Not a stupid question at all. This is a new part of my EZTubeMixer design. This uses 12V heaters and the main amplifier PCB uses a 12AX7 plus a couple of 6922s with their heaters wired in series.
This new board is a relatively simple low gain buffer using a single 6922 configured as an SRPP stage with anode follower style NFB. All the boards plug into a motherboard carrying the heater supply buses. I really want this board to be plug compatible with the others so it need to run from 12V heater supply. I could add a dropper but that's wasteful or make some arrangement so adjacent boards share a 12V supply but that's not ideal. The ideal is a 12V heater 6922.
Cheers
Ian
Ian,
Here is a really stupid question, is there room for a 7806 + heat sink on the card? I'm thinking of long term availability issues with the 8416.
A schottky diode in the ground lead of the 7806 would get you 6.3V out. I know from an efficiency standpoint this is not optimum..
Edit: Perhaps on second thought this no better than just a dropping resistor, but I had to throw it out there. lol
Here is a really stupid question, is there room for a 7806 + heat sink on the card? I'm thinking of long term availability issues with the 8416.
A schottky diode in the ground lead of the 7806 would get you 6.3V out. I know from an efficiency standpoint this is not optimum..
Edit: Perhaps on second thought this no better than just a dropping resistor, but I had to throw it out there. lol
Yes, dissipation would be the same. To drop 6.3V at 0.3 amps needs a 21R resistor. The dissipation would be nearly 2W so it would need to be a 5W type. That's simpler and probably uses less PCB space than a regulator + heatsink.
I think you now see why I am looking for a 12V heater version.
I had thought of using other tubes that are readily available with 12V heaters like the 12AT7 and 12AU7. I need this circuit to be able to drive 20V rms into a 5K load with less than 0.5% distortion. The gain required is only 10dB so the 6922 with a mu around 35 has enough open loop gain for the NFB to reduce both the distortion and the output impedance to meet this spec with an idle current around 5mA. The 12AT7 can run happily at 5mA and has a mu of 60 so I would have expected it to perform better than the 6922 but simulations indicate it is actually worse. The 12AU7. I am still puzzled by that. The 12AU7 is well known to have a high intrinsic distortion and with a mu of only 20 I expect it to be even worse. I think I might look at the 12AV7 next.
Cheers
ian
Thanks, Ian for the comprehensive explanation, I can see the thought you have put into this and think your design choices are about as close to optimum as possible. Curious what if any effect does cranking up the quiescent current slightly (<= 10mA) have on linearity assuming adequate supply margin and dissipation in the 8416?
Not really for a couple of reasons.
1. There are a lot of the boards out in the field already so backwards compatability would be a big headache
2. In even a small 6 into 2 mixer there are approxiamtely 30 tubes with a total heater current draw of about 5 amps at 12V. At 6.3V that would be 10 amps
Cheers
ian
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