The Care and Feeding of Linear Amplifiers

The Care and Feeding of Linear Amplifiers Marc C. Tarplee, Ph.D. N4UFP ARRL Technical Coordinator South Carolina Section

What Is A Linear Amplifier? • In amateur radio parlance, a linear amplifier (also known as a linear or a brick) is an RF amplifier designed to amplify the output of a transmitter to a higher power level without introducing distortion. • Linear amplifiers are available for frequencies from 1.8 MHz to above 1 GHz. • Linear amplifiers may have an output power of 20 to 1500 W PEP

Why use a linear amplifier? • Working DX on the lower HF bands and 160m may require high transmitter power to get through the high level on noise on these bands. • Certain modes, such as EME, have very high path losses (> 200 dB) and high power is required to make a contact.

Types of Linear Amplifiers • Solid State • Active device is a power BJT or MOSFET • Available in powers of 20 – 1000 W • Commercially available for any amateur band (1.8 – 1296 MHz and higher) • Vacuum Tube • Active device is a triode or tetrode vacuum tube • Available in powers of 300 – 1500 W • Commercially available for amateur frequencies between 1.8 MHz and 144 MHz

Solid State Linear Amplifier Examples ICOM PW-1 1 kW 1.8 – 54 MHz Mirage B34G 35W 144 MHz

Basic Operating Controls Status Indicators Power Amp Switch Mode Switch Receive Pre-Amp Switch

Vacuum Tube Linear Amplifier Examples The-Tec Titan III 1.5 kW 1.8 – 28 MHz Ameritron AL-80 1 kW 1.8 – 28 MHz

Basic Operating Controls Band Switch Metering Plate Tuning Power Switch Load Tuning Metering Switch Standby Switch ALC Level

Solid State Linear Amplifier (Brick) • Advantages • Requires no adjustments when changing frequencies within the amplifier’s design range • Generally RF switched • Generally include a receive pre-amp • Disadvantages • Requires high current (20 – 140 A) LV DC power supply • Designed to operate into a 50 ohm load • Relatively expensive ($1 - $4 per W )

Vacuum Tube Linear Amplifier • Advantages • Requires no external power supply • Can operate into a wide range of loads (30 – 150 ohms) • Relatively inexpensive ($1 - $2 per W) • Disadvantages • Requires adjustments when operating frequency is changed, even within a band. • May some sort of external TR switching relay • Difficult to find for frequencies above 28 MHz

Setup/Operation of a “Brick” • Connect the RF output of the brick to a dummy load. • Select the proper RF mode. • Turn of the power amp. RF Mode Switch Pre-Amp Switch Power Amp Switch • Transmit into the dummy load, increasing the drive until the proper output power is reached. • Shut down the brick Connect it to the antenna. • Turn on the brick.

But it’s not working….. • Is the DC power supply connected?? • Check to make sure that the SWR at the RF output of the brick is below the maximum allowable value (typically 1.5 to 2.0) • Check to make sure that the proper mode is selected. Trying to operate SSB in the FM mode can create problems.

Operation of a Vacuum Tube Linear Plate Tuning • Connect antenna, turn on linear and set the “Meter Function Switch” to “Plate Current” • Apply ~ 30 W of drive and adjust the “Plate Tuning” for minimum plate current. • Set the “Meter Function Switch” to “RF Out” and adjust the load tuning for maximum power output. • Increase the drive until the output power reaches the desired level and repeat the plate and load tuning steps Load Tuning Meter Function Switch Band Switch

I tried all that and it didn’t work…. • Check to make sure the antenna is connected. • If the plate current does not show a dip and the antenna is OK, one of the amplifier tubes may be bad. • Use the amplifier’s metering to check the output of the HV supply. No HV = no RF output! • Check the antenna impedance. A vacuum tube linear cannot generally match impedances less than 30 or greater than 150 ohms. • Make sure that the amp is in the right mode (CW/SSB)

Which Amplifier Should I buy? • If money is no object, it is hard to beat a good solid state linear amplifier for HF/VHF work. • If money is a concern, vacuum tube linear amplifiers are widely available on the used market at prices as low as $0.75 per W.

Selecting A “Brick” • Decide whether or not a receive pre-amp is important – this can add significantly to the cost of the brick. • Make sure that the brick’s output will not exceed the rating of the antenna or any mast-mounted pre-amps • Remember that a DC power supply will also need to be purchased. The power supply can be sized according to the following equation: • I = Pout/6.3 • Good bargains – Mirage units and rfConcepts units for VHF/UHF, Ameritron ALS-600 (w/pwr supply)

Selecting A Vacuum Tube Amp • Triode Amplifiers • Generally grounded grid design • Do not require neutralization (unconditionally stable) • Gain limited to 10 - 12 dB • Triode transmitting tubes are very robust (and relatively inexpensive) • Tetrode Amplifiers • May require neutralization • Gain can reach 20 dB • Tetrodes may be destroyed by excessive grid current • Sweep Tube amplifiers • Gain limited to ~ 6 – 8 dB • Sweep tubes can be destroyed by full duty cycle operation (FM, RTTY, PSK31, even CW) • Best bang for the buck – grounded grid triode linear such as an SB-200, SB-220, TL-922A

For More Information • Linear Amplifier Design • http://www.cpii.com/eimac/PDF/C&F2Web.pdf • HF Linear Amp Construction • http://users.knoware.nl/users/veldman/frans/english/hflinear.htm • http://i5uxj-2.cln.it/amp/hfamp.html • VHF/UHF Linear Amp Construction • http://www.qsl.net/dl4mea/2g35.htm • http://www.svetlana.com/docs/TechBulletins/technoteNo42.html

Demo Time! • We will get some hands-on experience tuning the club’s Kenwood TL-922A linear amplifier. • Letting the smoke out of any of the components is not an acceptable outcome!

The Care and Feeding of Linear Amplifiers