Pentode/Triode Operating Points (6BL8,6U8,6GH8,7199 etc)

Tubes Asylum

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I've been following the thread linked below on the Vintage board and it sparked my curiosity about the differences in operating points of some of the commonly used pentode/triode combos (7199, 6BL8, 6U8, 6GH8, etc.) Figured it would be better to ask about it here so as not to hijack the other thread.

I have a Scott 222C that I found at a flea market a while back and now that I've finished with the Sherwood S-5000 I'll probably start on it before long. My 222C is apparently the 3rd version - it uses 6U8s (or 6GH8s) in that position while the earlier versions used 7199s like the Sherwood.

One thing that struck me as strange when I looked at the Scott 222B schematic posted on the other thread is that the G2 (screen) voltage on the 6BL8 is higher than the plate. It specifies 60v on the plate and 105v on the screen. Can that be right?? I always thought screen voltage was supposed to be close to the plate voltage and preferably less.

In contrast, the schematic for my version of the 222C puts both plate and screen at 65v for the 6U8. The schematic for the Sherwood S-5000 that uses 7199s puts the plate at 95v and the screen at 45v, although I dropped those a bit after some research.

So why would they put the screen of the 6BL8 so much higher than the plate?

I know the scarcity of 7199s has led many folks to substitute one of the other tubes, either by using an adaptor or by rewiring the socket. But since the specs are different, what other circuit changes do you recommend? Do any of you have particular "preferred" operating points for these tubes?

Questions About Scott 222B Phase Inverter (Open in New Window)

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Topic - Pentode/Triode Operating Points (6BL8,6U8,6GH8,7199 etc) - FlaCharlie 11:00:38 08/30/16 (8)

RE: Pentode/Triode Operating Points (6BL8,6U8,6GH8,7199 etc) - Tom Bavis 19:47:47 08/31/16 (2)

Thanks Tom - FlaCharlie 10:05:19 09/01/16 (1)

RE: Thanks Tom - Eli Duttman 11:26:37 09/01/16 (0)

RE: Pentode/Triode Operating Points (6BL8,6U8,6GH8,7199 etc) - Jim McShane 13:13:13 08/30/16 (3)

Any Suggestions for Circuit Changes After Conversion? - FlaCharlie 09:26:05 08/31/16 (0)

Mullard also - Triode_Kingdom 19:57:09 08/30/16 (1)

RE: Mullard also - Eli Duttman 06:35:02 08/31/16 (0)

RE:It might be a typo . nt. - DAK 11:50:17 08/30/16 (0)

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