Download HF Power Amplifiers featuring the Heath SB-220

HF POWER AMPLIFIERS
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A LOOK INSIDE HF POWER AMPLIFIERS
Featuring the Heathkit SB-220
By John White
VA7JW
2 July 2009
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Some Choices
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ACOM 1010 Price unknown
ALPHA $5,900 US
Heathkit $500
AMERITRON $3,800 US
Commander $5,000
TOKYO $5,900 US
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Tube or Solid State?
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Tubes still widely used and sold, but Solid State becoming
more competitive
Power Output
Tubes still reign, up to 2500 W output
S.S. just achieving 1200 W RF output
Cost
lower for Tube that S.S.
Good used market for tube amps
Tuning
Tube amps typically manually tuned
S.S. are wideband – no manual tuning
Size and Weight
Tube = bigger, heavier
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Basic Specifications
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Bands Covered
RF Power Out
RF Drive Power In
Power Input
RF Output Power
AC Power
Heat and Duty Cycle
Output Impedance
Distortion
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Bands
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Bands
HF = 160 / 80 / 40 / 30 / 20 / 17 / 15 / 12 / 10 meters
Not all amplifiers cover all bands
Typically WARC bands missing from Tube amps
Solid State amps are broadband 1.8 MHz to 30 MHz
Older Tube amps require manual band switching
Newer products may be automatic switched and tuned.
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RF Power Output
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Not always specified
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Depends greatly on efficiency which is not spec’d
Manufacturer specifies DC POWER INPUT to amp circuit
Labels amp as something like model “..2.5K…”
This refers to the DC power
At 60% efficiency, RF out is just a bit better than one half the
DC rating
Legal Limits per I.C. RIC-2 section 10.2, (a & b) allows
DC input of 1000 watts or
CW Carrier output of 750 watts or
PEP of 2250 watts
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RF Drive Power
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Typically 100 watts required from the Exciter
Exciter means “your rig”
Most all transceivers over last 30 years will
provide adequate “drive” to the amplifier so that
full output power from the amp can be realized.
less drive, less RF output
Tendency to “overdrive” to realize more output power is bad
practice as distortion will occur = splatter
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AC Power
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230 VAC mains power recommended
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power available 15 amps (limited by circuit breakers) x 230
V = 3450 watts
At 60% efficiency, RF out = 3450 x 0.6 = 2070 watts
good for “legal limit” amps
Some amps can be run from 115 VAC mains
AC power is limited to 15 amps x 115V = 1725 watts
Smaller amps with about 1725 x 0.6 = 1035 watts max
Dedicated AC power circuits for amps is a must
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Power Input means DC
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Refers to the DC POWER supplied to the amplifying
tubes or transistors by the POWER SUPPLY
This is not equal to RF Power Out
Amplifier efficiencies (class B) are theoretically 65% max
Typical is less than 60%
2500 Watts DC input ~ 1500 RF watts out
The remaining 1000 watts is HEAT !
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Heat
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For an amp to put out 1500 RF Watts, it will require 2500
watts of DC power
1000 watts of waste heat is generated in Transmit
Heat kills electronic components
every 10 degree rise in temp will halve component life
Internal temperatures must be controlled
ALL amps have FANS
Allow generous space around amp for cooling
Only the best heavy duty ($$) amps can provide full power
indefinitely - “key down” operation.
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Duty Cycle
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Duty Cycle is ratio of Tx time to Rx time
Max heat developed when transmitting
Heat is minimal (but not zero) when receiving
RTTY is Key Down, full carrier operation in transmit
Rx periods, little heat, whereas Tx maybe 30% of time
Duty cycle = 30% so heat load is reduced by 70% on average
Still, QSO’s of 5-10 minutes can cause tremendous heat buildup that can exceed amplifier heat tolerance
SSB, by nature of voice, has a low duty cycle,
perhaps ~ 20% produces much less heat on average
QSO’s can be prolonged without heat issues arising.
CW even less.
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Output Impedance
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All amplifiers designed to work into a 50 ohm impedance
as presented by a correctly matched 50 ohm coaxial line /
antenna system.
If Zin to the coax is not 50 ohms, destructive currents and
voltages may be developed on the coax or in the amplifier.
At high power, coax & components etc can be destroyed
quickly and dramatically!
Arcing of tuning capacitors is common.
Antenna Tuners (kilowatt ratings) are often required to
ensure proper matching to non-resonant antenna systems
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Distortion
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Power amps designed to maximize efficiency at the
expense of introducing distortion
Class A amps – Not used for amps
< 50 % efficient - very low distortion
Class B amps – This is amp operating class
to 65% efficient - acceptable distortion
Class C amps – Not used
to 75% efficient – OK for RTTY or FM but too much distortion
for SSB
High Distortion causes excessive bandwidth (splatter)
Overdriving input to amp results in same effect
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Linear Amps
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Most ham amps referred to as LINEARS
Linear means that the output signal is the same as input
signal in terms of its “waveform purity”
Non Linear operation means that the output is distorted
with respect to the input
Transmitted signal does not sound “good”
causes excessive occupied bandwidth, bad practice.
Linears find best trade-off between efficiency and distortion
Linears are designed as a Class AB which produce
efficiencies of about 60% with acceptable distortion
RF sidebands better than -30 dB below carrier
I.C RIC-2 section 4.2 spec requires 26 dB / 6 kHz
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Tube Amplifier Types
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Classic Grounded Grid Amplifier using 3-500Z tubes
50+ year old design still common today
Least expensive implementation
3-500Z triode tube (3 elements – 500watt dissipation) by Eimac (USA)
Modern Tube Amplifiers
introduced over last 15 -20 years
modern tubes as grounded grid or grounded cathode for more gain
Commonly Eimac 3CX800 triodes or 4CX1200 or 4CX1500 tetrodes
Solid State Amplifiers
First Solid State PA was Icom 2KL (’81) and later Yaesu FL-7000 (’86)
RF power output has not been as high as tube amps but getting close.
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Heathkit SB-220
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Classic Design of a Grounded Grid amplifier
Introduced 1970 and sold for $370 US
Sold about 1,300 kits up to 1978
Well featured for its price
Many still in service - very popular, not expensive
Operates 80, 40, 20, 15, and 10 Meters
Manually operated and tuned
Requires 100W of drive
Typical RF power out 1100 watts, bit less on 10M
Operates on 115 or 230V mains
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Station Set Up
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Transmit – Receive Relay
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switches antenna from Tx to Rx
Enables power amp by removing tube cutoff bias
Exciter
LINEAR AMPLIFIER
TRANSCEIVER
T/R relay ALC Input
50 ohm
coax
ALC
Output
50 ohm
coax
Optional
ANTENNA TUNER
kW rated if needed
ANTENNA SYSTEM
T/R
230 VAC Mains Power
ALC = Automatic Level Control
feed back loop that minimizes overdrive.
keeps amplifier in a Linear operating mode
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Amplifier Block Diagram
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Transmit – Receive amp bypass
Rx
RF Input
100 watts
Tx
Rx
TUNED INPUT
CIRCUITS for
EACH Band
GROUNDED GRID
AMPLIFIER
CIRCUIT
AC L.V.
Filament
Typ 5V
Rig
T/R
Bias
TUNED OUTPUT
PI- NETWORK
Tx
RF Out
1100 watts
to antenna
+ H.V. DC
Typical 2500 V
Amp antenna
Relay driven
by rig T/R
POWER SUPPLY
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Simplified Amp Circuit
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Tuned input
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one “Pi” network per band – switched in
matches 50 ohm transceiver output to tube input
Tube
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Triode, grid element grounded, hence grounded grid amplifier
RF is fed into Cathode
High Voltage + 2500 VDC is fed to plate via 50 uH RF Choke
Tuned Output – the TANK CIRCUIT
Pi network – switched per band
Resonant Circuit
Matches high Z of tube to low Z (50 ohm ) of coax
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Amplifier Circuit
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C in
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Band switched
tapped L
2 tubes in parallel
to develop 1 kW of Power
Each tube rated 500 watts
Output PI Network
The TANK CIURCUIT
Filament RF Choke
Filament Power
Tuned input PI networks
One for each band
Antenna T/R Relay
ALC circuit
RF rectified voltage
T/R Connector to rig
RF In - Out
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Simplified Power Supply
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High Voltage Doubler circuit
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Main Power Transformer
High Voltage DC
115VAC
Winding
RF Plate
Choke
HV
Winding
DC blocking
Capacitor
RF OUT
+ +
900Vac
Plate
~ 1,250VDC
Grid
TRIODE
TUBE
Filament / Cathode
115VAC
Winding
- +
~ 2500VDC
+
~1,250VDC
RF IN
RF Filament
Choke
Filament
Winding
115VAC
Winding
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+130VDC Bias
5 Vac at 15 Amps per tube
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Power Supply Circuit
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115 /230 V
Terminal board
Strapping field
Main Power
Transformer
Capacitor Bank
Fan
“Top” Caps
4 in series
HV Rectifier Chain
7 in series
“Bottom” Caps
4 in series
Filament and Bias
Transformer
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Diodes and caps placed
in series to withstand
high voltages
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Chassis Topside
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Filter Capacitor bank
Rectifier board
Power Transformer
Metering circuits
Safety Interlock
Grounds HV when
Cover removed
Input PI Networks
Tank Inductor
LOAD Variable
Capacitor
Fan
TANK Circuit
Output PI Network
Metal enclosed
High Power RF
Compartment
TUNE Variable
Capacitor
Tubes (2)
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Chassis Bottom Side
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Output Coax
ALC Circuit
Fan
Antenna – T/R relay
RF Filament choke
Tube Sockets (2)
Circuit breaker
115 / 230 V strapping
terminal board
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+130V Bias
power supply
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Controls
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Tube Plate Current
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Relative Power &
High Voltage Meter
Sensitivity for relative
power measurement
“TUNE”
Tunes Output
Tank Circuit
from tube
Meter switch for HV,
Relative Power &
Grid current
“LOAD”
Tunes Output Tank Circuit to antenna
Tune / CW, SSB Select
“BAND”
switches input PI networks and
taps on Tank Circuit
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Summary
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Tube amps are High Voltage / Low Current
2500 VDC / 1 amp
Extremely dangerous – electrocution for HV
Never defeat interlocks
Solid State amps are Low Voltage / High Current
48 VDC / 50 amps
No so dangerous but burns from high currents possible
Ensure all coax and equipment to the antenna side is in
very good working order to avoid melt-downs and fires
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