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Old 21st April 2008, 05:18 PM   #1
Jeb-D. is offline Jeb-D.  United States
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Default Partial feedback confusion

After reading the TCJ and some previous threads, the Partial feedback idea still isn't completely clear. It is my understanding that the output tube acts as an ICVS. With the driver tube acting as a VCIS. So it would seem that the only voltage that shows up on the grid of the output tube is error AC voltage(the difference between the driver and output tubes dynamic characteristics), since the driver tube is theoretically working into a 0ohm load. Is this correct or does the grid of the output tube still need the same grid drive voltage as if it were an open-loop system?
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Old 21st April 2008, 07:12 PM   #2
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The grid of the output tube will need the full grid drive voltage as it would in a non local feedback amplifier. Where an ideal current to voltage stage would have an input impedance of near 0 ohms, the output stage of a local feedback amplifier is many K ohms.

The P-P circuit used in the 47 P-P and Tabor amps has an input impedance around 6.5K. The impedance looking into the output stage also changes with amplifier loading. The lighter the load on the output stage is (the speaker), the lower the input impedance of the output stage is. The driver stage effectively sees a load that is the inverse of the speaker load.

On the Solid State Tabor the input impedance of the output stage varies from ~50 ohms with the amp un-loaded to ~800 ohms when driving a 4 ohm load from the 16 ohm tap.

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Old 21st April 2008, 10:24 PM   #3
Jeb-D. is offline Jeb-D.  United States
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That makes things more clear, thanks! What kind of output impedance are you getting from the Tabor amp? I am planning to do a Pentode amplifier, but am looking into this as an alternative to global feedback.
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Old 22nd April 2008, 01:11 AM   #4
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I don't have notes on the output impedance of the Tabor. Can't test it as it is partially disassembled in the slow process of attempting to add an interstage like the SS version has.

The 47 is in current use in the living room. A quick test shows that the output impedance is 1.7 ohms on the 8 ohm tap. Damping factor is 8/1.7=4.7.
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Old 22nd April 2008, 05:20 PM   #5
Jeb-D. is offline Jeb-D.  United States
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Quote:
The 47 is in current use in the living room. A quick test shows that the output impedance is 1.7 ohms on the 8 ohm tap. Damping factor is 8/1.7=4.7.
Great. I'll try to implement this feedback method in the next build.
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