Tube DIY Asylum

Posted by VoltSecond (A) on October 29, 2001 at 21:42:13

In Reply to: Durn it... -> CCS? posted by str8aro on October 29, 2001 at 21:13:10:

From my C4S page:

"How much B+ do we need to run a Constant Current Source (CCS) on a tube? An choke fed output tube can swing above B+ to make output power. A CCS can not. So we have to plan for a higher B+ to bias a tube with a CCS. Also, we should plan on having the plate to cathode voltage of the output tube move about 10% because the plate is current fed instead of voltage fed when determining the bias point.

Version 1 (my favorite)
B+ = ( 115 V AC/ 104 V AC ) * [ V_cathode_bias + V_plate_to_cathode + I_bias * R_plate_load + Plate_margin + CCS margin]

Version 2
B+ = ( 115 V AC/ 104 V AC ) * [ V_cathode_bias + V_plate_to_cathode + Max_V_swing + Plate_margin + CCS margin]

Note: Max_V_swing is the voltage swing from the plate curves between the bias point and the plate voltage with zero volts on the grid.

The standard Bottlehead C4S hard clips at about 3V across it. I've been playing with the MJE3xx models and curves. I've noticed that the (each) MJE3xx likes to see 10-20V instantaneous minimum across it instead of just 3V.

For an example, I'm going to use a 2A3 at 50 mA with about 15W plate dissipation feeding a 5K load. This CCS has three series pairs of MJE3xx transistors.

Version 1
B+ = ( 110 %) * [ 60V + 310V + 50 mA * 5K + 10% of 310V for plate margin + 3 * 15V CCS margin ]
B+ = ( 110 %) * [ 60V + 310V + 250 V + 31V + 45V ]
B+ = ( 110 %) * [ 696 V ] = 766 V B+

Version 2
B+ = ( 110 % ) * [ 60V + 310V + (310V - 80V) + 31V plate margin + 3 * 15V CCS margin]
B+ = ( 110 % ) * [ 60V + 310V + 230 V + 31V + 45 V]
B+ = ( 110 % ) * [ 676 V ] = 744 V B+

If we were building a standard Parafeed, B+ would have been about 382V. (60 V cathode + 310 V plate to cathode + 12 V drop in the choke) "