7F7 loctal vs. 6SL7GT

I recently received an email from someone who built adapters to use 7F7 loctal tubes in place of 6SL7GT’s. He said that he didn’t like the sound of the 7F7 because the gain was too low. This was news to me because RCA’s RC-22 tube manual states:

For maximum ratings, typical operation as A1 amplifier, and curves refer to refer to glass-octal type 6SL7GT

A quick check in Tube Test Conditions for Hickok Cardmatic Tube Testers, published by Hickok in 1962, shows that they are tested with exactly the same electrode voltages and have the same full-scale gm. The Triplett 3444A, however, does test these with different parameters. The 7F7 is tested in a region of the curves where the gm is lower than the conditions where 6SL7GT is tested. This is an anomaly and gives beginners the impression that the 7F7 has lower gm. Gm is not a fixed number for any and all conditions. If it were the characteristic curves would in fact be straight lines.

In RCA tube manuals you can find nomographs to estimate gm, rp, Ib, etc at voltages/currents other than those listed as the test conditions.

A Sylvania JAN 7F7 loctal with its close relative, an RCA JAN 6SL7GT

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