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Old 23rd November 2010, 01:53 PM   #21
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Originally Posted by artosalo View Post
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.
I see this, but what is the performance with no feedback and only a 100 ohm grid stopper? I thought your claim was that adding local feedback to a stage extends the frequency response. For the no-feedback case the grid resistor would be ~100 ohms.

Please read my entire question from the previous post "Compared to a circuit using only a grid stopper, how can plate feedback with an added input resistor extend the frequency response?"

Cheers,

Michael

Last edited by Michael Koster; 23rd November 2010 at 02:11 PM.
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Old 23rd November 2010, 02:08 PM   #22
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Originally Posted by Wavebourn View Post
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.
So the load on the driver increases because it has to swing more voltage into the distributed winding capacitance of the coupling transformer... That's creative!

Cheers,

Michael
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Old 23rd November 2010, 04:59 PM   #23
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I see this, but what is the performance with no feedback and only a 100 ohm grid stopper? I thought your claim was that adding local feedback to a stage extends the frequency response. For the no-feedback case the grid resistor would be ~100 ohms.
This seems to be quite depleted topic now.
However I still want add that nobody should be concerned about worse frequency response when adding shunt feedback to tube stage, even when the grid series resistor is some 33...100 kohms, provided that the anode-grid resistor is also added and is some reasonable level, say below 1M.
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Old 24th November 2010, 04:21 AM   #24
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Originally Posted by artosalo View Post
This seems to be quite depleted topic now.
However I still want add that nobody should be concerned about worse frequency response when adding shunt feedback to tube stage, even when the grid series resistor is some 33...100 kohms, provided that the anode-grid resistor is also added and is some reasonable level, say below 1M.
Yes, depleted, dead, fini... Whatever you say.

Bye
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Old 24th November 2010, 04:54 AM   #25
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Originally Posted by Michael Koster View Post
So the load on the driver increases because it has to swing more voltage into the distributed winding capacitance of the coupling transformer... That's creative!
It needs more voltage to drive tube's input impedance, since feedback is applied in an opposite polarity to the opposite side of the winding.
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Old 24th November 2010, 05:45 AM   #26
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That means, I was wrong: it is feedback applied in series, actually, thanks to the transformer.
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Old 16th April 2013, 06:29 PM   #27
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one pot for screen voltage adjust and second for adjusting of "transformer % tap"
will it fly?
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Old 17th April 2013, 01:12 PM   #28
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In my experience guitar amplifiers with feedback to reduce Zout simply don't work well with large cones. Triode-like or low Zout can work in specific cases where I don't want an "open" sound but not as a rule. High Zout is desireable with 10" and 12" cones. If you can find another way to reduce the hum will surely be beneficial to the sound. Otherwise you will have to use small cones to get some true trebles.
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Old 20th April 2013, 05:01 PM   #29
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one pot for screen voltage adjust and second for adjusting of "transformer % tap"
will it fly?

This works, but the supply voltage for the NFET must be higher than Ua if the DC-potential at the screen is near to Ua, like is the case with EL34 and 6L6GC..
Think about the voltage swing at the anode; there is the DC-component (about Ua) + AC component. Then the screen must follow above Ua level.

With sweep tubes when the Ug2 is some 150 V your circuit works as it is now.
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Old 20th April 2013, 05:11 PM   #30
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yes,i forgot about the anode swing
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