The rebuild will add significant improvements, features, and updates including:

1) Changing the Plate Tank from Pi to a Pi-L network

2) Adding the 160, 17 and 12 meter bands

3) Adding tuned input circuitry

4) Update the RF section with tighter shielding

5) Update T/R control from an old open-frame relay to vacuum relays

6) Change the high voltage plate transformer to a plug-in unit for easier transport

The next two shots are rear chassis views of the new PA compartment shielding. The small center compartment will house the tuned-input circuitry which will be band-switched with relays. On the right hand side note the green vertical G10 fiberglass panel with banana plugs for the plate transformer to slide (and plug-in) in from the rear of the chassis - a back saving must have since the all up weight of the amplifier complete and in its desktop mini rack cabinet will be just north of 90 lbs.

Photo (below) is of the new Pi-L Plate Tank early during the assembly process. The plate TUNE capacitor has been upgraded to a 500 pF vacuum-variable driven by a home brew turns counting DIAL and DRIVE mechanism.

A closeup of the plate tank's band switch going in. The front wafer switches DC for the relay band-switched tuned input circuitry. The second wafer wiring is complete, and switches the Pi-L's output inductor L-2, constructed of a stack of three glass tape insulated T200 toroids wound with FormVar enameled wire double insulated with Teflon tubing. The third band switch wafer (not yet installed in this photo) will go in next and switch taps on the Pi-L's input (L-1) inductor.

With the chosen L-C values, the Pi-L tank circuit will efficiently match a wide range of Plate impedance from 2000 to 2500 ohms. That gives the option of running a pair of 4-400A tubes at 1000 watts PEP output, or a pair of 3-500Z/3-500ZG tubes at 1000 to 1500 watts PEP output. A handy option should acquiring replacement 4-400As' turn out to be problematic. I.e. when the amplifier was originally built years ago, 4-400A tubes were readily and inexpensively available as BCB Transmitter "pulls", and even in new condition on the surplus market. To buy NOS 4-400A tubes from commercial suppliers these days tends to be cost prohibitive, thus the option of going with the Asian mfg. 3-500ZG.

Photo of the completed Pi-L tank with L1, a composite of three inductors in series, those being the top horizontal coil of 1/4" O.D. silver plated tubing for the 10, 12m, 15m and 17m bands. Next is the vertical coil of AWG No.8 heavy duty AirDux stock adding the 20m and 40m bands, and finally the FormVar (dark enameled wire) coil wound on a grooved ceramic form for the 80m and 160m bands:

Flat-black painted vertical metal work, aids cooling of the power tubes.

A turns counter and dial mechanism for the plate TUNE vacuum variable capacitor, fashioned from junk box parts and 1/8" aluminum plate stock:

Under chassis view; rework is 90% complete. The AC mains input, inrush current limiting step-start, and control relays with associated circuitry is complete, along with the solid-state BIAS control board, T-R vacuum relays, and the filament and control transformers.

More to come as the project nears completion.