12 volts DC from 6.3 heater tap

[LeftyStrat]

I'm building a preamp with seven 12ax7s, and thinking about supplying 12 volts DC to the heaters. My PT has a 6.3 volt 3 amp tap. I could voltage double this to 12.6 volts 1.5 amp, rectified 17.8 (actually closer to 18.5, since the primary is 115 volts).

Seven tubes at 12 volts require 1.05 amps current, so the PT has enough capacity.

Any problems using an LM317 in this application?

[Phil_S]

The doubler is "rectified" double or 2 * sqrt(2) = ~2.8.
So, from 6.3 you get about 17+. IMO you need to start with about 4.5VAC to double it to 12.6VDC. I think you need to find a 9VAC source. to produce 12.6VDC.

I'm clueless about the sand.

[martin manning]

The regulator will limit its output voltage to 12VDC, no problem there. The TI LM317 is rated at 1.5A, so it would handle seven tubes.

[LeftyStrat]

There is a LM317T that's rated to 1.5 A:

http://www.mouser.com/ProductDetail/STM ... 52b3mJJw==

I can't seem to find the forward voltage drop on that datasheet, but googling it shows ~3 volts.

So 2.8*6.3 = 17.64

My line voltage never drops below 120 volts, so 120/115 = 1.04.

1.04*17.64= 18.34

Should be enough. I suppose I should look for a to-3 package and add a big heatsink.

[Reeltarded]

If you do a few really shitty solder joints maybe you could leave the regulator out!?

[LeftyStrat]

If you do a few really shitty solder joints maybe you could leave the regulator out!?

Yeah, an intermittent connection with some caps, SMPS!

[VacuumVoodoo]

Watch your heater current budget, Voltage doublers have poor current efficiency and work fine for bias supply but have hard time with heaters. You might be just within margins.

[martin manning]

Use a full-wave doubler for sure.

[JazzGuitarGimp]

If you wind up with 18.34V at the input to the 317, then the 317 will need to disipate (18.34V - 12V) x 1.05A = 6.657 Watts. A TO-3 on a small-ish heat sink should do nicely.

[LeftyStrat]

Thanks for all the advice.

I've got an email to Suhr to see if they will sell me the transformer they use in their upgrade of the CAE 3+ SE. If I can get that I won't try to shoehorn this Hammond 270DX in there.

[ampdoc1]

Why go to all this trouble? Tie your 4&5 pins together and ground them. Then hook your pins 9 to a simple 6V supply. (FWB rectifier and filter cap). I've done this on several amps. The loss of .3 volts on those preamp tubes is nothing.
Check the Peavey Classic 15 schematic for this simple heater supply.

[LeftyStrat]

Why go to all this trouble? Tie your 4&5 pins together and ground them. Then hook your pins 9 to a simple 6V supply. (FWB rectifier and filter cap). I've done this on several amps. The loss of .3 volts on those preamp tubes is nothing.
Check the Peavey Classic 15 schematic for this simple heater supply.

Mainly because I've never built a rack mount high gain three channel preamp, so I was trying not to deviate too far from the scheme used in what I'm copying (Custom Audio Electronics 3+ SE).

I suppose I could start simple and only get more involved if I have problems.

[Kagliostro]

If you decide to stay with the doubler

I suggest you to use an LM338T (not LM338k which is very expensive) instead of the LM317

the LM338T is rated at 5A continuous (7A peak) and the price is near the price for one LM317

K

[Leo_Gnardo]

Somethin' to keep in mind: start up conditions in other words cold filaments. As I'm sure you know, cold filaments draw (A LOT) more current than when heated to operation temp, so make sure your SS DC supply circuit can get past the critical startup moment when fil current is maxed. Note some commercial companies (Peavey, Marshall) simply rectify & filter, and even they have problems with rectifiers & filter caps pooping out occasionally.

[M Fowler]

I would get a Hammond PT 229B120 or 229B230 and filament PT 229B12 that would make things so much easier.