That means, during first decade of Hi-fi sound reproduction was taken "rather seriously...", no more. 😀
people do misunderstand "negative feedback" - its use in circuits predated Black's "invention" - triodes have built in negative feedback from the plate electric field and it was in fact a limitation since however "linear" you couldn't get V gain higher than the tube mu - positive feedback was one attempt to overcome the built in negative feedback, later tetrodes, pentodes were developed to get higher V gain in one tube by interrupting the internal negative feedback
cathode followers were happily used before the intellectual framework of modern negative feedback theory make it glaringly obvious that they have "100%" negative feedback
so early tube amps did benefit from local negative feedback in common circuits before Black showed that you could deliberately build high gain amps and reduce their distortion, stabilize their gain with a Global negative feedback that "threw away" the gain you just built into the amp - the latter step of reducing the gain deliberately with feedback was the "unobvious" part because from the beginning of vacuum tube circuit design the fight was to get enough gain in the 1st place
cathode followers were happily used before the intellectual framework of modern negative feedback theory make it glaringly obvious that they have "100%" negative feedback
so early tube amps did benefit from local negative feedback in common circuits before Black showed that you could deliberately build high gain amps and reduce their distortion, stabilize their gain with a Global negative feedback that "threw away" the gain you just built into the amp - the latter step of reducing the gain deliberately with feedback was the "unobvious" part because from the beginning of vacuum tube circuit design the fight was to get enough gain in the 1st place
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There is a book published on the history of vacuum tube technology in Japan (can't recall the name exactly) that mentions research on linearity before neg. feedback came along. One of the techniques was the VT current mirror. All such schemes were abandoned after Black published neg. fdbk. I think one can assume that a cathode degeneration resistor was well known for linearity then also. And certainly some understanding of how to make the tubes themselves more linear by construction. Once neg. fdbk became established it would have become more important to get optimum gm tubes, which we see in abundance, rather than the optimum linear Mu tubes.
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Audio amplifier development 1927–1937
Actually yes, that is quite accurate interpretation.
Actually yes, that is quite accurate interpretation.
The tube history book: "History of Electron Tubes" by S. Okamura
Now that I remember it, I think it was the "voltage mirror" circuit that was mentioned for pre-feedback linearity research in the above book. (and some other techniques too)
The "voltage mirror" is the circuit used as the input stage of the modern Aikido circuit. This uses a non-linear plate resistor as a load resistor for a tube to get linear voltage gain. The top loading tube in the Aikido has the same variable plate resistance as the bottom triode, giving constant Mu/2 gain as a result. One can also scale this for a different rp on top to get a higher V gain. And a pentode can be used bottom side as well to remove the bottom tube's rp from the gain equation.
Presumeably, someone who imagined the "voltage mirror" form back then would have also imagined the "current mirror" form too, but it is not mentioned in the book. The current mirror circuit form uses a biased thermionic diode across the grid to cathode points of a tube (pentode or high mu triode, but pentodes came later though). The diode performs a 2/3 power current to voltage transform, and the subsequent tube performs a 3/2 power voltage to current transform. Net result being linear current gain if the tube gm is greater than 1/rp of the diode.
One also gets a similar effect when operating a tube in positive grid territory (only). The grid acts as a forward biased thermionic diode with input current determining its voltage. The grid voltage then determines cathode current. Can also look at it as the grid intercepting a constant fraction of the cathode current (at least until plate voltage - triode case- gets low enough to impact emission and the grid current interception ratio.) This results in a current gain "Beta" just like a transistor, and Beta drops off as plate voltage gets too low. -Beta droop- similar to how bipolar transistors do but versus collector current. Screen driven pentode tubes fit this scenario as well.
Now that I remember it, I think it was the "voltage mirror" circuit that was mentioned for pre-feedback linearity research in the above book. (and some other techniques too)
The "voltage mirror" is the circuit used as the input stage of the modern Aikido circuit. This uses a non-linear plate resistor as a load resistor for a tube to get linear voltage gain. The top loading tube in the Aikido has the same variable plate resistance as the bottom triode, giving constant Mu/2 gain as a result. One can also scale this for a different rp on top to get a higher V gain. And a pentode can be used bottom side as well to remove the bottom tube's rp from the gain equation.
Presumeably, someone who imagined the "voltage mirror" form back then would have also imagined the "current mirror" form too, but it is not mentioned in the book. The current mirror circuit form uses a biased thermionic diode across the grid to cathode points of a tube (pentode or high mu triode, but pentodes came later though). The diode performs a 2/3 power current to voltage transform, and the subsequent tube performs a 3/2 power voltage to current transform. Net result being linear current gain if the tube gm is greater than 1/rp of the diode.
One also gets a similar effect when operating a tube in positive grid territory (only). The grid acts as a forward biased thermionic diode with input current determining its voltage. The grid voltage then determines cathode current. Can also look at it as the grid intercepting a constant fraction of the cathode current (at least until plate voltage - triode case- gets low enough to impact emission and the grid current interception ratio.) This results in a current gain "Beta" just like a transistor, and Beta drops off as plate voltage gets too low. -Beta droop- similar to how bipolar transistors do but versus collector current. Screen driven pentode tubes fit this scenario as well.
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