The AA8V Wingfoot 813 Amplifier
High Technology Of The 1950's In The 2000's
by Greg Latta, AA8V

Interior and Back Views

Wingfoot 813 Amplifier Pages
 Main Page and Front and Side Views  Tank Coil Information
 Interior and Back Views  Typical Operating Conditions
 Circuit Description and Schematic Diagram  Power Supply Photos
 813 Tube Information  Power Supply Circuit Description and Schematic Diagram

 Amplifier Rear Panel:
The rear panel of the amplifier contains connections for the power supply, meter, and control circuits. The connector at the upper left supplies 117 VAC for the filament transformer. The BIAS adjustment just to the right sets the operating bias. The two SO-239 connectors in the middle of the rear panel are for RF INput and OUTput. To the right of them are the High Voltage (HV) and ground (GRND) connections for the 2000 volt plate supply, which is fed to the amplifier through a coaxial cable. The connector at the upper right (MTR) is for the meter panel, and the connector at the lower left (CTRL) provides connections for the bias supply and control circuits. The antenna changeover relay is visible on the top of the chassis at right.

Amplifier Back
Click on the image for a larger view.

Click here for a super detailed view.
 Amplifier Underside:
Compared to the power supply, the amplifier is very sparse underneath. In this photo the rear of the amplifier is to the left. The bottom of the antenna changeover/bias relay is visible at the upper left, and the bottom of the filament transformer can be seen at the lower left. The filament RF choke is the long coil at the bottom of the photo. The mode switch, which is mounted on the front panel, is visible at right, and just below it are the green and red standby/operate indicator lamps, also mounted on the front panel. Output from the plate tank circuit is taken through a feed-through bushing visible just behind the mode switch, and a coaxial cable carries the output to the changeover relay.

Amplifier Uniderside
Click on the image for a larger view.

Click here for a super detailed view.
 Amplifier Input Transformer:
When originally built, no input matching network or transformer was used with the amplifier. This was because the Wingfoot Exciter had no trouble driving the amplifier directly. However, it was later found that solid state transmitters had trouble driving the amplifier. This is because the input impedance of a grounded grid amplifier is typically several hundred ohms. Though most vacuum tube transmitters will have no trouble driving such an impedance, solid state transmitters, which are designed to drive loads that are very close to 50 ohms, will usually refuse to drive such a load. The best (and most complicated) solution is to use a tuned matching network on the input. However, a simple 1 to 4 unbalanced to unbalanced transmission line transformer is very easy to make and will lower the input impedance by roughly a factor of four, usually bringing it within range of most solid state transmitters.

In 2013, some years after its original construction, an input transformer was added to the amplifier consisting of seven turns of RG-174 coax or similar small coaxial cable on an FT-50A-61 toroidal core. (If the coaxial cable is not available, 11 turns of #24 enameled magnet wire, bifilar wound, will also work.) After the addition of the input transformer it was found that both solid state and tube transmitters could drive the amplifier without difficulty.

Input Transformer
Click on the image for a larger view.

Click here for a super detailed view.
 Amplifier Tube Socket Close-up:
Circuit wiring must be done carefully in amplifiers to avoid parasitic oscillations. Although the tube in this amplifier operates between 3.5 and 20 MHz, it can (and will) oscillate on spurious frequencies if given the opportunity to do so. In this view of the tube socket, notice how the grid bypass capacitors (the two, blue, dipped capacitors) are connected with very short leads to the tube socket pins and then grounded directly at the tube socket mounting lugs. Also notice how the G3/BF pin (for the beam forming plates) is grounded directly to the tube socket mounting lug. Such connections keep stray inductance to a minimum and help to avoid parasitic oscillations. The large brown capacitors on the left side of the tube socket connect the RF input to the cathode/filament of the tube. The large filament RF choke at bottom permits the filament voltage to reach the cathode/filament, while preventing the RF from flowing back to the filament supply.

Amplifier Tube Socket
Click on the image for a larger view.

Click here for a super detailed view.

Wingfoot ExciterClick here for pictures and information on the matching Wingfoot VFO 2E26 Exciter

meterBack to Dr. Greg Latta's Electrical Engineering and Amateur Radio Pages

Questions, Comments, and E-Mail

LetterIf you have any questions or comments, you can send E-Mail to Dr. Greg Latta at

This page is under constant revision. Please check back often.

Thanks for stopping by!