I try to figure out what happen to the mu, gm and ra when a tube gradually wear out from internet and books but no detail information on have been found. I guess as tube gradually wear out the mu will be the only parameter that stay constant. Is that correct?
But among many other factors, distortion is a function of the open loop gain versus the closed loop gain (for an amp where there is negative feedback).
The open loop gain goes down as the tube weakens.
As the open loop gain goes down, the distortion in closed loop goes up.
And whenever the ratio of open loop gain is close to the closed loop gain that is 'dictated' by the feedback ratio,
then the feedback math dictates that the closed loop gain is not exactly according to Rf and Ri formulas.
So, that gain might vary.
How about that?
The open loop gain goes down as the tube weakens.
As the open loop gain goes down, the distortion in closed loop goes up.
And whenever the ratio of open loop gain is close to the closed loop gain that is 'dictated' by the feedback ratio,
then the feedback math dictates that the closed loop gain is not exactly according to Rf and Ri formulas.
So, that gain might vary.
How about that?
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If you want to use (and calculate negative feedback), be sure to have open loop gain that is Much More than the feedback'a "dictated" closed loop gain.
Otherwise, the "dictation" effectively gets ignored by the circuit.
Otherwise, the "dictation" effectively gets ignored by the circuit.
I think Rayma's post is the correct answer and everything else is off topic.
By the way, the heater to cathode insulation also slowly gets worse if there is a large voltage between the heater and the cathode. That has something to do with electrolysis. But that's also off topic, as the question was about transconductance, rp and mu.
By the way, the heater to cathode insulation also slowly gets worse if there is a large voltage between the heater and the cathode. That has something to do with electrolysis. But that's also off topic, as the question was about transconductance, rp and mu.
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MarcelvdG,
You are absolutely correct.
I got carried away.
If januaryabc wants to know what happens to an amplifier performance when its tube(s) wear out . . .
Then he can ask that additional question too.
You are absolutely correct.
I got carried away.
If januaryabc wants to know what happens to an amplifier performance when its tube(s) wear out . . .
Then he can ask that additional question too.
From you answers I can imagine that the loadline curve will gradually shift horizontally to the right, and as mu is more or less keep constant, the horizontal spacing between different gird should still be the same. Just like the Vg=0 curve will shift to the position of Vg=-1. Is my understanding correct?
If that is correct, do anybody know when a tube nearly comes to the end of life, Vg=0 will approximately shift to which Vg curve? Take 300b as an example, I guess Vg=0 curve's position will gradually goes to Vg=-40's position at nearly the end of its lifespan. Is that a good estimation? I am asking because I think I should take this into account when setting the operating point of a cathode biased amp so as to let the tube to perform as best as it can till the end of its life.
If that is correct, do anybody know when a tube nearly comes to the end of life, Vg=0 will approximately shift to which Vg curve? Take 300b as an example, I guess Vg=0 curve's position will gradually goes to Vg=-40's position at nearly the end of its lifespan. Is that a good estimation? I am asking because I think I should take this into account when setting the operating point of a cathode biased amp so as to let the tube to perform as best as it can till the end of its life.
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It seems your view is correct. The slope is less steep. But when considering horizontal loadline the spacing between different grid voltage should remains the same?
A Load-line always has the slope of the Load.
Load, Ohms = Voltage/Current.
The output transformer and loudspeaker do not change over time (unless the speaker suspension wears out, etc.)
The slope of the Grid Line is plate resistance, rp.
As the tube ages, . . .
The Grid Line Slope becomes Less Steep.
As plate resistance goes up, the grid line slopes more to the right.
I am talking about the graph that has lines with 0, -1V, -2V, . . . etc.
I hope that helps.
Load, Ohms = Voltage/Current.
The output transformer and loudspeaker do not change over time (unless the speaker suspension wears out, etc.)
The slope of the Grid Line is plate resistance, rp.
As the tube ages, . . .
The Grid Line Slope becomes Less Steep.
As plate resistance goes up, the grid line slopes more to the right.
I am talking about the graph that has lines with 0, -1V, -2V, . . . etc.
I hope that helps.
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It's hard to find tube curves with mu, rp and gm, but Sylvania had them in their manual for the 12AX7.
The mu is not real constant with operating point variation, but is better and more linear at smaller Vgk voltages.
These curves will vary even more with aging, but it's doubtful whether anyone has ever made aged curves.
http://www.philbrickarchive.org/12ax7_sylvania.pdf
The mu is not real constant with operating point variation, but is better and more linear at smaller Vgk voltages.
These curves will vary even more with aging, but it's doubtful whether anyone has ever made aged curves.
http://www.philbrickarchive.org/12ax7_sylvania.pdf
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