HF POWER AMPLIFIERS A LOOK INSIDE HF POWER AMPLIFIERS Featuring the Heathkit SB-220 By John White VA7JW 2 July 2009 NSARC 1
Some Choices ALPHA $5,900 US ACOM 1010 Price unknown Heathkit $500 AMERITRON $3,800 US Commander $5,000 TOKYO $5,900 US NSARC 2
Tube or Solid State? Tubes still widely used and sold, but Solid State becoming more competitive Power Output Cost Tubes still reign, up to 2500 W output S.S. just achieving 1200 W RF output 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 NSARC 3
Basic Specifications Bands Covered RF Power Out RF Drive Power In Power Input RF Output Power AC Power Heat and Duty Cycle Output Impedance Distortion NSARC 4
Bands 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. NSARC 5
RF Power Output Not always specified 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 NSARC 6
RF Drive Power 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 NSARC 7
AC Power 230 VAC mains power recommended 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 NSARC 8
Power Input means DC 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! NSARC 9
Heat 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. NSARC 10
Duty Cycle 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. NSARC 11
Output Impedance All amplifiers designed to work into a 50 ohm impedance as presented by a correctly matched 50 ohm coaxial line / antenna system. If Z in 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 NSARC 12
Distortion 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 NSARC 13
Linear Amps 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 NSARC 14
Tube Amplifier Types 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. NSARC 15
Heathkit SB-220 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 NSARC 16
Station Set Up Transmit Receive Relay switches antenna from Tx to Rx Enables power amp by removing tube cutoff bias Exciter TRANSCEIVER T/R relay ALC Input 50 ohm coax LINEAR AMPLIFIER ALC Output T/R 50 ohm coax Optional ANTENNA TUNER kw rated if needed ANTENNA SYSTEM 230 VAC Mains Power ALC = Automatic Level Control feed back loop that minimizes overdrive. keeps amplifier in a Linear operating mode NSARC 17
Amplifier Block Diagram Transmit Receive amp bypass RF Input 100 watts Rx Tx TUNED INPUT CIRCUITS for EACH Band GROUNDED GRID AMPLIFIER CIRCUIT TUNED OUTPUT PI- NETWORK Rx Tx RF Out 1100 watts to antenna Rig T/R Amp antenna Relay driven by rig T/R AC L.V. Filament Typ 5V Bias POWER SUPPLY + H.V. DC Typical 2500 V NSARC 18
Simplified Amp Circuit Pi Tuned input one Pi network per band switched in matches 50 ohm transceiver output to tube input Tube 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 NSARC 19
Amplifier Circuit 2 tubes in parallel to develop 1 kw of Power Each tube rated 500 watts Filament RF Choke C in L Band switched tapped L Output PI Network The TANK CIURCUIT Filament Power Tuned input PI networks One for each band Antenna T/R Relay T/R Connector to rig RF In - Out ALC circuit RF rectified voltage NSARC 20
Simplified Power Supply High Voltage Doubler circuit Main Power Transformer 115VAC Winding HV Winding 900Vac + - High Voltage DC + ~ 1,250VDC RF Plate Choke DC blocking Capacitor Plate Grid TRIODE TUBE RF OUT 115VAC Winding - + ~ 2500VDC Filament / Cathode RF IN Filament Winding + ~1,250VDC RF Filament Choke 115VAC Winding +130VDC Bias 5 Vac at 15 Amps per tube NSARC 21
Power Supply Circuit 115 /230 V Terminal board Strapping field Main Power Transformer Fan HV Rectifier Chain 7 in series Capacitor Bank Top Caps 4 in series Bottom Caps 4 in series Filament and Bias Transformer Diodes and caps placed in series to withstand high voltages NSARC 22
Chassis Topside Filter Capacitor bank Power Transformer Safety Interlock Grounds HV when Cover removed Fan Metal enclosed High Power RF Compartment Tubes (2) Rectifier board Metering circuits Input PI Networks Tank Inductor LOAD Variable Capacitor TANK Circuit Output PI Network TUNE Variable Capacitor NSARC 23
Chassis Bottom Side Output Coax ALC Circuit Fan Tube Sockets (2) Antenna T/R relay RF Filament choke Circuit breaker 115 / 230 V strapping terminal board +130V Bias power supply NSARC 24
Controls Tube Plate Current Relative Power & High Voltage Meter TUNE Tunes Output Tank Circuit from tube Sensitivity for relative power measurement Meter switch for HV, Relative Power & Grid current LOAD Tunes Output Tank Circuit to antenna BAND switches input PI networks and taps on Tank Circuit POWER ON / OFF Tune / CW, SSB Select NSARC 25
Summary 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 NSARC 26