Help with e180f and ixys current reg

[duderduderini]

Hi there
Ok I have done my load lines with triode curves for the 6688.
As far as I know the ixys provides a 100k plate load for the tube (according to pete millet)
If I do a load line for 100k, at 150V the current through the device is like 1.5 mA at the maximum current point.
Judging by the curves, to get say 10mA at 150V i need a 15k plate load.
This will put the grid at -1 V. Rk for this will be 100 Ohms.
So do I ditch the ixys plate load and just use a resistor?(and put up with the increased distrortion)
I know the load is dynamic for the Ixys but I am a tad flummoxed by trying to marry load lines and a dynamic load.
thanks for any help
Nick Mega

[dsavitsk]

You are thinking backwards. The ixys is a constant current chip, so draw a line on the curves that is a constant current. It should be horizontal. Ignore the 100K remark as it isn't helpful here -- instead, think of the load as infinite. I explain how to think with a ccs on this page: http://www.ecp.cc/meha.html I also cover it on this page: http://www.ecp.cc/less-pressivo.html and a bit on this one: http://www.ecp.cc/c4s.html

[duderduderini]

Thant makes sense and thanks.. I have been to your website before and thought it excellent.
According to the datasheet for the current reg 300 ohms will give me 10mA or isnt it as simple as using the graph they provide on it?
Thanks again

[tubelab.com]

According to the datasheet for the current reg 300 ohms will give me 10mA or isnt it as simple as using the graph they provide on it?

The graph should get you close, but every chip is slightly different. I have been puting 330 ohms in my SimpleSE and TubelabSE amps. The current falls between 9 and 11 mA.

As far as I know the ixys provides a 100k plate load for the tube (according to pete millet) ..... It should be horizontal. Ignore the 100K remark as it isn't helpful here -- instead, think of the load as infinite.

The DC current is set by the (330 ohm) resistor at 10 mA. The AC load line would indeed be horizontal if the CCS were perfect, and there was no other load on the tube. In reality the CCS isn't perfect. Its "dynamic impedance" changes depending on the current setting. The 100K number is the minimum specified by IXYS. I made a rather crude measurement under these (10 mA) conditions and got about 650K ohms. The AC load seen by the tube is 650K in parallel with the input impedance of the next stage, which is usually the grid bias resistor in parallel with the tubes capacitance, which changes with frequency.

For amplifier design, assuming a horizintal load line is usually close enough.

[dsavitsk]

It is, except they are not terribly accurate. A trimmer can help if you want to be precise, though I don't think it that important. It can also help with finding a good operating point.

For the 6688, 10mA might be a little low. I think I'd shoot for more like 14 or 15mA at the low end, but try different currents and see what works.