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Michael Koster
diyAudio Member

Join Date: Jan 2008
Location: Sunnyvale, CA
Quote:
 Originally Posted by artosalo 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.

Michael Koster
diyAudio Member

Join Date: Jan 2008
Location: Sunnyvale, CA
Quote:
 Originally Posted by Wavebourn 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

artosalo
diyAudio Member

Join Date: Feb 2010
Quote:
 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.

Michael Koster
diyAudio Member

Join Date: Jan 2008
Location: Sunnyvale, CA
Quote:
 Originally Posted by artosalo 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

Wavebourn
Designer & Technologist
diyAudio Member

Join Date: Sep 2006
Location: Pleasant Hill, CA
Quote:
 Originally Posted by Michael Koster 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.
__________________
Nothing in the universe is perfect. The ideal things are the ones that are most optimal. Optimization criteria, what matters. When I hear "No Compromise Design", I want to take a sledgehammer and test how impact-proof it is.

 24th November 2010, 05:45 AM #26 Wavebourn   Designer & Technologist diyAudio Member     Join Date: Sep 2006 Location: Pleasant Hill, CA That means, I was wrong: it is feedback applied in series, actually, thanks to the transformer. __________________ Nothing in the universe is perfect. The ideal things are the ones that are most optimal. Optimization criteria, what matters. When I hear "No Compromise Design", I want to take a sledgehammer and test how impact-proof it is.
 16th April 2013, 06:29 PM #27 hpeter   diyAudio Member     Join Date: Feb 2013 Location: Electrostats or bust one pot for screen voltage adjust and second for adjusting of "transformer % tap" will it fly? __________________ BLOG. XMOS_U8/PCM5102A > LL1544A > 6H9C+gyrator > 2A3_SOV/JJ > LL1660 > STAX SR207 _ SR L300 _ SR007 # LL2748 > 394A > LC 12H/1000µ
 17th April 2013, 01:12 PM #28 45   diyAudio Member     Join Date: Dec 2008 Location: UK 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.
artosalo
diyAudio Member

Join Date: Feb 2010
Quote:
 Originally Posted by hpeter 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.

 20th April 2013, 05:11 PM #30 hpeter   diyAudio Member     Join Date: Feb 2013 Location: Electrostats or bust yes,i forgot about the anode swing __________________ BLOG. XMOS_U8/PCM5102A > LL1544A > 6H9C+gyrator > 2A3_SOV/JJ > LL1660 > STAX SR207 _ SR L300 _ SR007 # LL2748 > 394A > LC 12H/1000µ

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