IXYS 10M45 CCS IC question
 

Quick (and hopefully easy to answer) questions :

How many volts at minimum does this chip need to drop to work satisdfactorily? I'll have about 200 V B+ available and want to drive a 12AT7 . Will this suffice .

Likewise, will this CCS happily regulate at 2 - 5 mA or is it more stable at higher currents (I'd have to look at a different tube)?

Thanks

I have found they are happiest with 20V minimum headroom. That includes the signal voltage, not just operating point.

The datasheet seems to imply operation as low as 1-2 mA, but I haven't used them this low. I think, based on the Idss rating, the DN2540 is able to run higher currents in cascode, so possibly the IXYS is better at low currents ?

Thanks. In that case, I should be able to do -1.7V bias @ 5mA easy . Perfect for a single Red LED on the cathode
http://www.nj7p.org/tube/graph/12at7.png

If current was truly held constant, needs only a resistor under the cathode.
I suppose if you are driving a low enough impedance, cathode current might
not correllate exactly the same as the plate....

Either way (LED or Resistor), I'd bypass with a good cap if the voltage is be
held constant.

We'll find out soon enough

I'll bring this contraption around in a few days

I got my Edcors... You gots to seee!

Plenty more AX/AU/AT/DJ tubage in my junk box if
you wanted to roll some others for comparison.

On the bench I've seen no obvious issues below 1 ma.

The IXYS data sheet for this part is pretty brief.  They list an Id of 2-100mA, and no minimum voltage (max voltage 450).  I'm using them at 9mA - 68V, and they're perfectly stable.  Pete Millet tested on a 6SN7 section at 5mA (no voltage given, but the plate curves indicate 80-85V), and reported better technical performance and sound than with a 100K resistive load.  In practice, you'll want to have the swing of your plate plus maybe twenty volts across the thing, I would think, and if you're dissipating over a watt or so in it, slide a heatsink onto it.  I'm burning 0.7W in mine, and they hardly get warmer than my hand.

A caution; there's some example-to-example variation, about 10% or so, in Id/Rg-k, so you need to put a little pot in there as well. I use a 250Ω metal film and a 100Ω 12-turn little pot.  It's fun to adjust your OP with a screwdriver and hear what you can hear; very instructive.

Aloha,

Poinz

I forgot to mention. This project is a 12AT7->RC->12B4A->old 5K/8ohm console transformer . Should be good for a watt or so.

I'm using a single red LED on the tail of the 12AT7 and am starting off with plate resistors. Once everything is working, I'm going to substitute them with my IXYS CCS and compare the sound.

Future plans include a "powerdrive" circuit on the 12B4A grids. Just using boring resistors for now.

The 12B4A can be turned near fully on with only +0V on the grid.
I'm not sure how much grid conduction you are fighting? Probably
only capacitance... If you superdive with a MOSFET for no reason,
might be worse capacitance than your 12B4A grid... 5 picoFarads
times Mu of 6.5, your superdrive gate would have to be 32pF or
less to beat it.

Your 12AT7's plate resistance is 10K-15K pending on voltage...
Roll off (-3db) into your 12B4A's grid*mu at only 331,741Hz!...
So unless you ARE forward conducting, I'm not sure the point.

You might try a cathode follower. 6AU6 Pentodes I got lots of
spares. Would do just fine in that application. Or use your 12AT
as the follower. I got more 6DJ8's than you could shake a stick
at (Used perfectly good HP/Amperex, and Tektronix/Telefunken),
you could have up front... I may have a 12AU6 if you want 12V.

I also have an excess of 6U8A's if you can figure out what they
might be good for.... Triode + Beam Pentode in one Noval...

And EF95's go figure...