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Old 24th October 2005, 01:10 PM   #1
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Default Long Tail and Local feedback

Hi all,

I just won a quartet of matched JJ-Tesla KT88 on ebay, at a very low price; so I have thus decided to build a PP amplifier.
In the past two weeks I read a lot about tube amplifiers, and I love long tail (I'm a sand guy, and I'm used to use them in semiconductors projects), so this is the way I want to go.
I learned something about negative feedback, and as far I've understood it is not possible (and not suggested) to have a lot of global negative feedback; so why do not have local feedback, instead?
I ended up with two solutions, and I like to ask you if my ideas can work: attached two schematics.
Please do not care about the components values (resistor and caps), I have not yet calculated them, those values are only "casual".
The first schematic (1.pdf) uses local feedback directly from the plate of the differential pair, the second one from the output tubes (both using the R5, R17, C6, C7 networks).
Could they work, or are these stupid ideas? In case, which is the best, in your opinion?
Remember, I'm a newbie in this field, I'm still thinking with "sand" in my mind...

1.pdf
2.pdf

Ciao,
Giovanni
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Old 24th October 2005, 03:51 PM   #2
anatech is offline anatech  Canada
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Hi Giovanni,
Many good amps use overall negative feedback as well. The amount is far lower than you see in solid state designs due to transformar phase shifts. The feedback is typically returned to the cathode of the voltage amplifier tube, but you could use the grid of a diff pair. I've been thinking of trying that. Local feedback in the form of ultra linear or unity coupling may be used in the output stage to further improve operation.

-Chris
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Old 24th October 2005, 04:16 PM   #3
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For what it is worth, I like it best applied it from the transformer through a voltage divider into the bottom grid of the LTP since I don't like feedback in the input stage....it sounds less organic to my little tin ears. I usually form the voltage divider by splitting the "tail" resistor, but with THE CCS down there, it might be tougher to do it that way. I have fed it into the both grids, cross feedback from the plates before, but i didn't like it as much that way, but maybe I just used too much FB.
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Old 24th October 2005, 04:25 PM   #4
ilimzn is offline ilimzn  Croatia
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There are other ways to apply local feedback, and in general, if you need to apply feedback even if only to reduce the overal amplifier gain, it is a good idea to apply it locally in order to reduce the required amount of global feedback to get the same effect.

High(er) valuse of global feedback are possible if phase shifts can be made smaller. In particular, direct couping of stages helps on the low end of the spectrum, however, as Chris said, the transformer is the element that is not easily avoided.

One more way to apply feedback locally is plate to plate feedback between output and drivers. However, since we are generally talking triodes in the drivers, the feedback also acts within the driver stage, thus avoiding the extra phase shift of a coupling cap in the feedback network from plate to grid.

Other methods involve partial local degeneration by only partially bypassing cathode resistors. For LTPs, this is commonly done by splitting the tail resistor into two of twice the value, one for each triode cathode. Then the cathodes are joined by a smaller value resistor, which can even be made adjustable. This way it is possible to vary the 'differential' action from full (small resistor = 0) to none (small resistor = infinite), without appreciable change in bias. BTW this is also possible for a tail CCS but requires a slightly inspired combination of CCS and current mirror for the double tail. It is also possible to use a capacitor instead of the small value resistor in order to make the LTP act as a LTP only for AC (for instance in order to enforce DC balance).
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Old 24th October 2005, 04:31 PM   #5
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Quote:
Originally posted by ilimzn
For LTPs, this is commonly done by splitting the tail resistor into two of twice the value, one for each triode cathode. Then the cathodes are joined by a smaller value resistor, which can even be made adjustable.

Just curious, what would the benefit be in splitting the tail between the two cathodes as opposed to the shared tail resistor? If something that simple will provide a marked improvement, then I might just try it. Is it a balance issue?
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Old 24th October 2005, 06:03 PM   #6
Yvesm is offline Yvesm  France
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My two pence:

http://www.dissident-audio.com/PP_ECL86/Schema.gif

Here local feedback is around the power stage.
It also improves balance.

Yves.
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Old 24th October 2005, 10:10 PM   #7
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by aletheian

Just curious, what would the benefit be in splitting the tail between the two cathodes as opposed to the shared tail resistor? If something that simple will provide a marked improvement, then I might just try it. Is it a balance issue?
No, there is no improvement only on the account of the double tail, if you use triangle to star network conversion you will see that it's really the same thing.
The advantage of a split tail becomes apparent when you need to add adjustable differential gain through local degeneration. Conventionally, you would put two extra unbypasssed resistors into each cathode of the pair. The problem here is that adjustment requires both to be adjusted. In the split tail configuration, only a single component is adjusted. In addition, differential gain can be varied down to nearly zero if the cathode to cathode resistor is made very large, without any change to the bias conditions. Differential gain is maximized when the cathode to cathode resistance is zero, at which point you have a standard LTP.

Now, if the tail resistor is replaced by a current sink, making that a split tail configuration requires an unusual dual current sink, but it does indeed improve DC balance. In fact, using BJT composite sink + mirror, insures DC balance to within fractions of a percent. AC balance can also be independently adjusted by including a plate to plate pot that has the wiper connected to +B via a cap.

Amazing what one can learn looking at schematic diagrams of tube oscilloscopes
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Old 24th October 2005, 10:13 PM   #8
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by Yvesm
My two pence:
http://www.dissident-audio.com/PP_ECL86/Schema.gif
Here local feedback is around the power stage.
It also improves balance.
Yves.
Well worth a look at the very least! One of the most inspired designs I've seen here, another take at plate to plate feedback. I love the added refinement of R12, feedback adjust without change to bias conditions as it only acts on differential signals - brilliant!
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Old 25th October 2005, 04:37 AM   #9
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Hi Croccodillo,

I would like to suggest to check the driver requirements - 12AX7 probably will have a very hard time to drive the Miller capacitance of triode strapped 6550/KT88 at the required swing.

Tom
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Old 25th October 2005, 06:12 AM   #10
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Quote:
(...) another take at plate to plate feedback.
Yes, but in the case of the original poster, 12AX7 definitely has not enough guts to cope with the additional loading by plate shunt FB AND driving triode strapped 6550. Compute the plate load imposed on 12AX7 in this case of the original poster and see yourself.

A beefier high gain alternatives to 12AX7 would be for example EC91 (single system 7 pin miniature, =100). Maybe 12AT7, too.

Tom
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