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Old 11th August 2010, 12:32 PM   #1
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Default 12AX7 heaters from 9VAC transformer

I am trying to feed the heaters of a single 12AX7 from a 9VAC supply. After rectification and filtering, I get 10.8VDC.

I need 6.3V @ 300mA. Is there a simple way to achieve this with just a series resistor? Do you have any similar schematics I can take a look at?

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Old 11th August 2010, 12:55 PM   #2
DF96 is offline DF96  England
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Ohm's law says you need 15 ohms, but this includes the output impedance of your DC supply.
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Old 11th August 2010, 12:55 PM   #3
OneyedK is offline OneyedK  Belgium
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You need to drop (10.8-6.3) 4.5V @ 300mA.
Assuming the 10.8VDC will not drop under load,
Rseries = Vdrop / I = 4.5V/0.3A = 15 Ohm
Power = Vdrop * I = 4.5V * 0.3A = 1.35W ==> take at least 3W to be safe.

If the 10.8VDC drops under load, Rseries has to be smaller too...
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Old 11th August 2010, 01:04 PM   #4
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another idea - could voltage double the 9VAC supply and then bump it down to 12.6V via resistor or regulator. Of course that puts you in the same boat as before!
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Old 11th August 2010, 02:31 PM   #5
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Wire the heaters in series and run on 10.8VDC, no problem...

If you really need higher emissions, was probably wrong tube
for the job anyways...

Last edited by kenpeter; 11th August 2010 at 02:36 PM.
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Old 11th August 2010, 03:52 PM   #6
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What Kenpeter suggests, but using Shottky diodes to rectify.
The Devil is not so terrible as his math model is!
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Old 11th August 2010, 04:10 PM   #7
TheGimp is offline TheGimp  United States
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Use large (5X your current draw) schottky diodes to minimize the voltage drop. You should be able to get near 12V that way. For a single 12AX7 1A diodes will work fine, but 3A might give you less forward drop.

What topology rectification are you using? Half wave? Full Wave center tapped? Full wave bridge?

What is the ampere rating of the transformer? You may have too much internal IR Drop.

What size capacitor do you have?

How much ripple (AC) riding on the DC?
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Old 11th August 2010, 05:53 PM   #8
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You can also put a resistor in the primary with similar results.
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Old 11th August 2010, 06:49 PM   #9
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Default heater center tap

There is a center tap on the heater of the 12AX7, making it two six.threes (nine). I've been rectifying the AC heater supply of my H182 organ to try to get some dc to bias a NFET gate, I cant get more than 5 VDC, so I think Hammond runs their heaters at about 5 vac. The tubes lasted 42 years so far, and they don't hardly glow. Go the the flea market, buy a 6VAC wall plug transformer, they are $1 here at the charity resale shops.
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