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Class A Operation/Bias Points with PP 6B4G

Posted by FlaCharlie (A) on August 18, 2007 at 09:20:18

Still trying to grasp some of the tech theory of all this. So, first, another basic question: What constitutes Class A operation? Can anyone give me a quick, easy to understand, tutorial?

In a post below there was a discussion concerning the optimum bias point for PP 6B4Gs between old buddies Tre' and drlowmu. It somehow degenerated into namecalling (imagine that) and I think a few of the less productive posts were removed. I'm not trying to start THAT up again and I've never been able to figure out why people can't just accept that this is ALL subjective in terms of preferences and just leave it at that. In the PP 6B4G I'm building I'm trying out a few different ideas so I can arrive at MY preference. In this case being ignorant of theory may be a blessing since it creates a willingness to try different things. Some would just say I'm gullible.

Sorry for the diversion . . . back to the technical: Tre' suggests 250v plate to cathode and 60mA of bias (which is considered pretty standard and is also what Pete Millet uses on his schematic) and says this will produce the maximum Class A output. Jeff prefers (and in a deleted post he listed a few other builders who concur) 45mA. Tre' says this bias point will, in some situations, result in Class B operation and higher distortion. I plan on trying both and maybe even something in between too, but I'd like to get a handle on the technical side of this. So, a few questions:

If you run the bias at 45mA, can you safely increase the plate voltage (I am using NOS 6B4Gs not the Sovtecs) enough to maintain the 15 watt maximum plate dissipation? If so, is this still Class A operation or will it slip into Class B using 45mA regardless of the plate voltage used?

And Jeff, in your opinion, is the more "relaxed" sound you prefer dependent only on the bias or on a combination of 45mA bias with a particular plate to cathode voltage and the resulting plate dissipation?

Thanks . . . Charlie

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