500W GI-7B 23cm PA

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This amplifier consists of a pair of GI-7B cavity PAs combined with 90 degree hybrids (the output one is homebrew). Power output is 500 Watts with little thermal drift. Water cooling is used for the plates, and a blower for cathode/grid.

The cavities are constructed using 100% copper tubing readily available in the USA. No fingerstock is used. All copper pieces have been soldered with silver and have a light silver plating to prevent corrosion. No lathe is necessary; just a hacksaw, drill press, and a lot of filing.

More information, diagrams, schematics, etc., will be added as it becomes available. See the link to construction details at bottom of this page.

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GI-7Bs (center front), with and without air cooler, and cavities, one assembled and one awaiting assembly. PAs are neutralized with a piece of 0.005" Kapton 3/4" wide inserted between tube and grid line. Note water cooler, a 2" copper pipe end cap (with 1/2" coupler in center) soldered to plate blocking capacitor on assembled PA.

Top view of PA chassis, showing two completed GI-7B PAs installed with a 90 degree hybrid combiners on both input (low power) and output (high power). Fifty Watts of drive easily yields 500W output. Grid current must be kept under 100mA per tube to control thermal drift.

Bottom view of PA chassis, showing radiator for anode water cooling system. Air from the cathode and grid cooling blower on top of chassis exits through this radiator. The water reservoir is a five gallon fishing bucket and the submersible pump is a "Little Giant" pool pump.

Top view of the PA, with PA chassis installed in the rack cabinet, as seen through the top "hatch" of the cabinet.

Top view of the power supply for the two GI-7B cavity amplifiers. It is 2.5KV no-load, 2.2KV @ 0.8A (400mA Per tube), key down. It's a doubler with huge capacitors (8 x 1900 uf) and small transformer. The PAs are biased for 100mA plate current each, no drive (idle).

Front view of the completed PA rack assembly, PA above power supply.

Hybrid Combiner
    Details of PA construction
Hybrid Combiner
    Details of output hybrid combiner

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