Wiring up 3 6.3 Volt Heaters to a 12V supply - diyAudio
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Old 18th January 2013, 04:56 AM   #1
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Default Wiring up 3 6.3 Volt Heaters to a 12V supply

Hi there,

My tube amp project involves 3 6BQ7A tubes, and I will use a 12VDC supply for the heaters. I'm wondering if the best choice would be to:

A) Use a voltage divider with high Wattage resistors to drop the voltage down to 6.3 and wire the tubes in parallel from that supply.

B) Wire two tubes' heaters in series, and have one tube's heaters in parallel with one of those two tubes. I am unsure of the consequences this approach, and it seems unconventional.

Thanks for your advice in advance!
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Old 18th January 2013, 05:55 AM   #2
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Just use a 12BQ7A, same tube but uses a 12.6 Volt heater..........

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Old 18th January 2013, 06:03 AM   #3
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Hi!

If you want to use option B, do it like this: wire two tubes heaters in parllel.
Wire a resistor in parallel to the single tube which is left, which draws the same current.
Then put these two proups in series

Thomas
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Old 18th January 2013, 06:46 AM   #4
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Quote:
Originally Posted by Richard Ellis View Post
Just use a 12BQ7A, same tube but uses a 12.6 Volt heater..........

__________________________________________________ _Rick...
I tried searching eBay, but there don't seem to be any for sale... These I also have in my hand, so I'd like to use them if possible

http://www.r-type.org/pdfs/6bq7a.pdf

Each heater takes 0.4 amps, so at 12 volts, I should use a 30Ohm, 5Watt resistor, just to confirm:

+12V ---^--^--- GND
+12V ---R--^-- GND

Where ^ = 6.3V heater, R = 30Ohm, 5Watt resistor.

or

+12V ---^--\/--^--- GND
+12V ---R--/\--^-- GND

Where ^ = 6.3V heater, R = 30Ohm, 5Watt resistor., and the wires cross at \/ and /\

Last edited by supersonik319; 18th January 2013 at 06:55 AM.
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Old 18th January 2013, 06:55 AM   #5
Elvee is offline Elvee  Belgium
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If your 12V is strong and healthy (comfortably sized transformer), you could try to rectify and filter, and feed the three tubes in series.
You should use schottky diodes in the bridge to minimize the losses.
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Old 18th January 2013, 09:35 AM   #6
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The filaments are not strictly identical between tubes. If you wire them in series, one tube may end up at 5.7v and the other at 6.3...
The right voltage if you want the maximum performance is 6.3V, not 6 or less. Yes, it's only a 5% difference, but it does have an influence on the tube's parameters and lifespan.
It didn't matter in the 60's because there were millions of spare tubes around and they didn't have good and cheap voltage regulation... But today we should do better!

Why not doing it the right way??
Buy an adjustable DC-DC LM2596 converter module ($3 on ebay), it's good for 2 Amperes and wire the heaters in parallel.
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Old 18th January 2013, 10:45 AM   #7
DF96 is offline DF96  England
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Use the first connection in post 4: one leg with two heaters in series, the other leg with heater plus resistor, and no cross-connection. Omitting the cross-connection will reduce the warm-up stress on the heater which would otherwise be placed in parallel with R, and will avoid any similar stress created if R goes open circuit.

Although (unlike 4BQ7A and 5BQ7A) the 6BQ7A does not have a controlled warmup heater it should be OK in short series heater strings.
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Old 18th January 2013, 02:10 PM   #8
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Quote:
Originally Posted by Vincent77 View Post
The filaments are not strictly identical between tubes. If you wire them in series, one tube may end up at 5.7v and the other at 6.3...
The right voltage if you want the maximum performance is 6.3V, not 6 or less. Yes, it's only a 5% difference, but it does have an influence on the tube's parameters and lifespan.
It didn't matter in the 60's because there were millions of spare tubes around and they didn't have good and cheap voltage regulation... But today we should do better!

Why not doing it the right way??
Buy an adjustable DC-DC LM2596 converter module ($3 on ebay), it's good for 2 Amperes and wire the heaters in parallel.
That's....actually a good point. LM317s could also go up to 1.5Amp..... I think I'll go with this method as I have voltage regulator chips lying around, and no 30 Ohm 5Watt resistors (I was planning on using 3 10 Ohm sandbars I already had). Thanks for the help!

I'll do some more research into heater power specifics to understand DF96's proposal to keep the knowledge for when I run out of chips
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Old 18th January 2013, 02:23 PM   #9
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Just be careful of relative voltages.

Don't put yourself in a position where one heater could be grossly positive with respect to the cathode, you might end up with conduction between the cathode and the heater.
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Old 18th January 2013, 02:52 PM   #10
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Would that be possible if I wire the three heaters in parallel with the output of the LM317 chip? How would I go about minimizing this risk?
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