{Collins} Re: 812 vs 811 Transmitting Tubes



The 812 cannot be substituted for the 811. If you do, the plate currrent will soar. The meter may peg and the tubes will commit suicide quickly.

The 812 was the rf tube, with substantial negative bias, while the 811 was designed for the modulator, to be used at zero bias. The old rf amps were class C and needed a lot of bias. Since the ssb amp needed to be linear, the zero bias tube, originally designed to be modulators, fit right in, and the 812 dropped right out of sight.

So, using the 812 requires you to add sufficient bias to limit the plate current during idle. It requires so much that you would normally obtain that by lifting the grid and applying it there. Substantial modifiction would be necessary.

Put the 812 back on the shelf and find some 811s.

73, Colin
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