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Old 1st September 2012, 05:57 PM   #21
pilovis is offline pilovis  Italy
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Quote:
Originally Posted by costis_n View Post
Υou also forgot to make the anode resistors 100K. With feedback, you are going to need the gain.
Actually I would like to maintain the 47k anode resistor and increase gain only when feedback is switched on.
I should find a simply way to do it
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Old 1st September 2012, 06:51 PM   #22
pilovis is offline pilovis  Italy
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Corrected schematic:

note: I increased anode resistor of 6N2P from 47k to 56k (small gain adjustment) but if feedback is used I'd suggest to increase it up to 100k.

Click the image to open in full size.

Click the image to open in full size.

Last edited by pilovis; 1st September 2012 at 06:56 PM.
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Old 1st September 2012, 09:10 PM   #23
jane is offline jane  Norway
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68k/150R will give you another feedback fraction than 68k/1k.
What is your target?
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Old 2nd September 2012, 09:00 PM   #24
jjman is offline jjman  United States
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With UL in this setup, what would the effect be of changing the value of the screen resistors? Is dropping the screens below the plates the major consideration? Is it merely a tradeoff between power and safety? I ask because I think I've generally seen lower screen resistor values on UL and I'm ignorant on UL's considerations.
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Old 2nd September 2012, 10:10 PM   #25
pilovis is offline pilovis  Italy
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Quote:
Originally Posted by jjman View Post
With UL in this setup, what would the effect be of changing the value of the screen resistors? Is dropping the screens below the plates the major consideration? Is it merely a tradeoff between power and safety? I ask because I think I've generally seen lower screen resistor values on UL and I'm ignorant on UL's considerations.
6L6 screen grid should not exceed 250V, this circuit has 290V plate voltage, the screen resistor is calculated for 50V voltage drop.

Last edited by pilovis; 2nd September 2012 at 10:14 PM.
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Old 2nd September 2012, 10:11 PM   #26
pilovis is offline pilovis  Italy
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Quote:
Originally Posted by jane View Post
68k/150R will give you another feedback fraction than 68k/1k.
What is your target?
Right, I think I shuld lower feedback resistors as low as 10K, do you agree?

Last edited by pilovis; 2nd September 2012 at 10:15 PM.
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Old 3rd September 2012, 12:07 PM   #27
jane is offline jane  Norway
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Originally Posted by pilovis View Post
Right, I think I shuld lower feedback resistors as low as 10K, do you agree?
If the open loop gain is something like 50x (I havnt calculated the Aol) then a 10k feedback resistor should give about 5dB feedback. Under the same conditions a 68k resistor would give you less than 1dB feedback.
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Old 4th September 2012, 05:42 PM   #28
rongon is offline rongon  United States
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Quote:
Originally Posted by jjman
With UL in this setup, what would the effect be of changing the value of the screen resistors? Is dropping the screens below the plates the major consideration? Is it merely a tradeoff between power and safety? I ask because I think I've generally seen lower screen resistor values on UL and I'm ignorant on UL's considerations.
Quote:
Originally Posted by pilovis View Post
6L6 screen grid should not exceed 250V, this circuit has 290V plate voltage, the screen resistor is calculated for 50V voltage drop.
270V is the max screen grid voltage for old 6L6 types, like the original metal can ones, or 6L6G, 6L6GA (from RCA Tube Manual).

The Tung-Sol 6L6WGB was rated for 300V on the screen grid (grid #2).

6L6GC can take 500V (!) on the screens "in push pull circuits where the screen of each tube is connected to a tap on the plate winding on the output transformer." (From GE Tube Manual)

So, with screen voltages above 300V, use a 6L6GC.

The screen grid resistor in a UL connection is a "screen stopper," is it not? It's not a decoupling resistor with a filter capacitor. The screen stopper goes from the screen grid to the 40% tap on the OPT. 2k2 is a large value in this spot, but I don't think it does any harm for it to be a large value. You could try the more usual 470R and see if it makes any difference at all.


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Last edited by rongon; 4th September 2012 at 05:46 PM.
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Old 4th September 2012, 06:14 PM   #29
TheGimp is offline TheGimp  United States
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The screen resistor value does have an effect on distortion.
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Old 4th September 2012, 06:31 PM   #30
rongon is offline rongon  United States
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Originally Posted by TheGimp View Post
The screen resistor value does have an effect on distortion.
The larger the value, the lower the distortion?

Any other limiting factors? I haven't seen this discussed, so am curious...

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