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Old 29th January 2009, 08:04 PM   #1
Bengali is offline Bengali  United States
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Default DC Heater Voltage

Hi,

when using DC voltages for heaters, if the tube calls for 6.3V for the heaters, what is the lowest DC voltage you can use?

I've read using a lower voltage helps increasing life of the tube.
I've read some post saying using lower voltages is not good for the tube.

Is there any performance hit by using DC heaters at 5.8-6V?

thanks.
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Old 29th January 2009, 11:48 PM   #2
alexg is offline alexg  Philippines
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On my projects, I try to stay within +/- 5% of the specified heater voltage of the tube. Most tube data specifies 5% variation.

Though, I have read favorable articles about starved heater and also read articles about how bad for the tubes are lower voltage heaters.
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Old 30th January 2009, 04:44 AM   #3
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You can starve the filament up to a point, but you should reduce cathode current as well since you will not be emitting as many electrons from the cathode. Depleting the space charge can damage the cathode on oxide coated cathodes. How will you know when you've reached the point where you are causing more harm than good?

Radiotron Design Handbook talks about this. It is available online.
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Old 30th January 2009, 04:15 PM   #4
Bengali is offline Bengali  United States
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thanks for your replies.

too bad you can't search the text for that book online.

so I guess the question is how low of a DC voltage can you go before damage occurs.

+/-5% is sound. by going lower, would the damage be drastic or over a long period of time?
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Old 30th January 2009, 04:40 PM   #5
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I would suggest going on the plus side but not on the minus side with DC heater voltage. The lower voltage will cause more current flow. This is usually not a problem, if you are rectifying 6.3AC you should get more then 6.3 DC?
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Old 30th January 2009, 05:25 PM   #6
Bengali is offline Bengali  United States
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thanks.

I got my answer. confirmed with a few experts.

http://www.tubecad.com/july2000/page10.html

I'm using a volt. reg. hence the voltage drop.

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Old 30th January 2009, 05:36 PM   #7
m6tt is offline m6tt  United States
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I've seen a commercial mic pre that had 11.85v across a 12ax7's full heater, DC. Sounded good. I've also run 8v tubes in 6.7v sockets, with the result that they wore out after about 2 years (still pretty good, but the hours weren't that high...now they crackle...probably damaged the cathode...they were $1...oh well).
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Old 30th January 2009, 11:26 PM   #8
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My fisher puts 11V across 12ax7 filaments. Again, the currents in these tubes are very low IIRC, well below the tubes max cathode current.

I think that Broskie's blog post is highly misleading. We have next to no information about the data he posts. Were tubes operated near max cathode current? Has Broskie ever tried it?(He implies that he hasn't in the post.)

There is a magic point where the voltage reduction starts to cause more harm than good. I wouldn't push the envelope with expensive tubes.

Quote:
I'm using a volt. reg. hence the voltage drop.
So am I, but correctly spec'd transformers are cheap enough.
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Old 2nd February 2009, 12:13 AM   #9
Bengali is offline Bengali  United States
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He says he has been running them at 6VDC for years, never any issues. I'll trust his experience on this one.

I'm working with what parts I have for now. Should I later go all out, then might as well spec it to the correct voltage.

btw, can large filter caps (>6800uF) on the heater supply cause thumps during turn off or that is strictly related to the +B voltage?

Thanks.
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Old 2nd February 2009, 04:19 AM   #10
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Default Heaters

Hey
I run mine at 6 V for 6.3 and 4.9 for 5 volt jobs.. dtat sheets almost always specify a 10% range but I know for sure that overdoing the voltages on heaters is a sure way to shorten their life.
Nick
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