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In Reply to: RE: 6Y6GT/6922 Link to larger schematics and PDF file posted by Tom Bavis on March 30, 2015 at 11:00:33
Hi, Tom,You wrote:
"Also there would be an advantage to running the phase inverter at the same current as the gain stage."1. Is the advantage you wrote of contained in your following statement ("net current drawn by the two stages is constant") or is it something else?
2. Also, would the IRFBC20 function well at 7 or 8 mA (the gain stage current)? It's running in the low 20s now.
Please also see my reply to Eli below, which is also addressed to you.
Thanks,
Jeff
Edits: 03/30/15Follow Ups:
The FET will be just fine at the lower current. However, transconductance (g fs ) will decline. Win some, lose some. Compromises will forever be with us.
Eli D.
Two questions:
1. What is the advantage in having the currents through the voltage amplifier and the splitter be the same?
2. Given that the voltage across the MOSFET itself seemed to increase slightly (and current decrease) when I lowered the anode CCSs, how do I figure the change in resistor vales on the legs? With the drop in CCS current, the voltage drop across the leg resistors went from about 94 and 94 to 87 and 87. The B2+ voltage rail dropped some too (220 to 215). But that doesn't account for all of the shift in the leg measurements. So there seem to be some moving targets here.
Would anyone ever put a CCS on the splitter? That might force a matched current, but I have no idea if that would work or just be overbuilding.
Thanks for any insights.
--Jeff
Don't put a CCS on the splitter. Remember, FETs and pentodes are "constant" current devices. Do you want "head butting? I think not. Also, the volts needed to operate a CCS are not present.Go back and reread Tom's post. He gave you a rationale.
Altering the total resistance the FET's drain current flows through is how you control that current's value. DC coupling the FET to the 6922 anode turns it on "hard". The internal resistance of the turned an FET is so small that it can be ignored.
Eli D.
Edits: 04/01/15
Okay, then, 6.5mA across 215 VDC B2+ gives a total of about 33 KOhms, or 16.5K per splitter leg. The1.5 KOhm pot can still have some effect on the total for fine tuning. Maybe 16.5 KOhm fixed on the drain leg and 16 KOhm fixed plus the pot on the source leg for starters. Tweak the leg values on the bench using the actual B2+ rail voltage to attain 6.5 mA, matching the CCS current.
Does this seem good?
Thanks,
Jeff
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