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In Reply to: Re: Thanks,that cleared it up for me! posted by Triode_Kingdom on April 30, 2007 at 06:01:01:
One solution to supplying two tubes - e.g. a diff pair - with one current source is to wire them in series. So if you remove one tube you break the circuit.
Follow Ups:
> ...wire them in series.Yes, that's a good option, although it does make it more difficult to know which tube of the pair has failed. I have to say, my preference in all this is a voltage regulator (voltage source) with a current limiter, not merely a simple current regulator. I believe tubes are spec'd and designed for a specific filament voltage, and that the filament current listed for most is only an approximation provided for use in determining supply requirements. Regulating the tube to that current is unlikely to be what the manufacturer intended.
my preference in all this is a voltage regulator (voltage source) with a current limiter, not merely a simple current regulator. I believe tubes are spec'd and designed for a specific filament voltage, and that the filament current listed for most is only an approximation provided for use in determining supply requirements. Regulating the tube to that current is unlikely to be what the manufacturer intended.I used to think that, I even wrote an article in AudioEXpress on the topic.
After adding series chokes to my battery supplied filaments I'm thinking the current reg has benefits above and beyond lighting the things correctly. It looks as though extra impedance in the supply line is beneficial - whether it blocks hash getting in or out I can't say, but it seems to do something.
BTW you can get around the issue with current v voltage by making the current reg adjustable and setting it so that the voltage drop across the filament is correct.
I agree with those who say that the high impedance keeps the audio signal out of the power supply. The last thing i want is a good portion of my audio signal finding its way to ground though the filament supply.How did you size your chokes for your battery supply? I pretty much size it so the Z at 20hz matches the filament resistance and see what that looks like.
For the 71As I used a pair of existing chokes which were about 1H and had about the right DCR to drop nominal 6.3V to 5V. I added a common mode toroidal which tops up the DCR.For the 211s I bought some big transformers off the shelf, unstacked it, unwound the primary and added a new winding. The winding roughly matches the DCR of the existing secondary to give me almost 0.8 ohms as required to drop nominal 12.6 to 10V with the 3.25A draw of the 211 filament. I will add an air gap with paper between the Es and Is and put it on the DCR bridge to see what I get and use the same trick with the common mode toroidals as above.
I have only just sorted the recharging for the 211 batteries so they haven't been installed yet. I'm using laptop power supplies (15V) and a silicon bridge rectifier to achieve 13.8V, the laptop supplies are current limited at 6A so this should work a treat for the 65 Ah batteries.
I'm curious, being as you're using DC on the filaments, why aren't you using SS devices for isolation? This seems like a very inconvenient application for chokes given the frequencies and currents involved. I should probably add that I'm on the outside of the current rage for SS CCSs, but this particular use would seem much less likely to color the sound. The impedance ratio of desired to undesired signal paths would be extremely high with a SS CCS.
The original circuits used fairly high spec SS devices (LT108X series regs). With the 71As I moved from there to series / shunt regulation using discrete devices, from there to SLA batteries and from there to SLA batteries plus chokes. I felt that each step was a worthwhile improvement. With the 211s I'm in the process of jumping straight from first to last.I tried SS current reg but didn't like it, YMMV etc etc.
I tried SS current reg but didn't like itYou probably have some theory about the reason why. No?
I never tried batteries, probably should give them a go.
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