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Old 16th January 2013, 09:19 PM   #31
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The last schematic you posted should work in principle, yes. But mixing parallel/serial heaters is still not recommended, though it works theoretically - just for one example, if one of the paralleled 12AX7 fails, it will probably lead to subsequent destruction of the second 12AX7 in that pair.

Greetings,
Andreas
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Old 16th January 2013, 09:24 PM   #32
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Originally Posted by Rundmaus View Post
The last schematic you posted should work in principle, yes. But mixing parallel/serial heaters is still not recommended, though it works theoretically - just for one example, if one of the paralleled 12AX7 fails, it will probably lead to subsequent destruction of the second 12AX7 in that pair.

Greetings,
Andreas
could add a small LED on the 12ax7's on the load side so if the heater dies than the led light goes out
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Old 16th January 2013, 09:30 PM   #33
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than this should work would you agree?
Yes, that should work. As I mentioned before, it might be a good idea to put power zeners of appropriately chosen voltage across each of the banks of 12AX7 tube heaters, to prevent the voltage across them rising too high at switch-on. (I am suspecting that the 7n7 tubes will tend to heat up considerably slower than the 12AX7 tubes, and that therefore in the first few seconds the 12AX7 heaters would tend to get a disproportionate share of the voltage.) 6.8V is a standard zener voltage, so that might be suitable. The zeners would also protect against a failure of of one of the paralleled 12AX7's.

It would also be good to check that in the steady state, after they have all warmed up, that the voltages across each 7n7 and 12AX7 bank really are about right.

Series-connected heaters may not be the most highly recommended way of doing things, but if it works, then why not? I've got away with it so far, for a set of fairly disparate tubes hooked up in a chain running from 120V AC.

Chris

Last edited by cnpope; 16th January 2013 at 09:37 PM.
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Old 16th January 2013, 09:59 PM   #34
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[QUOTE=cnpope;3329047]As I mentioned before, it might be a good idea to put power zeners of appropriately chosen voltage across each of the banks of 12AX7 tube heaters, to prevent the voltage across them rising too high at switch-on. 6.8V is a standard zener voltage, so that might be suitable./QUOTE]

Thx

So one zener per 12ax7 bank or one per 12ax7 tube? I would think bank? Also a 25vdc supply will have a + and - side so would all anodes on the zener point toward the positive side?

Thx again
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Old 16th January 2013, 10:21 PM   #35
cnpope is offline cnpope  United States
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[QUOTE=desperateaudio;3329075]
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As I mentioned before, it might be a good idea to put power zeners of appropriately chosen voltage across each of the banks of 12AX7 tube heaters, to prevent the voltage across them rising too high at switch-on. 6.8V is a standard zener voltage, so that might be suitable./QUOTE]

Thx

So one zener per 12ax7 bank or one per 12ax7 tube? I would think bank? Also a 25vdc supply will have a + and - side so would all anodes on the zener point toward the positive side?

Thx again
One per 12AX7 bank. Mouser has 5W zeners at 36 cents. Should be OK, I think. They should be connected the way round that means they allow the 6.8V across the heaters. I think that means cathode more positive than anode.

Chris
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Old 17th January 2013, 01:42 AM   #36
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[QUOTE=cnpope;3329106]
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Originally Posted by desperateaudio View Post

One per 12AX7 bank. Mouser has 5W zeners at 36 cents. Should be OK, I think. They should be connected the way round that means they allow the 6.8V across the heaters. I think that means cathode more positive than anode.

Chris
So u mean this for the zeners.
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Old 17th January 2013, 01:59 AM   #37
cnpope is offline cnpope  United States
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[QUOTE=desperateaudio;3329320]
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So u mean this for the zeners.
Zeners the other way round, I think. The cathode (barred end) should be more positive than the anode.

Chris
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Old 17th January 2013, 02:06 AM   #38
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[QUOTE=cnpope;3329340]
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Zeners the other way round, I think. The cathode (barred end) should be more positive than the anode.

Chris
my bad not thinking again. Looking at it like a regular 4007 setup.
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Old 17th January 2013, 01:11 PM   #39
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just to close the loop heres the final. wiring it today
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Old 17th January 2013, 01:54 PM   #40
cnpope is offline cnpope  United States
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just to close the loop heres the final. wiring it today
Yes, that should work. And it would be a good idea to measure the voltages across each heater, just to make sure the voltage is dividing out as it should.

Chris
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