B+ required w/ ccs load

[lt_texan]

I understand that the rule of thumb is to have at least the same voltage across the anode load resistor as across the valve.

Is there such a rule for a ccs load?

I replaced the load resistor with a ccs on the first stage of my phono stage to good effect. I did not change the HT supply. Just wondering if it is generally the case to keep the same drop?

(I battery biased the cathode which kept the time constants correct, not an approach I can take credit for.)

Thanks
Dan

[SY]

There will be some minimum voltage required by the CCS, depending on its design. Add some safety margin (account for line variations!), add the maximum expected swing of that stage, and you've got a reasonable drop.

Experimentally, you can hook up a variable-voltage supply in place of the regulate B+, feed the stage with the biggest signal you want it to handle, then watch the output on an oscilloscope as you reduce the B+. At some point, the CCS won't work (you'll see some clipping). Bring the B+ back up 10V or so.

[lt_texan]

thanks SY, that was most helpful

Dan