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Old 1st December 2002, 06:30 AM   #1
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Default regulated 5V DC supply

I would like to try to heat my 300B's DC. Now there seem to be several possibilities to tackle this. I could use a bridge rectifier and a couple of 10.000 uF caps. Or two schottky diodes with the same caps. Or two caps and a choke. Those are the possibilities I've seen so far. Which of those would be the best sonically ? Or should I make it a regulated supply ? Could someone point me to a schematic for this ?
Thanks for your help.

Martin
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Old 1st December 2002, 06:53 AM   #2
mirlo is offline mirlo  United States
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Without actually addressing your question (I apologize; I don't know the current requirements of 300B filaments), I would like to throw one other question out to whomever picks this up:

I suspect the best performance is probably had from a regulated filament supply.

But does it make sense to voltage regulate it or current regulate it?

I would imagine that an advantage of a current regulated supply is that it would automatically limit inrush current drawn by cold tubes, and it might compensate better for aging.

http://ken-gilbert.com/images/tubes/lm350.gif

http://ken-gilbert.com/images/tubes/DC_current_regs.gif
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Old 1st December 2002, 07:09 AM   #3
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Thanks, these are certainly helpfull. The filament current rating is 1.9 A (KR300BXLS)
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Old 1st December 2002, 07:13 AM   #4
Colt45 is offline Colt45  Serbia
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I dont think DC is a good idea on filaments, especially directly heated ones.. as one side will be hotter than the other.

I believe that also leads to premature aging..
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Old 1st December 2002, 07:22 AM   #5
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Yes, by what I've heard I tend to agree, and I'll be doing it reluctantly, but I can't stand the residual hum from the direct heated filaments. I want to listen to music, not hum.
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Old 1st December 2002, 07:29 AM   #6
mirlo is offline mirlo  United States
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So if AC is good for filaments, because it promotes even heating, should it be 50 Hz or 60 Hz, or should it be some high frequency >> 20 kHz, which is inaudible along with all of its harmonics and all of its low order intermods with anything audible?

Of course, building a 100 kHz sinewave generator perhaps isn't practical, but might it not be theoretically better?
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Old 1st December 2002, 08:37 AM   #7
SHiFTY is offline SHiFTY  New Zealand
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Quote:
I dont think DC is a good idea on filaments, especially directly heated ones.. as one side will be hotter than the other.
I have seen this asserted before, but I don't understand how it could work! Why would one end be hotter than the other, assuming the filament is a uniform resistance the whole way?

If one side was hotter than the other, you would be able to see it- one end would be white hot and the other would not be glowing!

If this was the case, the local resistance in the white hot region would increase until it reached equilibrium, and vice versa.

I imagine that if you could measure the voltage along the length of the filament it would be a regular drop along it, and the power dissipation would be constant over the entire filament.

Anyone ?
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Old 1st December 2002, 09:51 AM   #8
dhaen is offline dhaen  Europe
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Quote:
I dont think DC is a good idea on filaments, especially directly heated ones.. as one side will be hotter than the other.
I think what is meant is that the emission of one end will be higher than the other, rather than the temperature.
It might make sense to reverse the polarity at least once in the life of the valve.
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Old 1st December 2002, 09:56 AM   #9
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Somebody (Jocko, i think) said a while ago that the life of a
lamp bulb is shortened if run on DC instead of AC. The reason
had to do with migration of material from one end of the filament
to the other. I guess this would apply to valve filaments also.
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Old 1st December 2002, 10:06 AM   #10
dhaen is offline dhaen  Europe
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Quote:
Somebody (Jocko, i think) said a while ago that the life of a
Yes he did say that recently. So did I a couple of months ago. Try doing a search for "notching".
What is not clear, is whether this occurs at lower the temperatures of valve filaments.
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