pentodes with CCS load?

mrkramer · 23rd January 2012, 04:49 PM

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

ErikdeBest · 23rd January 2012, 05:08 PM

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

jcx · 05:14 PM

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

DF96 · 23rd January 2012, 05:45 PM

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.

Osvaldo de Banfield · 23rd January 2012, 06:03 PM

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.

Best regards.

DF96 · 23rd January 2012, 06:07 PM

The OP was asking about using a CCS as a pentode load, not using a pentode as a CCS.

Yvesm · 23rd January 2012, 06:25 PM

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.

Osvaldo de Banfield · 23rd January 2012, 06:30 PM

Quote:

Originally Posted by DF96

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.

23rd January 2012, 04:49 PM	#1
mrkramer diyAudio Member Join Date: Mar 2008 Location: San Francisco bay area	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

23rd January 2012, 05:08 PM	#2
ErikdeBest diyAudio Member Join Date: Mar 2003 Location: Itoculo	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 __________________ my surname is indeed 'de Best': neither misspelling nor snobbism! Ask SY!

23rd January 2012, 05:11 PM	#3
jcx diyAudio Member Join Date: Feb 2003 Location: ..	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.

23rd January 2012, 06:03 PM	#5
Osvaldo de Banfield diyAudio Member Join Date: Dec 2011 Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina	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. Best regards. __________________ LW1DSE

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23rd January 2012, 05:45 PM	#4
DF96 diyAudio Member Join Date: May 2007	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.

23rd January 2012, 06:07 PM	#6
DF96 diyAudio Member Join Date: May 2007	The OP was asking about using a CCS as a pentode load, not using a pentode as a CCS.

23rd January 2012, 06:25 PM	#7
Yvesm diyAudio Member Join Date: Jul 2004 Location: Ardeche	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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