Pentode current sink

[abj1]

Thanks SY

I really would prefer a little more voltage on the diff end (better operating points.....) and a little less across the sink - especially across the 6J6 splitter stage. Should be able to get good results from the sink at 120 to 150V across it so it makes sense to allow for this in the design rather than just take the simple option and return the grid leaks via 0V. (Can't really increase the B+ with the trannies I have available for this project).

One more question - if I raise the grid potential above B- for the sink with a potential divider - does this allow me to use a larger cathode resistor and increase the impedance of the sink?

Thanks all, I feel much less confused now and I can begin to put together a final draft of the circuit over the weekend.....

Just love this hobby and the support available on this forum.

Cheers,
Andrew.

[SY]

Hmmm, that should work at the cost of a bit of compliance. As a practical matter, getting the impedance ultra-high doesn't gain you much since you end up being limited by capacitance at the cathodes (e.g., cathode-to-heater as well as stray).

Gary P has done some excellent work on current sources- check out his website.

[abj1]

Pretty much got this all under control now - thanks everyone.

Question for Gary P I guess as he alluded to it in his post - is it really necessary to send the current for the diff amp screen grids through the CCS. Normally with pentodes (both output and small signal although CCS-less to date) I generally like to use a VR tube to regulate the potential (IMHO this really sorts pentodes out and makes them behave well). Would it be reasonable to do this with a sink in the tail and remove the screen currents from the picture (ie connect the cathode of the VR to 0V rather than using a cathode reference as would be more typical with a normal series resistor and bypass arrangement). The one disadvantage I can see with this would be losing the ability to easily adjust the balance within the pair by adjusting the screen potential. On the upside would be consistent screen postential with changes in B+ (our line voltage varies from around 250V to less than 230V when the whole suburb uses their airconditioning on hot evenings).

Is it also possible to similarly fix the screen potential of the pentode sink (and thereby make the the sink current much less sensitive to line voltage shifts)? Or should I stop asking and just try it - as I will have plenty of current available to 'waste' with shunt regulation.....and just love a tweak or three

Cheers,
Andrew.

[Gary P]

Here is an idea that might work well. It's on my project list but hasn't been tried yet.

Set the VR tube up with the cathode on the cathodes of the diff pair, the anode to the screens, and feed the VR tube from a good CCS. Configured in this arrangement the sum of the screen and VR tube current is always constant. The main tail CCS is now set for the current drawn by the diff pair + screen circuit current.

The advantage is the screen circuit current is constant and won't upset the differential pair's operating point. The down side is you need another CCS in the circuit.

For balancing the circuit you can use some series resistance between the VR tube and screens.

With the series resistance on the screens the thought comes up about linearizing the transconducance of the diff pair pentodes. Pentodes, like triodes have increasing transconductance with decreasing grid bias (increasing current). In a pentode when the plate current increases, the screen current will also increase. With series resistance in the screen lead this will lower the screen voltage slightly as plate current increases. Decreasing the screen voltage will decrease the transconductance. With the right amount of screen resistance one should be able to get the overall transconductance to be very linear over the operating current range of the input signal. Another form of local feedback.

The series screen resistance is in the Tabor and 47PP amplifiers but the screen circuit is not running in constant current mode. None of has not been optimized in any way. Just another item on the list of things to optimize...

On the tail CCS you could do the same screen VR tube arrangement but feed the VR tube from a "long tail" resistor to B+. This might get the current change from line voltage fluctuations down far enough, or you could feed it from a CCS also...

Gary

Gary P's DIY page
Link to Tabor schematic. The schematic is not linked from the web page yet.

[abj1]

Thanks Gary for some very interesting food for thought.

Particularly like the idea of the series resistance to balance the pentode diff pair via the screen potential with VR tube regulated supply. And hadn't even considered the likely benefits from the partial feedback in linearising the transconductance with the appropriate resistor values (hours of tweaking here alone ) - brilliant - would never have thought of that in a million years. What a bonus - will definitely explore the possiblilities with this.

Thanks also for the opinion on the possibility of controlling the screen current for the sink also.

Cheers,
Andrew.