Push-Pull 814 Rack Transmitter
The original PP 814 transmitter used for Field Day 2017 was rebuilt into a 6ft wooden rack with new power supplies and some changes to the RF sections. The amplifier’s HV supply uses a UTC S-48 transformer (refinished in gloss black with a wrinkled top plate) with taps for 1kV, 1.25kV, and 1.5kV. These taps can be selected via banana jacks/plugs on a strip of bakelite between the transformer and the 866 mercury vapor rectifiers.
There are additional supplies for the filaments (6.3VAC and 10 VAC), the oscillator and doubler (75V and 225V regulated), buffer (600V using 816 mercury vapor rectifiers), amplifier screen (300V using 816s), and amplifier bias (-90V regulated with a 874), all of which, including the HV supply, populate the lower four decks. Lockouts are included to prevent application of screen voltage on the final amplifier tubes if B+ is not present as well as locking out B+ if there is no grid bias voltage.
The exciter line-up is an 89 ECO oscillator choke coupled to a 42 Class A buffer (plate tuned to second harmonic of ECO grid) followed by an 802 buffer/driver. Operation is much more stable than with the originally tried two-stage circuits.
The amplifier runs a pair of 814s in push-pull, providing more output power than the PP 804s originally used.
The antenna tuner was reconfigured from a previous version by putting the RF current meter in the center of the panel and the tuning capacitors on either side, similar to many period tuners. The circuit itself is unchanged from it’s original configuration, facilitating switching between series and parallel tuning via a DPDT knife switch behind the front panel.
Below are various images of the project (click any image to enlarge):