I've been trying to learn what I can about how push/pull pentode outputs work, and I've carefully read (and I believe I've understood) what is presented at valvewizard.uk site on this subject.
I can see that some tube datasheets provide very specific operating points as examples. For instance, the sheet from GEC for the KT-88 helpfully suggests that you can run a pair of these with 560V B+, a 4500 ohm load, G2 supplied with 300V, G1 fix-biased to about -34V, each plate idling around 60mA. These conditions do correspond to a point on the plate characteristics chart, yay.
Now, and finally getting around to my question: when I plot the load lines as described by The Valvewizard, they go across the maximum plate dissipation curve, and into that forbidden zone rather deeply... Valvewizard says it's ok for the class-b part of the load line to cross. In this case the class-a part of the line also crosses, and it just doesn't look okay to me. Granted, I don't know shirt from shinola with this stuff. What's going on here? Am I just being a scaredy cat?
Because this confused me, I also looked at the datasheet for 6L6GC, it looks to me like it has somewhat more sane ideas. The load lines skirt the max dissipation curve closely.
Most likely, I won't ever embark on building any such thing, but still I'm the curious type...thanks for any insight on this.
I can see that some tube datasheets provide very specific operating points as examples. For instance, the sheet from GEC for the KT-88 helpfully suggests that you can run a pair of these with 560V B+, a 4500 ohm load, G2 supplied with 300V, G1 fix-biased to about -34V, each plate idling around 60mA. These conditions do correspond to a point on the plate characteristics chart, yay.
Now, and finally getting around to my question: when I plot the load lines as described by The Valvewizard, they go across the maximum plate dissipation curve, and into that forbidden zone rather deeply... Valvewizard says it's ok for the class-b part of the load line to cross. In this case the class-a part of the line also crosses, and it just doesn't look okay to me. Granted, I don't know shirt from shinola with this stuff. What's going on here? Am I just being a scaredy cat?
Because this confused me, I also looked at the datasheet for 6L6GC, it looks to me like it has somewhat more sane ideas. The load lines skirt the max dissipation curve closely.
Most likely, I won't ever embark on building any such thing, but still I'm the curious type...thanks for any insight on this.
Only just, not enough to be worth clutching your pearls just yet. Besides, the HT voltage is expected to sag under load, so average dissipation will remain comfortably within limits even if the load line looks a bit 'hot' when drawn with idle voltages.
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Wow my question went right to the boss. Your reply is much appreciated sir.
I'm half-assedly fantasizing about a purpose-built amp for playing jazz guitar, as I do every so often. Trying to tamp my usual ambition though, and think more about having someone else build it. Thank you for the information.
I'm half-assedly fantasizing about a purpose-built amp for playing jazz guitar, as I do every so often. Trying to tamp my usual ambition though, and think more about having someone else build it. Thank you for the information.