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Old 10th May 2012, 01:53 AM   #1
Junm is offline Junm  Philippines
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Default Tube VAS stage questions???

Last night, I,ve notice again my Luxman LV103 hybrid amp since the other channel is not working and I was again came accross to the schematics, and by simplifying the schematic attached i've noticed that the tube section is SRPP I think.....? Since I'm still a newbie on tubes so I got these questions for you....anybody is welcome for their answers and comments...please help..

1. Is the tube really a VAS or semi buffer?
2. Is this good and are the tube running on their linear region? what is the current draw or bias?
3. I want to replace the tube with 6N1P will it suit without modding the cathode resistors?
4. Why the cathode resistors value are not equal?
5. What are the function or benefit of the 15pF ceramic cap between plate and grid of the lower tube?
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File Type: pdf LuxMan-VAS.pdf (97.0 KB, 77 views)
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Old 10th May 2012, 05:18 AM   #2
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1. Its not a buffer, its not a VAS, it is a gain stage.
2. Do your math with Ohms law and check Ua/Ia curves.
3. Forget it. See 2
4. See 2
5. HF-compensation

And it is not SRPP the way its drawn here.
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Old 10th May 2012, 07:05 AM   #3
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ok thanks....

i double check the schematics but its drawn this way or am i mistaken....AFAIK the output should be on top of the 3.9k or cathode of upper tube.

Im not yet familiar with tubes since i was born on ss era so i want to learn it..Im still confused on gain and curves of this devices so i'm asking...anybody kind enough to explain the configuration and since most hybrid i have seen uses tube as input stage...is it better this way?
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File Type: jpg Luxman LV-103_1.jpg (573.2 KB, 141 views)
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Old 10th May 2012, 08:21 AM   #4
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If componentvalues and voltages are right, the working point is really bad choosen. Ua=116V and Rk=8.2k gives us Ia=0,49mA. Ua should be higher and Ia at least 10 times higher for this tube to act linear.

The schematic is not readable to my old eyes.
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Old 10th May 2012, 08:31 AM   #5
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I can't upload bigger size than that.....SM is around 22mb....voltages are read from the manuals referance list...

one channel is oscillating even putting a series gate resistor at the ouput Mosfets from 100 to 1.5k....the only things i dont replace is the tubes and the input section,,,

how about replacing the tubes with other type that fairly suit the gain stage....any reccommendation?

i only have 6n1p and 6n6p at the moment...
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Old 10th May 2012, 08:56 AM   #6
DF96 is online now DF96  England
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The circuit you have drawn is a grounded cathode with active load, not SRPP. The full official circuit you supplied is too badly drawn for clarity, so I can't be bothered to check what it really is.

To learn about the SRPP see the Valve Wizard website. If the existing circuit is correctly designed then substituting different valves will degrade performance.
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Old 10th May 2012, 10:08 AM   #7
DF96 is online now DF96  England
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I have had a closer look, and the circuit appears to be an active load. In this sort of application the active load and SRPP have similar performance.
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Old 10th May 2012, 01:58 PM   #8
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Quote:
Originally Posted by Junm View Post
I can't upload bigger size than that.....SM is around 22mb....voltages are read from the manuals referance list...

one channel is oscillating even putting a series gate resistor at the ouput Mosfets from 100 to 1.5k....the only things i dont replace is the tubes and the input section,,,

...
Did you tried the Drain to Gate 20pF trick on it?
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Old 14th May 2012, 01:06 AM   #9
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Quote:
Originally Posted by DF96 View Post
I have had a closer look, and the circuit appears to be an active load. In this sort of application the active load and SRPP have similar performance.
Thanks for the explanation.....i'm attaching a cropped image ,i hope it's a little bit clearer so as for you and other to verify ..I'm really intrigue about this since it does not look as those I see on this forum...
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File Type: jpg Luxman LV-103 amp section.jpg (815.5 KB, 79 views)
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Old 14th May 2012, 01:10 AM   #10
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Quote:
Originally Posted by miklos View Post
Did you tried the Drain to Gate 20pF trick on it?
Not yet, i will try to tweak on it when i got through it... i substituted the output with IRFP240 and 9240's.
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