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Tube VAS stage questions???

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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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  • LuxMan-VAS.pdf
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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?
 

Attachments

  • Luxman LV-103_1.jpg
    Luxman LV-103_1.jpg
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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...
 
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.
 
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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  • Luxman LV-103 amp section.jpg
    Luxman LV-103 amp section.jpg
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The way you have it drawn is in error. The top tube's grid will not be happy without some sort of DC reference.

I couldn't make head nor tail of the "official" drawing, which seems calculated to prevent you from determining that the tubes don't do much in this amp other than allow it to be sold as a tube amp.
 
Yes gentlemen,,,,i'm really intrigue why the schematics is drawn like that...when i read it looks like the one I drawn which is unusual.....what will be the correct schematics will be? i look on the other luxman LV 103u is not like that it's an LTP...so i think this must be srpp...only a mistake on schematics...

Actually what i want to know is if I can substitute the 6cg7 tubes with 6n1p or 6n6p for tweaking purposes...with reference to the original schematics...never mine what i drawn?

Thanks miklos for the clarification....
 
^only one way to find out, the 6n1p have the same pin-outs as the 6cg7 only question is, is the filament supply capable to support the higher filament current required by the 6n1p...?

i had an AMX preamp with a burned out traffo simply because the owner replaced all the 6dj8 tubes with 6n1p's....
 
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i had already changed the filament trafo with bulkier one so no worry since its capable of 1.5amp 13v....filament is series connected.

one thing more when i inspected the tubes...i've noticed that the other tube is labeled 6FQ7 and the other one is 6CG7 both GE, even though they are almost identical i think there's still a difference on them.
 
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6FQ7/6CG7 are extremely similar, and later tubes often carry both markings. The 6FQ7 has significantly higher absolute maximum voltage ratings in a number of areas in particular relating to TV service than the 6CG7.

Chances are both of your tubes are identical or close to it.

Not sure why you want to replace the 6FQ7/6CG7 one of the most linear tubes out there with the 6N1P which is IMHO nothing special although OK. More to the point if these tubes are inside the feedback loop you may have stability issues.

I would be surprised if you could hear any difference between tubes in this design - there is so much needless solid state complexity that it can hardly even be considered a hybrid.. No real tube sound here.
 
Thanks kevin... I just want to tweak it....since the supply rail is +/- 114v (228V total) i Doubt that 6cg7 are in linear mode with this rails...so i thought of 6n1p since it can work with lower rail voltages and I have it....also 6n6p...however tha dissipation of 6n1p is lower 2.2w compared to 3.8w of 6cg7...thats why I ask what is the actual dissipation or can 6n1p withstand the dissipation....

one thing more...why are the values of cathode resistors are not the same.....upper has 3.9k and lower is 8.2k?
 
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