HF Amplifier Design
Better than ever?
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.
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
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.
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
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
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.
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
Bob Hutchinson, N5CNN
President and Founder
Wireless Industry Association
If you would like to publish an article here contact Bob Hutchinson,