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Old 23rd January 2012, 04:49 PM   #1
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Default pentodes with CCS load?

Hi all,
I keep reading that pentodes cannot be used with a CCS load.
Is this actually true?
It seems regularly repeated on this forum and elsewhere but not regularly explained.

MrKramer
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Old 23rd January 2012, 05:08 PM   #2
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Hi,

here is a link from Gary Pimm about CCS loade pentodes. Hope it helps a bit!
http://www.pimmlabs.com/web/drivers.htm

Erik
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Old 23rd January 2012, 05:11 PM   #3
jcx is offline jcx  United States
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it is a fine gain stage - but the gain is so high small device variations cause relatively large changes in the exact value - so it needs to be used inside a feedback loop - and global feedback loops are not popular with some tube circuit designers
the output Z is high too so it needs a hi Z input in the next stage to realize the high gain

Last edited by jcx; 23rd January 2012 at 05:14 PM.
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Old 23rd January 2012, 05:45 PM   #4
DF96 is offline DF96  England
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Both the pentode and the CCS have high output impedances, so both are trying to set the current. The likely result, unless you include a bias adjuster or use DC NFB, is one of them will 'bottom' so then you get low gain and high distortion.
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Old 23rd January 2012, 06:03 PM   #5
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I had tried a CCS a EF183 as cathode biasing for a differential amplifier consisting in the two triodes of a two ECL82, and it is great work. And tried as active loading for a 12AX7, the work is properly AT LOW AUDIO FREQUENCIES. As capacitances are in parallel with so high impedances, they effectively shunt the CCS. As an example, this circuit work with a gain of almost 100 times, but the -3dB was at only 8KHz. Not for high quality audio reproduction.

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Old 23rd January 2012, 06:07 PM   #6
DF96 is offline DF96  England
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The OP was asking about using a CCS as a pentode load, not using a pentode as a CCS.
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Old 23rd January 2012, 06:25 PM   #7
Yvesm is offline Yvesm  France
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As shown in Garry Pimm implementation, the CCS is shunted by a resistor wich finally sets the gain of the pentode.
The same result (hi gain and lo distorsion) could be obtained with the same resistor value and no CCS at the price of a very huge B+

Yves.
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Old 23rd January 2012, 06:30 PM   #8
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Quote:
Originally Posted by DF96 View Post
The OP was asking about using a CCS as a pentode load, not using a pentode as a CCS.
I gave him my two points of view, from my own experience. He will use which he believe best for his case.

Cordially.
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