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EL84 triode wired bias and operating points
EL84 triode wired bias and operating points
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Old 22nd May 2020, 08:45 PM   #1
operater is offline operater
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Default EL84 triode wired bias and operating points

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.

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VS

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What am I missing
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Old 22nd May 2020, 09:21 PM   #2
Ketje is offline Ketje  Belgium
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In SE the current can swing from 0 to Ibias to 2xIbais.
As a triode the tube can't get as much max current as in pentode.So it's no use turning up the bias that high.
Mona
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Old 22nd May 2020, 10:02 PM   #3
operater is offline operater
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Quote:
Originally Posted by Ketje View Post
In SE the current can swing from 0 to Ibias to 2xIbais.
As a triode the tube can't get as much max current as in pentode.So it's no use turning up the bias that high.
Mona
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%
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Old 23rd May 2020, 12:42 AM   #4
20to20 is offline 20to20  United States
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The data sheets generally show snapshot examples of performance, not necessarily recommended operating points. There is a difference in the load impedance shown between those two examples, also. The bias point is @9.5v for the triode example. Just snapshots.
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Old 23rd May 2020, 05:45 AM   #5
PRR is offline PRR  United States
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EL84 triode wired bias and operating points
> 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.)
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Old 23rd May 2020, 08:02 AM   #6
6A3sUMMER is offline 6A3sUMMER  United States
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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.

Last edited by 6A3sUMMER; 23rd May 2020 at 08:04 AM.
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