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Old 7th January 2005, 04:19 PM   #1
yagas is offline yagas  Greece
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Question Line stage question

My line stage consists of a grounded cathode amp cascaded into a cathode follower.
The tube is ECC99 and the operating point is 150V@18 mA each section.
B+ is 360V, load resistor and CF cathode resistor are 10k.
My measurements showed that the GC section draws 18mA but the CF section draws 16,5 mA.
Why ?
Should I reduce the CF cathode resistor at 9,7K to achieve equal current draw and cathode
to plate voltage ?

Thanks
Mic
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Old 9th January 2005, 03:35 AM   #2
PRR is offline PRR  United States
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> achieve equal current draw and cathode to plate voltage?

Why? What possible advantage could there be?
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Old 9th January 2005, 05:59 AM   #3
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Quote:
Originally posted by PRR
Why? What possible advantage could there be?
Perhaps to cancel some distortion by operating both valves (of the same type) at the same operating point?
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Old 9th January 2005, 09:11 AM   #4
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Hmm, I seem to remember something about this from way back.
Wasn't it something about this on the Audio Genesis website? ( now gone )
Something to do with the power supply and subjective dynamics, I believe.
I may be wrong.

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Old 9th January 2005, 12:08 PM   #5
mcs is offline mcs  Denmark
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Quote:
Originally posted by slowmotion
Something to do with the power supply and subjective dynamics, I believe.
I remember reading something like that in an old issue of the "Tube CAD Journal" also... I think it was the cathode follower issue.

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Mikkel C. Simonsen
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Old 9th January 2005, 12:19 PM   #6
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Hi mcs

You're right it was in "Tube CAD Journal",October '99, page 3.
http://www.tubecad.com/october99/page3.html

cheers
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Old 9th January 2005, 06:24 PM   #7
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Pff... sounds like BS to me...

Besides, a CF doesn't produce distortion so it can't cancel any.

Tim
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Old 9th January 2005, 06:30 PM   #8
SY is offline SY  United States
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Tim, the idea was that there is a cancellation at the supply end; at AC, as the inverting amp's current drops, the CF's current increases by a near identical amount. So it's a valid way of optimizing a circuit to use with weedy supplies and inadequate bypassing. Of course, it begs the obvious question...
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Old 9th January 2005, 09:30 PM   #9
yagas is offline yagas  Greece
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As I'm not an expert, I would like to know if this non-symmetrical operation between the two sections of the same tube is something to worry about.
Normally, each triode shares the same load resistance and current draw. So, in my case I expect to see each section to draw 18mA.
But no, they don't. As I said before the GC section draws 18mA@153V and the CF draws 16.5mA@158V.
Maybe it is not such a big issue and to my ears it performs quite good.

Thanks for your replies.
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Old 9th January 2005, 09:37 PM   #10
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Besides, a CF doesn't produce distortion so it can't cancel any.
Good one Tim. You are very wrong though...
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