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Old 22nd January 2014, 01:35 AM   #1
beeah is offline beeah  United States
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Default Tubelab SE - 6A3?

I'm wondering if there's any way to accommodate the 6A3's 6.3V filament. I'd be running it at the 45's operating point - i.e. 275V @ 35mA.

The 6A3 is attractive as my power transformer can't handle the 2A3 filament's 2.5A current draw, whereas the 6A3's 1A is no problem.

I've read that a couple diodes in series from the regulator's common leg to ground would cause a 2*0.7V drop, essentially getting us there at 6.4VDC.

Is there a snowball's chance this'll work?

Last edited by beeah; 22nd January 2014 at 01:37 AM.
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Old 22nd January 2014, 04:40 PM   #2
rknize is offline rknize  United States
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I've never tried that trick with this regulator before, but it should work. It won't leave much headroom for the regulator, though. You can get a bit more by using Schottky diodes in place of D4 and D5. You may have to float pin 3 and the divider that feeds pin 4 as well.

You might be better off running it on AC?
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Old 23rd January 2014, 01:35 AM   #3
beeah is offline beeah  United States
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Thanks for the reply, Russ.

I figured I'd have to come up with a way to float pin 3, but could you please elaborate on pin 4? Also, any thoughts on bypassing the diodes to ground with a cap?

As far as AC, I've considered it. However, the 6A3 is notorious for hum with AC filaments, though I'll never know unless I try for sure. I imagine I'd need hum pots, which I have, though I'm loathe to butcher my pretty top plate more than I need to.

Might be time to bust out the clip leads and give AC a go...

Cheers,
Joe
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Old 23rd January 2014, 03:10 PM   #4
rknize is offline rknize  United States
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If you look at George's schematic:

http://www.tubelab.com/images/Assemb.../TSE_board.jpg

The regulator is U1. He doesn't have the pin numbers labelled, but it is easy to guess. The bottom leg is pin 3 and the lower-left leg is pin 4. In order to float the entire regulator up, you'll need to float its GND reference (pin 3) and the Vadj reference (pin 4). That would mean you'd want to place the diodes under both pin 3 and the lower legs of R1 and R35. C1 and C3 can stay as they are.

I imagine you'd want to bypass the diodes with a small cap, yes.
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Old 25th January 2014, 02:59 AM   #5
beeah is offline beeah  United States
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Got it - your explanation makes it crystal clear. Thanks, Russ!
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Old 25th January 2014, 03:45 AM   #6
Tubelab_com is offline Tubelab_com  United States
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I have not tried to actually float the filament regulator above ground, so I don't know if it will work. I did try to feed the regulator a little over 8 volts in an attempt to run a #50 tube.....it fried. The Sharp regulator has a 7 volt maximum input voltage spec. It doesn't like more than 8 volts.
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Old 25th January 2014, 05:20 AM   #7
beeah is offline beeah  United States
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Thanks for weighing in, George.
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