pentodes with CCS load?

[mrkramer]

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]

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]

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]

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]

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]

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

 

[Yvesm]

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]

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.