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Old 25th October 2007, 04:21 PM   #1
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Default 6SN7 plate voltage, current and lifespan

My PSU tweaking on my Aikido is reaping tangible results with regard to hum.

I added 30K across the final cap to bring current draw up to around 32mA, then added an extra 0.1uF to my PSU input cap (to give a total of 0.78uF on the tuning cap). Today, I will look for a 1uF motor-run cap as well as 80-100uF Motor-run caps for my filtering caps.

All of this brings me to questions of voltage, current, wattage and lifespan of my 6SN7GTA's.

I currently run the inputs at 4.5mA and the outputs at 7.2mA (or thereabouts). At it's present config, B+ is around 278VDC. When I do further tweaking with the "tuning" cap and maybe remove the 30K current sink, I expect that B+ will rise to somewhere in the region of 360VDC. Anyway, this would mean that each plate of my 6SN7's will be seeing 360/2 = 180VDC.

The tube specs give maximum ratings as: 450VDC per plate at 20mA cathode current with power dissipation of 5W for a single plate, 7.5W if both plates are used.

By my reckoning, if each plate does end-up seeing 180VDC, even at 10mA current, each plate would be required to dissipate around 1.8W. This is well within the tubes specs.

OK, I noticed when I added current-draw, that sonically, the amp, with 278VDC (as opposed to 317VDC) sounded a tad more sluggish - still amazing, but somehow the polish was dulled slightly. Sorry for the poor adjectives.

Now my QUESTION: with the high cost of NOS 6SN7s, to what degree does would running the tubes hotter (ie. higher wattage) effect lifespan? Of course, I am in no way going to get close to their max. I had read that many tubes were rated for 10,000 hours, is that running at max wattage?

Thanks,
Charlie
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Old 25th October 2007, 08:25 PM   #2
SY is offline SY  United States
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At 180V, you can run these at 10mA or a little less for about forever.
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