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The Care and Feeding of Linear Amplifiers. Marc C. Tarplee, Ph.D. NCT N4UFP ARRL Technical Coordinator South Carolina Section. What Is A Linear Amplifier?.

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the care and feeding of linear amplifiers

The Care and Feeding of Linear Amplifiers

Marc C. Tarplee, Ph.D. NCT N4UFP

ARRL Technical Coordinator

South Carolina Section

what is a linear amplifier
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 can have an output power of 20 to 1500 W PEP
why use a linear amplifier
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
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 frequency above 1.8 MHz
  • 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
Solid State Linear Amplifier Examples

ICOM PW-1 1 kW 1.8 – 54 MHz

Mirage B34G 35W 144 MHz

basic operating controls
Basic Operating Controls

Status Indicators

Power Amp Switch

Mode Switch

Receive Pre-Amp Switch

vacuum tube linear amplifier examples
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 controls8
Basic Operating Controls

Band Switch

Metering

Plate Tuning

Power

Switch

Load Tuning

Metering

Switch

Standby

Switch

ALC

Level

solid state linear amplifier brick
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
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.
    • Requires some sort of external TR switching relay
    • Difficult to find for frequencies above 28 MHz
setup operation of a brick
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
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
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
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
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
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
Selecting A Vacuum Tube Amp
  • Triode Amplifiers
    • Generally grounded grid design
    • Do not require neutralization
    • Gain limited to 10 - 12 dB
    • Triode transmitting tubes are very robust
  • 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
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
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!