Linear Amp Tuned Input

The biggest single improvement that can be made to the simple GG RF amplifier is the addition of a tuned input.

The tuned input improves linearity while also reducing the drive requirements.

I recently found a tuned input design by ZL1AXB that does this as well as eliminating the filament choke required by tubes using directly heated cathodes.

A full description of an HF amplifier using this tuned input circuit can be found HERE.

The single 813 amp shown is designed to cover only 40 thru 10 meters. So the specifications for the input circuit will not work down to 80 meters. However, all that is required is to increase (probably double the turns) the inductance of the input coil to cover 80 meters.

The input is deceptively simple. For the original design, the input coil is composed of 7.5 turns of 3/16th inch copper tubing wound on a 1 inch diameter form. A length of teflon insulated #12 wire is threaded through the tubing to form the second connection to the filament. The tube filament pins are connected to the tubing and to the #12 wire at the tube socket. Three turns down from that provision is made to insert driving power through a 0.01mf disk ceramic capacitor. Claims are made that such a tap provides a 50 ohm input across all bands.

At the other end of the input coil, the tubing and center conductor are connected to the secondary of a center-tapped filament transformer. The center-tap is grounded to provide a return for the plate current. Both filament leads at the transformer are bypassed to ground through 0.01mfd disk ceramic capacitors.

A 1000mfd variable capacitor is connected across the tubing coil to allow tuning 40 thru 10 meters.

A 2.5kohm 2watt resistor is connected across the variable capacitor to reduce the Q of the input circuit.

It seems like a very neat and painless way to significantly improve amplifier performance while eliminating the need for a filament choke. The only down side is fabricating the tubing coil and threading the center conductor through the tubing. Obviously the center conductor should be threaded through the tubing before trying to bend the tubing into a coil. That should help prevent collapsing the tubing during bending and also simplify installation of the center conductor.

An even simpler approach might be to use coax in place of the tubing and center conductor. After all, the tubing and center conductor are really just a length of homemade coax.

A length of RG8 with solid dielectric should work well. The smaller coaxial cables like RG59 and RG58 may not handle the filament current very well. A single 813 needs 5 amps at 10 volts. A quad of 811s require 6.3 volts at 16 amps. Or 12.6 volts at 8 amps depending on how they are wired. In either case, the #20 center conductor of the smaller cables may not be sufficient to handle the current.

For an 80 meter application, approximately 14 turns of RG8 will be required. At a 1 inch diameter this amounts to 42 inches of cable. The capacitance of the center conductor to the shield will not be inconsequential and it may cause problems when trying to tune the 10 meter band but it should work without problem across frequencies below 10 meters. Running high power on 10 meters is usually not necessary in any case so that should not cause any real problem.

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2 Comments on “Linear Amp Tuned Input”

  1. Frank Says:

    ZL1AXB credits his single 813 amp with 600w pep output. At first glance that appears a little on the high side but on further consideration that seems about right.

    An 813 has about twice the power dissipation of a pair of 811s. The Tiny Tim using a pair of 811s can run 300 watts output on CW with 100 watts input. That is 300 watts continuous or 600 watts pep.

    I think the ZL1AXB design is not only neat but exceptional.

  2. Mike Lord Says:

    I have built the single 813 and it works beautifully. His input circuit is an old idea that should be reborn to the general ham population. I am now in the process of building the 2X813 which will cover 80 – 10. Thanks for your input. 73

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