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Old 13th January 2013, 10:46 PM   #1
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Default OTL Heater Questions

Sorry guys but I am having a brain fart while working on my latest OTL. I have a 25v dc heater supply for all the power tubes and want to use it on the front tubes as well. I have 2 - 12ax7's and a 7n7 in the front. I want to wire one 12ax7 heater as a 12v heater, one as a 6.3v heater and the 7n7 is 6.3 and run them in series to get 24.6v as is shown in the picture attached. will this work? Thx
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Old 13th January 2013, 11:30 PM   #2
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NO NO NO ! If you tried it you would probably end up with something like 18V across
that first 12ax7 about 4 v across the second 12ax7 and about 3v across that 7n7 .
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Old 13th January 2013, 11:31 PM   #3
piano3 is offline piano3  United Kingdom
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Only if all 3 require the same current
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Old 13th January 2013, 11:40 PM   #4
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Quote:
Originally Posted by piano3 View Post
Only if all 3 require the same current
Could you elaborate please?

Thx
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Old 13th January 2013, 11:41 PM   #5
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NO NO NO ! If you tried it you would probably end up with something like 18V across
that first 12ax7 about 4 v across the second 12ax7 and about 3v across that 7n7 .
Why do you think this would be the case?

Thx
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Old 14th January 2013, 12:19 AM   #6
piano3 is offline piano3  United Kingdom
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Woody thinks this because he knows that they do not require the same current(I don't know the details because I haven't used them). Valve heaters wired in series are no different than any other resistors wired in series. It will not work unless you work out some complex scheme with resistors in parallel with 2 of the valves.
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Old 14th January 2013, 12:21 AM   #7
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Quote:
Originally Posted by piano3 View Post
Woody thinks this because he knows that they do not require the same current(I don't know the details because I haven't used them). Valve heaters wired in series are no different than any other resistors wired in series. It will not work unless you work out some complex scheme with resistors in parallel with 2 of the valves.
And this is solely due to the varied voltages. Meaning if it were 2 - 12ax7's set to 12v in series there would be no issues correct?
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Old 14th January 2013, 12:24 AM   #8
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For a moment don't think of them as tubes just resistors. The current all parts of seriese
resistors is always the same. The 12at7 with heaters in seriese has about 40 ohms resistance that one in parall has about 20 ohms and the 7n7 about 10 ohms. Now just
think about it as a simple voltage devider problem. In practice it will even be a little
more extreme with the 12ax7 drawing more curent than rated so it's resistance will be
even higher and that 7n7 will be a lot colder so it's resistance even lower. By the way
this is covered in the RCA receiving tube manual.
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Old 14th January 2013, 12:27 AM   #9
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Quote:
Originally Posted by woody View Post
For a moment don't think of them as tubes just resistors. The current all parts of seriese
resistors is always the same. The 12at7 with heaters in seriese has about 40 ohms resistance that one in parall has about 20 ohms and the 7n7 about 10 ohms. Now just
think about it as a simple voltage devider problem. In practice it will even be a little
more extreme with the 12ax7 drawing more curent than rated so it's resistance will be
even higher and that 7n7 will be a lot colder so it's resistance even lower. By the way
this is covered in the RCA receiving tube manual.
gothca but again this is solely due to the varied voltages. Meaning if it were 2 - 12ax7's set to 12v in series there would be no issues correct?
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Old 14th January 2013, 12:30 AM   #10
piano3 is offline piano3  United Kingdom
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The 25 volt supply only knows that it has a certain total resistance across its terminals, will obey Ohm's law and a certain current will flow. They voltage drop due to this current across the 3 different valves will also be determined by Ohm's law and in each case will simply be the product of the current flowing and the heater resistance of the individual valve. Series heater connection is tricky enough even with identical valves. Personally I would never recommend it be attempted with the very different valves you are using.

And yes, 2 x 12ax7 in series should be more or less fine

Last edited by piano3; 14th January 2013 at 12:33 AM.
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