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HF Amplifier Design Now
Better than ever?
by Bob Hutchinson, N5CNN

OK, let's get this straight up front. The real power in HF Amateur radio comes from the linear amplifiers. As young men many of us were taken with powerful, loud, rotating machinery, such as the internal combustion engine type devices - cars, boats, motorcycles, airplanes, etc. Since the average Ham today is about sixty years old, I might suggest that we don't appreciate the same type of power and noise as in our younger days.

Now we appreciate power in a different way. I love these amplifier things and still enjoy solder melting on occasion. I am examining here the design changes since Big Brother FCC changed the legal power limit rules, effectively increasing the allowable power output of these wonderful electronic appliances. I also will suggest changes that will help us continue to enjoy.

Times past
Under the old rules the limit was 1,000 watts input to the final amplifying stage for CW. Well, if you tuned up on the air, which nobody ever did, the theory was that you calculated the power input from B+ voltage and total of grid and plate current. At 65% efficiency you could legally pump 650 glorious hot watts into your antenna. Halleluiah! But, for phone I believe the limit was 2,000 watts peak envelope power (PEP) input to the final amplifying stage. God help me if I make an error here.

Another but, nobody had the testing or measuring equipment to properly do this so the law abiding radio amateur was expected to tune the amplifier in CW mode as above and then flip the switch to SSB mode for about 25% more B+ voltage. If enough milliamperes were available the proper power and no more would be available for SSB chatter and Big Brother would not jail the radio amateur as an amateur criminal. Halleluiah!

Not a single Ham ever illegally tuned an amplifier with the switch in the SSB position, thereby applying more than 1,000 watts to the final amplifying stage as that would have been good for a Kansas vacation at the Ft. Leavenworth Spa.

Bigger is heavier?
Big Brother was lobbied by somebody, or they blinked or caved and legal limit was reset the to 1,500 watts output, CW or PEP, at the amplifier output connector. Halleluiah! Another but, isn't there a correlation between the power of these wonderful amplifiers and gravity? Let's see - 650 watts to maybe 1,500+ watts. I dough no, twice, thrice the weight?  They used to make some of em in two pieces, with separate HV power supply on the floor and RF deck on the table. Hell - they're twice, thrice as heavy now and they put every thing in one big box and I can't pick the son-of-a-bitch up.

We still don't normally have the means of measuring PEP, unless you have some extra bucks for sophisticated equipment, so we're still ready for the Kansas Spa as the amplifiers today don't have the old method of avoiding illegal power settings. So how do we stay legal?

Tune the later legal amps
Some writings actually suggest this method: Tune while speaking into the microphone while saying "ahhhhhhhhh" or "hellllllooo" in a normal voice and observing voice peaks on your wattmeter. Perhaps your manual says something similar relating to ALC control. Well, it's the switch again. Nobody would ever tune in the CW mode to the new legal limit of 1,500 watts output and then switch to SSB for operation as this also would be an illegal act.

Heathkit knew
Back when most of us were becoming infatuated with the loud rotating machinery, new kit maker Heathkit designed an amplifier with a light weight low voltage plate transformer and a light weight voltage doubler rectifier/filter circuit. Many in Ham Radio spoke of this arrangement as sissy little stuff and not big and rough and tough enough for Hams. Ha!. The SB220 became one of the finest successes of amplifier design. The basis of the Heathkit design is alive and well today as engineers struggle to keep weight down so us aging Hams can lift the son-of-a-bitch.

It's not the wind, its the wind for reducing the weight
Most amps today have one of these two types of HV power supplies:

  • Full wave bridge rectifier/filter utilizing computer grade/type electrolytic capacitors.
  • Full wave voltage doubler rectifier/filter utilizing computer grade/type electrolytic capacitors.

Using single phase 220- 240 VAC mains -
Winding ratio for the plate transformer of a full wave bridge 2940 VDC B+ PS is approximately 9:1. Transformer voltage is 2,100 VAC, no load.
Winding ratio for the plate transformer of a full wave voltage doubler 2940 VDC B+ PS is approximately 4.5:1. Transformer voltage is 1,050 VAC., no load.

Yes, it's all about money
In transformers it's the turns, not the length of wire that are important. An outer turn uses more wire for a turn than an inter turn around the iron core. Diminishing returns as turns, resistance and weight increase. The full wave doubler rectifier uses half the diodes of the bridge. It also has a little more voltage sag under load but this is somewhat offset by the dramatically reduced resistance in the much shorter total wire length in the windings.

Less wire in the low voltage transformer, half the voltage of the bridge type, means less weight of components and less cost. The performance is still there for Amateur service or IACS as the engineering community calls it. For legal limit + amplifiers the weight savings is about 33%.

See later information about core type transformers and their benefits in size and weight,

Currently manufactured amplifiers using doubler rectifier/filter circuits are: Command Technologies HF-2500. QRO Technologies HF-2000, Ameritron AL-800, AL-800H, AL-572 and maybe AL-80B, AL-811, AL-811H and others I can't recall right now.

Lighten up
The amplifier maker, in struggling with cost and probably liability problems, puts everything in one box. Since some of these wonderful appliances are just too heavy now I will soon have an article with plenty of information and pictures about moving the transformer out of the box and on to the floor. Unlike the older two piece amps which had the rectifier/filter components inside the box with the plate transformer, today I suggest that only the the plate transformer is moved. This requires two HV wires coming into the deck instead of one. It's really much more agreeable and comfortable for us average aged Hams to move an amplifier that does not have a 46+ pounds of transformer inside. Manufacturers take notice.

Bob Hutchinson, N5CNN
President and Founder
Wireless Industry Association
n5cnn@wirelessindustry.com
713 467-0077
 

If you would like to publish an article here contact Bob Hutchinson, N5CNN: mailto:mailone@wirelessindustry.com.