Circuit
Improvements for the
Heath
SB-220
Amplifier-Part
7
The venerable SB-220 is one of the most popular Amateur
Radio amplifiers ever made-and for good reason. But it isn't
perfect. Here's how to make it better.
By
Richard
L.
Measures,
AGGK
6455
La Cumbre
Rd
Somis,
CA
93066
T
he Heath" SB-220/221 amplifier'
made a notable impact on the world
of Amateur Radio. It was the first
reasonably priced
and
intelligently designed
HF SSB/CW amplifier sold to the Amateur
Radio community. Unfortunately, this
amplifier is no longer manufactured. The
SB-220 (and its successor, the HL-2200) has
some excellent design features and a few
easily corrected design weaknesses. In this
two-part article, I'll discuss both topics,
and some cures for the amplifier's weak-
nesses.
The
High-Voltage Power Supply
Before the arrival of the SB-220, there
was a popular notion that legal-limit SSB
amplifiers needed heavy-duty power sup-
plies that required two grown men to move
them. Heath engineers knew that this idea
was based more on folklore than on sound
engineering princi~les.~ They also knew
that the average duty cycle of a human
voice is only about 15%. Why build a
100% duty cycle "lock-to-talk" power
supply when one wasn't required? So, they
designed a power supply that would do
the job at hand. That resulted in consider-
able size, weight and cost savings, which
Heath passed along to SB-220 buyers.
At first, some people in the ham com-
munity had negative comments about the
SB-220's "wimpy-looking" power supply.
With time, it became apparent that the
power supply did the job well. It had a low
failure rate and no detectable ripple. This
was no accident. 'Heath engineers wisely
chose an HV-transformer design with an
exceptionally low secondary resistance
(only about 12.2
Q).
This minimizes the
voltage drop under full load in the supply's
full-wave voltagedoubling rectifier circuit.
Such circuits have an extremely high peak-
to-average output-current ratio, so mini-
'Notes
appear
on
p
29.
mizing the transformer-winding resistance
is essential for good voltage regulation and
reducing 12R (heat) losses in the trans-
former's windings.
The voltage-doubling rectifier circuit has
some advantages over the traditional full-
wave-bridge rectifier circuit, including:
Low ripple voltage. As one capacitor
bank is charging, the other capacitor bank
is simultaneously discharging, canceling the
other's out-of-phase sawtooth waveform.
There is no safe
substitute for pulling the
electric-mains plug before
putting your fingers
inside any amplifier.
Half as many transformer secondary
wire turns as a comparable non-doubling
supply, which yields a more efficient trans-
former design. Here's why: One layer of
insulating paper is required between each
layer of wires, so fewer turns means fewer
layers of paper. The result is a transform-
er that has a high ratio of copper to paper,
and thus a relatively high power-to-weight
ratio.
Excellent voltage regulation during
current transients-exactly what's needed
for
CW
and SSB operation-because no
swinging-inductance filter choke is needed.
Cooling
Because about half of the power con-
sumed by a linear amplifier is converted
into heat, another important amplifier-
design consideration is cooling. Most of the
heat that a 3-5002 (or any other internal-
anode tube) dissipates is carried away by
heat radiation from its anode.
Here's how it works: During normal
operation, the anode gets so hot that it
glows a bright orange color. The surround-
ing objects are relatively much cooler, so
the anode loses most of its heat to its
surroundings by radiation, and a lesser
amount by conduction through the anode
stem and pins. Unfortunately, some of the
components to which the anode loses heat
are heat-sensitive parts of the 3-5002, such
as the tube's critical glass-to-metal seals and
the solder used at the pins in the tube's
base. These heat-sensitive parts must be
cooled by forced air.
Heath's engineers came up with a decep-
tively simple method of effectively cooling
the 3-500Zs. They realized that thqexpen-
sive Eimac" air-system socket/glass-
chimney cooling system had some Sefious
trade-offs, such
as:
the difficulty of forcing
enough air through the airflow restrictions
in the system to adequately cool the fila-
ment pins and seals; inefficient anode-cap
cooling (the horizontal fins on the standard
anode-cap coolers were obviously not
designed to be cooled by the vertical air-
flow through the Eimac air-system chim-
ney); and those airflow restrictions require
the use of a high-pressure centrifugal
blower (and all high-pressure blowers are
noisy). Heath needed a cooling system that
would quietly move high-velocity air past
the 3-500Z's hot filament pins,3 filament
and anode seals, and glass envelopes.
The Heath engineers knew that when
horizontal air flows across vertical
cylinders, such as a 3-5002 envelope and
its pins, the air follows the curves of the
cylinders, providing fairly uniform cooling
to all areas of the.cylinders (minimizing hot
spots). They concluded that, with horizon-
tal airflow, the cooling air has a direct path
to the heat-sensitive parts of the tube, and
allows the anode cooler's fins to take
maxi-
mum advantage of the flow of cooling air.
Because the filament pins are below the
chassis and the filament and anode seals are
above the chassis, the Heath engineers used
November
1990
25
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