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I'm working on a new 300B amp which will use a DC-coupled cathode follower driver. The benefits of a constant current sink here are substantial. The higher resistance will let me swing more voltage before my driver starts to draw grid current and really starts distorting. A CCS lets me adjust the current easily without changing the DC-coupled value from my voltage gain stage to feed a hungry 300B as needed. And of course, there is added linearity.

Depletion-mode MOSFETs like the IXYS 10M45S and Supertex DN2540 have really been boon for DIYers the last ten years or so. They provide a simpler active plate load versus BJT based CCSs, as they self bias with one resistor. Walt Jung published two landmark articles in 2007 in AudioXpress magazine. With their high voltage rating and more than adequate performance, these have been used in many tube DIY designs very effectively.

The main problem I have with using these in a design - particularly a DIY design - is that these need a trimmer pot to adjust the current. The trimmer needs to be adjusted to ensure minimum current on power up, and then the user then needs to adjust the current with a multimeter with the unit powered up (add one more resistor at this point - maybe a 100 ohm for sensing). FETs can die pretty easily plus you have exposed HV on the drain tab when you are fiddling with this adjustment.

The simple solution is to cascode with a LM334Z. The LM334Z gives us precision control at currents 10mA and less, and just needs 1V to 'turn on' (though at 10mA, I think we can swag this to be closer to 1.2V). The biasing region for the DN2540is about 1.8V to 2.2V (for all the currents 10mA and less). The biasing region for the 10M45Sis about 2.9V to 3.5V (for all the currents 10mA and less). We can even skip the "gate stopper" resistor, as the gate will be directly connected to ground. Since the LM334Z controls the current, you can co back and forth between these two different depletion-mode devices without re-biasing. So with three parts, we have about a perfect sink.



Here's my tests at 9mA comparing AC impedance. Note that the Supertex has better high frequency performance as recorded by Mr Jung in 2007.

IXYS 10M45S (300 ohm current set resistor)
20Hz 4.5M
1kHz 7.2M
20kHz 460K
100kHz 95K

IXYS 10M45S cascoded with LM334Z (8.2 ohm current set resistor)
20Hz 60M
1kHz 21M
20kHz 1.15M
100kHz 250K

Supertex DN2540 cascoded with LM334Z (8.2 ohm current set resistor)
20Hz 160M
1kHz 12M
20kHz 1.9M
100kHz 1.75M

Research Links
http://waltjung.org/PDFs/Sources_101_P1.pdf
http://waltjung.org/PDFs/Sources_101_P2.pdf
http://www.diyaudio.com/forums/tubes-va ... value.html

Shannon

Great information, Shannon, thanks!

I've used the dual BJT / LED CCS as current sources, and current sinks, in the past, but they are relatively complex, with a higher parts count,
and they tend to be somewhat "finicky".

If there should be a need for greater than 400V on the Anode / V+ lead, then just use the IXYS IXCP 10M90S, it's rated at 900 Volts.

/ed B

EWBrown wrote:I've used the dual BJT / LED CCS ascurrent source, and current sink, in the past, but they are relatively complex, with a higher parts count,
and they tend to be somewhat "finicky".

Yes, but bipolars will tend to pwn in the area of noise

separks wrote:I'm working on a new 300B amp which will use a DC-coupled cathode follower driver. The benefits of a constant current sink here are substantial.

Shannon

Bravo. A great read about CCSs. Looking forward to the 300B amp (as a kit or a driver board).

Two component CCS tests, with a Toshiba 2SK270 Blue that I had sorted previously to around 9mA. Gate shorted to ground.

Shannon


IXYS 10M45S cascoded with 2SK170 Blue
20Hz 54M
1kHz 24M
20kHz 1.18M
100kHz 261K


Supertex DN2540 cascoded with 2SK170 Blue
20Hz 20M
1kHz 22.5M
20kHz 3.6M
100kHz 747K

Looks like the D-FET is far superior for broadband operation, while the IXYS is best suited to PSRR.

I'm looking at a phase inverter design using 6ej7's and a circuit virtually identical to that used with Pete Millett's Engineer's Amp.

I see that Pete returns the tail to -60v supply as shown below. I see no difference or utility in returning the tail to anything other than ground, unless it has to do with the voltage drop across the DN2540 that, when I return to ground is only about 1 volt.

Since this is at the input dealing with small signals, I think you can get away with this. Maybe AC couple the input for a little extra safety.
With my LM334Z LTP phase splitters in my ST70 & Poseidon circuits, I needed at least 9V as margin to prevent an occasional oscillation (a "whoomp whoomp" motorboating IIRC). Of course it was dealing with much larger signals being post voltage amplifier.

Shannon

Shannon etc.,

A 300B driver -Something for your possible interest - see post #47 on
http://www.diyaudio.com/forums/tubes-va ... 2a3-5.html

Also discussed here:
http://www.bartola.co.uk/valves/2013/03 ... de-driver/

Ther Shunt Cascode Driver, another smart idea from Rod Coleman. It puts the drive tube into both constant current and constant voltage mode.

I have Output and Power trannies, a Hammomd Chassis and some JJ 300B sitting on the shelf waiting for me to "pull finger".

After the above SE project above I have PPP 300B Monoblocks on the cards, Plitron PAT4006 Toroidal Output Trannies with matching Plitron Torroidal Choke and Power Trannies, all parts + 10 off matched Electroharomonix 300B all in stock.

So many parts, so little time.

Cheers,
Ian

Thanks for the links, Ian - cool idea.
On my 300B driver, I've moved away from the of using the EZ Sink. I ditched my 6GH8/6U8/etc scheme and am using a 6GU7/6CG7/6FQ7 instead, in a cathodyne arrangement with a partially bypassed cathode resistor (for just the right amount of gain and lower distortion at power). Works very well at driving the 300Bs at 20kHz.

Shannon

1. Has anyone paralleled LM334's for higher current controlling?

2. Has anyone tried the LT3092 (attached) for current control to 200ma? Note they mention parallel operation in the datasheet.