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desperateaudio 13th January 2013 10:46 PM

OTL Heater Questions
 
1 Attachment(s)
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

woody 13th January 2013 11:30 PM

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 .

piano3 13th January 2013 11:31 PM

Only if all 3 require the same current

desperateaudio 13th January 2013 11:40 PM

Quote:

Originally Posted by piano3 (Post 3324656)
Only if all 3 require the same current

Could you elaborate please?

Thx

desperateaudio 13th January 2013 11:41 PM

Quote:

Originally Posted by woody (Post 3324654)
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

piano3 14th January 2013 12:19 AM

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.

desperateaudio 14th January 2013 12:21 AM

Quote:

Originally Posted by piano3 (Post 3324702)
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?

woody 14th January 2013 12:24 AM

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.

desperateaudio 14th January 2013 12:27 AM

Quote:

Originally Posted by woody (Post 3324708)
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?

piano3 14th January 2013 12:30 AM

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


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