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Bi-Grid - The Bi-grid Valve
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This beautiful valve is a very early double grid valve and dates from about 1922. It is a bright emitter and the only marking on the envelope is an etched 3.5 V and we take this to be the filament voltage. Anode voltage will be low especially if used as a space grid valve. Space charge grid valves originated in Germany with the SS1. The design operated so that the first grid suppressed the space charge to improve sensitivity.
Other Bi-grid designs were designed so that a signal and a local oscillator could be mixed together.
The construction still follows the coaxial pattern within a horizontal cylindrical anode, but the fixing supports have become more formalized, and thus the parameters more consistent. The second grid, used as the control grid, is also a wire helix, but with far fewer turns than the first grid.
The main picture features the valve on a base of the time.
Seen from the side the concentric structure is revealed.
Turning the valve allows the details to be seen.
Close-up the perfect tungsten filament becomes visible. Early tungsten wire is exteremly fragile and great care is required when handling these early valves.
At a greater angle the helical grids become visible. Also note the quality of the supports and the complete anode cylinder. Contrast with Bi-grid.
Close-up the folded filament support is exposed. A compression joint being an effective way of holding a metal that is difficult to join.
Further rotation indicates that the anode is attached by spot welds at each end of the flange.
This close-up shows the etching on the spherical envelope, the fixings to the pinch and the beautiful helical grids.
The balloon envelope is 51 mm in diameter and, excluding the B4 base pins, is 89 mm tall.
References: Observation & above article. Type Q(Bi-Grid) was first introduced in 1922.


Updated April 30, 2013.
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