise SSB design and all audio limitations that go with that 
including ALC issues. With current digital rigs it is often unknown how 
the AM is generated.

Unfortunately there is really no off the shelf solution. In a high level 
modulated transmitters 833s and 4-400s were the most common tubes used 
over the last 50 years or more for 1KW continuous duty broadcast band 
transmitters. A pair were used in the final to achieve 1KW carrier and 
4KW 100% peak. Those tube both have 400 watt plate dissipation. 500 watt 
version of those designs used a single tube. Almost any tube compliment 
discussed in available commercial amplifiers is more than adequate high 
level modulated AM if an external modulator is available to provide the 
additional power. A single 3-500 delivers almost twice the usual carrier 
power required for AM operation so many existing grid driven linears 
have enough tube capacity. The necessary modification to the amp would be:

A modulation choke between the HV and the tube(s) plate connection

Reduction of the HV supply voltage and a change of grid bias

and an external modulation source with it's own power supply. This would 
be homebrew but easier (and safer) to build than an RF amp. Staying 
within the realm of practicality, it an older RF amp could be converted. 
This means you use the chassis, tube sockets and power supply. Because 
there are two power supplies involved and the main amplifier is being 
run at reduced power, the light power supply problem is solved as well.

Take whatever transceiver is available an put it in CW mode to drive the 
final.

On 1/24/12 9:25 AM, Rick Poole wrote:
> Just a few thoughts to start the day...
>
> 1.  I know that the difference between a DX-100 and the legal limit
> is about 1 "S" unit.  But, an "S" unit is an "S" unit.  I have seen
> plenty of times, although on SSB, where an "S" unit made the
> difference between, say, phone-patch quality (for running traffic for
> boats at sea) and not.  Not that I'm going to be running phone
> patches on AM, just using that as an example.
>
> 2.  QSL on the part about using the best antenna... plan is to do
> that and THEN add power.  Current antennas are a 160-meter inverted V
> at 60 feet with open-wire feeders, and a half wave coax-fed
> off-center-fed dipole at 35 feet.  They both work pretty well, but
> the plan is to get both of those up higher, in the spring.
>
> 3.  I considered using a commercial pair-of-3-500's amp like the
> SB-220 or TL-922, but the ones I've looked at appear to be too
> power-supply limited to get true legal limit especially on AM.  I
> have a TL-922A and get 1200 watts PEP out of it at best, on
> SSB.  Yes, I know that there is no measurable difference between 1200
> and 1500 but per (1) above, if I can't get at least X4 amplification
> (1 "S" unit) then it's not worth doing.  Anyway, nothing I can buy
> for less than around a thousand bucks is likely to be very happy
> running on AM for very long.  Now, a Chippewa... :-)
>
> 4.  I don't "have to ask for a schematic to build one of the simplest
> pieces of ham gear" but it has been more than 40 years since the last
> time I home-brewed a high-power amp (or much of anything else, for
> that matter), so if an article was available on the web that
> describes a reasonable design AND perhaps offered suggestions on
> where to get the necessary parts, that would be very helpful.
>
> 5.  I'm acutely aware of the dangers of high voltage.  I'm currently
> restoring a 30L-1 including rebuilding its power supply, and I'm
> comfortable that I know what I'm doing and will still be here when it's done.
>
> Rick WA1RKT
>
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