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In Reply to: Re: Resistor as parafeed plate load? posted by Kurt Strain on October 29, 2001 at 19:56:09:
Thanks Kurt,
That was another thing I was wondering about, although I forgot to stick that in my first post. So with my output, I'd need about a 7.5K resistor to stay above a 2.5K total load on the 2A3, at which point things would get bad wrt voltage dropped and power dissipated.However, I've been doing some more reading and found Gary P's post on using a pentode CCS on the output tube. I have the parts to try that... The only thing that I'm not sure of is how much voltage I need to have across the CCS. Say I want my the plate-cathode voltage to swing 100V each way from the static condition voltage. Do I just need 100V plus some reserve across the CCS to cover when the tube needs a higher voltage than static conditions?
Thanks,
John
Follow Ups:
The rule of thumb I've been using on the output stage is... Set the voltage on the pentode CCS equal to the plate-cathode voltage of the output tube. This gives you the compliance you need plus some margin.On my 45 amp I have 180V across the cathode resistor to setup DC coupling, 275 Plate to Cathode on the 45, 275 across the 6bq5 pentode CCS.
Have fun!
Gary
Thanks guys - you've given me plenty of stuff to think about.John
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) "
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