12.6v+ vs 6.3v+/- heater supply

[mountainhick]

I can go with either 12.6 or 6.3 +/- with center tap.

The first pic below shows a circuit with 7812 regulator and a diode on the regulator ground to bump the voltage, but my understanding is the 7812 regulator needs a few extra volts on the supply side, hence the specified 15v transformer. Is this correct? (I already have a 12.6v transformer and would prefer not having to buy a different one).

The second pic shows a 12v +/- center tapped PS. I am wondering about adapting this to 6.3v by using my 12.6v center tapped transformer and 7806 and 7906 regulators. Certainly the 12.6v transformer will work. But, how can I bump the voltage from 6 to 6.3v? If diodes are added to bump the output of the regulators, won't the voltage be a bit too high? Or, can 12ax7 type tubes run well from the slightly lower 6v anyway?

[Tom Bavis]

78 series regulators need about 2.5V headroom to regulate. There are "low dropout" regulators that only need 0.6V or so - probably what you need. In any case, use Schottky rectifiers for their low forward drop (1N5822) to get the most from your transformer. 12AX7s CAN run well below rated heater voltage, but transconductance drops. OK if you don't have to deal with a lot of voltage swing as in a phono preamp.

[Svein_B]

6V for heater is within the specified +/- 5% and should be quite OK.

If you need to bump 0.3V you could try a shotkey diode. They have a low voltage drop of about 0.2-4V.

7812 is specified to require 14.6V input + 0.6V for your bumper diode = 15V. So you should be OK with the 12.6V PT.

Svein.

[mountainhick]

Originally Posted by Svein_B

7812 is specified to require 14.6V input + 0.6V for your bumper diode = 15V. So you should be OK with the 12.6V PT.

Svein.

I don't understand this. How is the diode used to bump 12.6v to 15v before the regulator?

[Zen Mod]

use LM317 as CCS for heaters

elevate (connect) heater supply to some positive voltage ( say 40V above cathode potential) , made with resistive divider from high voltage supply to gnd

give us more information about exact project , and I 'll tell you exactly how to make that

[Zen Mod]

Originally Posted by mountainhick

I am adding a separate PS to power one 12ax7 tube heater. I have a 12.6v transformer to use. It is that simple.

nope ;

difference between "it is that simple " and "nope " is what differentiate mediocre and good stage ; heating is important , not just helper circuit .

you see some tricks here : http://choky.diyaudio.rs/WOT_preamp_...y_charger.html

[mountainhick]

Originally Posted by Zen Mod

nope ;

Yep, 12.6v+ or 6.3v +/- regulated heater supply for a single tube is all I need. I do not need to complicate this issue.

[Zen Mod]

whatever ;

I was willing to draw exact schematic , for your exact project ;

[woody]

What is the voltage and current rating of your transformer?

[mountainhick]

Originally Posted by Zen Mod

whatever ;

I was willing to draw exact schematic , for your exact project ;

Please do. Really, the exact project is to add a transformer and regulation to supply one 12ax7 tube with regulated heater voltage. That's it.