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Need some education - Negative feedback - Too much ?
Need some education - Negative feedback - Too much ?
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Old 23rd February 2021, 10:01 AM   #11
IvesH is offline IvesH  Belgium
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Need some education - Negative feedback - Too much ?
Quote:
Originally Posted by Depanatoru View Post
That schematic is allmost hopeless , the gain of the pentode is
laughable , look at a practical schematic that I made and draw some conclusions .
Input sensitivity for 2x43Vrms is just 170mV of course without negative feedback .
Russian 6F1P was tried and is allmost the same .
Ordering components now, can you tell me the voltage rating for C2 please ?
Thank you.
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Old 23rd February 2021, 12:43 PM   #12
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by IvesH View Post
Hi Rayma, thank you for taking the time to answer, Yes I know that I deliberately cancel some input (neg feedback) I'm just surprised / not happy that it cancels so much. from x40 to x 13.
If you know the gain without feedback, then you know that the feedback cancels the input voltage to make it just right for the output voltage / no-feedback gain.

Example: no-feedback gain is 100. You add feedback to make the gain 10. Then the difference between the input voltage and the feedback voltage must be output voltage / 100. In this example, the feedback will need to cancel 90% of the input signal to make the result just right to support Vout.

The background is that whatever you do with feedback, the amp itself always continues to operate with its no-feedback gain. The amp doesn't change; you just manipulate the effective input signal (Vin-Vfeedback) to get the right gain.
If you think about it that way, it'll all fall into place!

Jan
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Old 24th February 2021, 05:39 PM   #13
IvesH is offline IvesH  Belgium
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Need some education - Negative feedback - Too much ?
Thank you jan.didden, a very clear explanation. All components are ordered, I will be building soon !
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Old 24th February 2021, 11:30 PM   #14
Depanatoru is offline Depanatoru  Romania
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Quote:
Originally Posted by IvesH View Post
Ordering components now, can you tell me the voltage rating for C2 please ?
Thank you.
C2 can be 16V as the voltage across cathode resistor is 1-2V
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Old 25th February 2021, 10:24 AM   #15
IvesH is offline IvesH  Belgium
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Join Date: Aug 2019
Need some education - Negative feedback - Too much ?
Quote:
Originally Posted by Depanatoru View Post
That schematic is allmost hopeless , the gain of the pentode is
laughable , look at a practical schematic that I made and draw some conclusions .
Input sensitivity for 2x43Vrms is just 170mV of course without negative feedback .
Russian 6F1P was tried and is allmost the same .
Thank you Depanatoru, was this simulation done in ltspice ? Would you mind sharing the file and libs please ?
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Old Today, 08:20 AM   #16
IvesH is offline IvesH  Belgium
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Join Date: Aug 2019
Need some education - Negative feedback - Too much ?
Quote:
Originally Posted by Depanatoru View Post
That schematic is allmost hopeless , the gain of the pentode is
laughable , look at a practical schematic that I made and draw some conclusions .
Input sensitivity for 2x43Vrms is just 170mV of course without negative feedback .
Russian 6F1P was tried and is allmost the same .
Hi Depanatoru, still waiting on components but the model gives a good amplification. Thank you. I did notice in my model that the 6u8a g2 sits at 354 Volt, going to work now. Will check with another tube simulation later today
Thx, stay safe
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File Type: png 2021-03-01 09_12_42-LTspice XVII - [TubeAmpIves.asc].png (10.4 KB, 28 views)
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Old Today, 08:33 AM   #17
Depanatoru is offline Depanatoru  Romania
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That is not a simulation , was built and measured in reality . With multiple tubes .

If your model shows no G2 current ( the same voltage across resistor ) then it is wrong .
But after all I don't understand why you do simulations if you have already the circuit build ... it is easier than soldering components ?

Last edited by Depanatoru; Today at 08:38 AM.
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Old Today, 09:33 AM   #18
IvesH is offline IvesH  Belgium
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Need some education - Negative feedback - Too much ?
No I'm dismantling and just waiting on components. Ordered a bunch of stuff at the same time, like a test high voltage transformer so I do not need to dismantle the case from the amplifier and wanted to build out my workbench and stock. At the same time I'm rebuilding to go from Kathode bias to negative voltage for the output tubes and a stabilized 350V for the pre-amp tubes. Including a little PCB to replace the original single 6U8 kathode resistor to your 2 resistors set-up and the bypass capacitor with a trimmer to set the NFB. Was just killing time with Spice.

Note, the bit strange layout of the PCB is to make it universal with left/ right on my main PCB so the trimmer always faces forward and both turn in the same direction and to compensate for the left / right direction of GND and Kathode in my set-up.

Thank you for your help I'm also looking forward to have it alive so I can see the output on my scope
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File Type: png 6U8 PCB.png (14.8 KB, 24 views)
File Type: jpg IMG_20210301_102010.jpg (529.0 KB, 23 views)
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Old Today, 10:59 AM   #19
Depanatoru is offline Depanatoru  Romania
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OK , just pay attention when you build it for auto oscillation or at least tendancy for that . Without NFB sometimes will oscillate itself or only when you touch the input .
That is the purpose of the capacitor on the pentode input . Others use a RC network in anode or capacitor in the grid of triode . But all designs old or new use some kind of HF filter to make it more stable .
So far I don't know the root cause , it's the PCB , the closeness of grid1 pentode with the anode of triode , or internal coupling .

Last edited by Depanatoru; Today at 11:04 AM.
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