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6BQ5 internal capacitances in triode mode

Posted by smoking-amp on February 28, 2004 at 11:14:07

Seeing the earlier "discussion" on EL84 G1 capacitance in triode mode, I went and measured a Westinghouse 6BQ5 (true pentode) I happened to have laying on the bench. I used a Heathkit digital LC meter on the 200 pf scale with 4 point leads and Kelvin clips. Subtracted lead capacitance readings from tube in circuit readings without moving leads to get results. Here's what I got:
G1 to Cathode: 7.2 pF (all other elements disconnected)
G1 to Screen: 3.2 pF ( " " )
G1 to Screen+Plate(alligator shorting clip,clip only) 5.6pF( " " )

If the supressor grid were grounded so as to provide shielding, the last cap. would presumably drop to the 3.2pF + 0.5pF = 3.7pF for screen + plate cap. (using the published Cg-p of 0.5pF)

Adding the 7.2pF + 3.2pF = 10.4pF is close to the published 10.8pF for Cinput, so numbers seem good.

Now, to compute input capacitance in triode mode: 3.7 * 19 + 7.2 = 77.5pF (using 19 for voltage gain)
or, worst case: 5.6 * 19 + 7.2 = 113.6pF

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