6V Regulated DC heater

[dennyg]

I'm sure this is out there somewhere but could use some basic input/cautions on creating a regulated DC heater circuit off of a 6.3V, 2A heater winding. My proposed approach is to use a doubling AC to DC circuit to yield 12VDC, use an LM7806 (rated at 1A with a minimum Vi of 8V) to feed 3 12AX7's (900mA total).  Since I'll be bleeding off 6V I'll use a heat sink.  I've also read that the tranny needs 40% more current to convert AC to DC but a 2A tranny should be well within tolerance to feed 900mA.  Am i missing anything or is there a better approach?
thanks!   

[EKDENTON]

If you just need enough power for filaments on three 12ax7 tubes and your wanting to operate them on 12dc maybe a small dc power supply would be the easiest way to go? I've made two small amps with these dc power supplies and a small board that you can dial the voltage down to  6.3vdc.

[PRR]

> a 2A tranny should be well within tolerance to feed 900mA.

There's a 2X factor for the doubler also.

I think 0.625A is the safe limit.

The doubler "should" give 6.3V*1.414*2 or near 16V. Wire the tubes for 12V. Add 8 or 10 Ohms series to drop to 12V, with another big cap there because 180+Hz buzz will be the big problem.

[dennyg]

I didn't realize the doubler would provide an additional 1.4 x increase - I'm assuming that's unloaded? If I want to regulate then I should have enough Vi to use an LM7812? I believe the minimum Vi is 14.5V but not sure if that's under load or not.

[DummyLoad]

I didn't realize the doubler would provide an additional 1.4 x increase

a voltage doubler is as it is implied - literally: output is approximately Vpk doubled. Vrms x 2xSqRt2 or 2.828 x Vrms UNLOADED. 

with a 300mA load - PSU designer shows ~9V output with a 2A winding. YMMV. i used 4,700uF caps for the stacked first filter pair and 10,000uF for the main filter. with a 600mA load output is just over 3V. i don't know much to trust PSU designer2 in regards to low volt simulations but it's been fairly accurate with HV simulations. doublers are terribly inefficient.

see attached SIM outputs.

if you're going to use a 7812 then use a 15V 2A transformer with a FWB. with a conventional Vreg (7812 types) you need at least 3V over the regulators output rating for regulation to function properly - e.g supply is 15V for a 12V regulator.

there are other regulators that allow the supply voltage to be closer to the regulated output voltage - they are called Low Drop Out or LDO types - typically you can get away with 1.5V over for the supply voltage. so a 12V regulator would require 13.5V supply - use caution though as the 13.5V should be maintained with the regulator operating at rated maximum current load.

a LDO with suitable Iout for this type of application are the Linear Tech, LT108x family. another made specifically for tubes is the TI LM1085A 3A

http://www.ti.com/lit/ds/symlink/lm1085.pdf

http://www.linear.com/product/LT1083

--pete 

[EKDENTON]

watch this and it will explain the 1.414 increase in voltage and the need for the capacitor. https://www.youtube.com/watch?v=lmwnCbwKjAk&t=939s
and also this video https://www.youtube.com/watch?v=dl729dEJ3To&t=305s


The filter cap will charge to the peak voltage amount which will be the 6.3v * 1.414 then if you double that if your making a voltage doubler.




Make sure you calculate the size of the voltage dropping resistor you will need if you do it like your planning.

[trobbins]

If you are doing this to lower hum, then you will need to be aware that rectifying the heater winding may introduce other hum.  You also need to ground reference the heater supply.