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Old 22nd November 2010, 05:56 PM   #11
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I think Michael probably was referring to the series resistor from the previous stage (maybe a triode, so needs this to establish a constant source R), not the grid stopper R. Does seem like one could skip the grid stopper here though.
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Old 22nd November 2010, 08:16 PM   #12
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Quote:
Originally Posted by artosalo View Post
If you just add a resistor in series with the grid, it will directly cut the high frequency response since it forms a low pass filter with Miller capacitance. However, when the feedback resistor is also added from anode to grid, the situation changes. The smaller the anode-to-grid resistor is, the lower the gain is, but also with the same the high frequency response will be extended. And distortion reduced, input impedance reduced, output impedance reduced, etc.etc....
Why would you add a large series resistor to the grid if not for the purpose of the feedback divider? Compared to a circuit using only a grid stopper, how can plate feedback with an added input resistor extend the frequency response? Also not sure about eliminating the grid stopper unless the input and FB resistors are right on the socket pin, or at least some portion of them.

My experience with these stages is that even at relatively low gain using a lot of feedback you don't get any better upper Fc than a full gain circuit with a low value grid stopper and careful layout.

At typical circuit values assuming a D3a in triode mode (Cgp 2.7 pF) and 5K-10K ohm input resistance, stage gain about 40, the upper Fc is on the order of 100-200 KHz. I think you can do much better with a low value grid stopper and no feedback, but for audio it's sufficient.

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Old 22nd November 2010, 11:36 PM   #13
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Quote:
Originally Posted by Wavebourn View Post
According to what we learned in Russia, it is "Parallel Feedback By Voltage". No confusion is possible: a fraction of output voltage is applied in parallel with input signal.
I'm confused. Two voltage sources in parallel?

The Schade paper shows two voltage sources, the input signal and the divided down output signal, summed by using an interstage transformer sitting on the feedback divider. I would call this a series connection.
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Old 22nd November 2010, 11:42 PM   #14
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Quote:
Originally Posted by Michael Koster View Post
I'm confused. Two voltage sources in parallel?

The Schade paper shows two voltage sources, the input signal and the divided down output signal, summed by using an interstage transformer sitting on the feedback divider. I would call this a series connection.
Effectively it is in parallel, but summed on grids, with transformer windings in series.
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Old 23rd November 2010, 03:20 AM   #15
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Wow! I'm really surprised to see the responses this has illicited.

Ok, I'll adopt the nomenclature "Shunt Feedback" if you prefer, but I'm not giving up "Thevenin's Equivalent".

Sorry, I gave up Cycles per Second in lieu of Hertz, and I'm not going any further.


Next you will be wanting me to concede that current flows from Positive to Negative.

Bah Humbug!

Back to the core of the question at hand, from everything posted here I still feel that what O.H.Schade described is now classical feedback with an input resistance and a feedback resistance. The equations can be derived if the Thevenin's equivalent of the preceding stage is used as the input to the gain stage with feedback.

The output impedance (including next stage grid resistor) becomes the input impedance of the feedback equation.

This to me looks like fig 33b for single ended or 33C for PP.


This is the central premise of what I was trying to convey.
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Old 23rd November 2010, 04:45 AM   #16
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Not so fast...

The circuit in 33(c) doesn't depend on the output impedance of the preceding stage to determine the feedback ratio. It subtracts the feedback voltage from the drive voltage at the node where the voltage divider tap connects to the coupling transformer winding. The feedback induces no current in the input circuit. This one generally requires a lot of drive voltage.

There are at least two other ways to get feedback from plate to grid. One of these is with an input resistance and a feedback resistance forming a divider, the tap of which is connected to the grid. In this circuit, the output impedance of the preceding stage is added to the input resistance. If there is a grid resistor, it's bootstrapped by the feedback and only a fraction of it's value contributes. This type generally needs a lower drive impedance and substantial drive voltage (to drive a pentode power amp stage with low output impedance).

The third topology I can think of is perhaps a special case of the second, where the driver dynamic load is mostly made up of the current through the feedback resistor. Examples are the Tabor from Gary Pimm and the DCPP amp from Pete Millett. The driver pentode acts as voltage-to-current converter loaded by the feedback resistors. Thus the grid-plate voltage of the output stage is a function of the input voltage to the driver pentode stage. The RH84 etc. do this using a triode driver. That's actually not so bad because the triode's Ri as well as any grid resistor or driver plate load resistor are bootstrapped by the feedback. This type is to me most efficient in terms of overall power gain for the circuit effort; the input pentode only needs to swing the output pentode's grid voltage. As a plus it offers the potential to get differential feedback in push-pull mode to smooth out the crossover distortion (or operate closer to class B). This one also produces the lowest effective plate resistance, close to 1/gm.

Last edited by Michael Koster; 23rd November 2010 at 04:51 AM.
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Old 23rd November 2010, 07:02 AM   #17
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Did a comparision sim quite a while ago in another thread.

Click the image to open in full size.

This showed a reduction in distortion when going the route of the lower two to half of the upper. Yet to be proven IRL.

In another thread I showed a 12ax7 buffered with a MOSFET SF also showed promising results using the "lower" method.

The 12AT17 has a Ri of ca 20k in the upper version while the idel driver, pentode, has infinite and can be looked upon as a current source.

In the sand-world the most wellknown application of "Schade" is NPīs Zen.

But hasnīt all of this been covered in many threads before???????

Last edited by revintage; 23rd November 2010 at 07:05 AM.
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Old 23rd November 2010, 07:46 AM   #18
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Quote:
Originally Posted by Michael Koster View Post
Not so fast...

The circuit in 33(c) doesn't depend on the output impedance of the preceding stage to determine the feedback ratio. It subtracts the feedback voltage from the drive voltage at the node where the voltage divider tap connects to the coupling transformer winding. The feedback induces no current in the input circuit. This one generally requires a lot of drive voltage.
Actually, load of the preceding stage on the grid's impedance is higher when feedback is applied. Let it be very high impedance, mostly capacitive, but anyway it is higher.
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Old 23rd November 2010, 08:13 AM   #19
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Quote:
...how can plate feedback with an added input resistor extend the frequency response?

Below is the figure of the results achieved with shunt feedback connection. Different traces represent different values of feedback resistor connected from anode to grid. The series resistor at the grid is held constant.

The topmost trace represent largest feedback resistor. The gain is 39 dB and fc (-3 dB) = 8 kHz. The lowes trace represent smallest series resistor. The gain is reduced 5 dB (to 34 dB) and fc (-3 dB) is extended to 14,4 kHz.

How this happens ? There are many good books to explain this so well that I do not even try put this in words.
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Old 23rd November 2010, 08:23 AM   #20
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Photo here:

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