Hi all, oprtr back.
I'd like to discuss a concrete example on EL84 operating points in triode vs pentode mode:
Why is it that all of the spec sheets insist that when those babies are triode operated that Rk should be higher and thus the bias lower. Aren't the current swings supposed to be lower and thus safer when running the tubes in class A, triode or not.
I have measured both plate and grid current and dissipation with the tubes cathode biased at 98% and it was perfect. I guess everyone worries about the screen grid, but even at max plate bias the screen was well below the 2W dissipation with no swings.
VS
What am I missing
I'd like to discuss a concrete example on EL84 operating points in triode vs pentode mode:
Why is it that all of the spec sheets insist that when those babies are triode operated that Rk should be higher and thus the bias lower. Aren't the current swings supposed to be lower and thus safer when running the tubes in class A, triode or not.
I have measured both plate and grid current and dissipation with the tubes cathode biased at 98% and it was perfect. I guess everyone worries about the screen grid, but even at max plate bias the screen was well below the 2W dissipation with no swings.
An externally hosted image should be here but it was not working when we last tested it.
VS
An externally hosted image should be here but it was not working when we last tested it.
What am I missing
I always thought that cathode biased class A (triode or not) was always at max amplitude and thus had no dramatic swings in current. Hence we never bias AB/PP above 70% and I always saw cathode biased class A as safe all the way up to 100%
> I always thought that cathode biased class A (triode or not) was always at max amplitude and thus had no dramatic swings in current.
You are thinking of DC current. The AC/audio current swings wildly. {as Mona says} Ideally from zero to twice DC. In a pentode, about 20% to 180%. In a triode, rarely past 50% to 150%. And since the max current is less in triode, the idle current is selected lower for bets total performance.
> data sheets generally show snapshot examples of performance, not necessarily recommended operating points.
The datasheet was intended to sell the tube. Small-signal tubes with a wide range of possible uses, the "Conditions" might be show-off. (In audio we never run 12AT7 per datasheet, we never use that much current, but a TV tuner will.) Tubes intended for audio power amps, the datasheet conditions are always excellent starting points; if they weren't, designers would not be inspired to use (and buy) that tube. (With a few marketing conflicts: for historical reasons when 6L6 ratings went up the Conditions were mostly copied-over, etc.)
You are thinking of DC current. The AC/audio current swings wildly. {as Mona says} Ideally from zero to twice DC. In a pentode, about 20% to 180%. In a triode, rarely past 50% to 150%. And since the max current is less in triode, the idle current is selected lower for bets total performance.
> data sheets generally show snapshot examples of performance, not necessarily recommended operating points.
The datasheet was intended to sell the tube. Small-signal tubes with a wide range of possible uses, the "Conditions" might be show-off. (In audio we never run 12AT7 per datasheet, we never use that much current, but a TV tuner will.) Tubes intended for audio power amps, the datasheet conditions are always excellent starting points; if they weren't, designers would not be inspired to use (and buy) that tube. (With a few marketing conflicts: for historical reasons when 6L6 ratings went up the Conditions were mostly copied-over, etc.)
EL84:
With a good output transformer, triode mode may be lower power.
But the frequency response and damping factor Are reasonable, even without negative feedback.
With a good output transformer, pentode mode may have higher power.
But the frequency response and damping factor are Not reasonable, so the circuit needs negative feedback for those to be reasonable.
With a good output transformer, triode mode may be lower power.
But the frequency response and damping factor Are reasonable, even without negative feedback.
With a good output transformer, pentode mode may have higher power.
But the frequency response and damping factor are Not reasonable, so the circuit needs negative feedback for those to be reasonable.
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