12v heaters supply [AC questions]

[petemoore]

  Little 2w 6SN7 amp sounds great, it's cobbled...but very nice sounds.
  Now I want to improve the SnR.
  I have a 12.6v transformer secondary connected, and wonder why I don't remember seeing AC tube heating with 12v supply.
  What value for the 'stabilizing?' [most 6.3vac heaters have the 100ohm resistors from ground from each side of the supply circuit].
  I don't know the formula, what value for these resistors @12vac supply? [I'm only guessing when I call them 'stabilizing'...that the keep the secondary floating voltage tied to something, ground]?
 

[Minion]

Well In the Preamp section of my Tube/Solid state hybrid amp I am running 12v AC heaters , there are no resistors on the Heaters , Just a twisted Pair of leads going from PSU to heaters , so I don"t think resistors are required for AC heaters .....

[frequencycentral]

Pete that tube has a 6.3v / 600ma heater right? I believe that elevating the heater above ground eliminates noise. Couldn't you elevate the heater by using 12v DC for the heater supply and using 9.5 ohm / 5 watt resistor in series between the heater and ground? Your 12v secondary is AC right? So rectifying it to DC and using a 7812 will get you your stable 12v DC. The 9.5 ohm / 5 watt will drop the 12v to 6.3v and dissipate the lost voltage as heat, hence the high wattage. I'm guessing that this might do you some good, I've only touched on elevated heaters in my research - maybe you could do some delving?

[JKowalski]

I believe you introduce alot of extra noise if you use AC voltage for your tube filaments. I think it's because the heated filament will give off it's own electrons inside the tube, and introduce a 60Hz hum into your signal.

If you want to lower your noise by alot, I think it would be a good idea to rectify your 12VAC and filter it, so as to provide a 12VDC filament supply.


EDIT: FC's got the right idea, sorry for the repetitive info.

[petemoore]

  Thanks for the replies...
  I looked and found a 6SN7GTA.
  Wiki states it's like a few others which require a 12v supply. ...the 'other version of the 6SN7.
  And found a nice transformer that is doing the honors, the amp is relatively low noise now, but I have to learn how to re-think the PI and bias of the other tubes.
  My B+ is about...78% of what the Moonlight is designed for.
  Yes, I should buy the correct transformer for the job,

[brett]

Hi Pete
While it's not essential for a good signal to noise ratio, I usually recommend a 12.6V or 6.3V DC supply for heaters. In the days of tube rectifiers DC was a lot of effeort, but these days 12.6 volts DC is as easy as getting/making a bridge and using an LM7812 with a diode on the com leg.

Tube bias is a bit of a black art, so you can have lots of fun experimenting.  For power tubes in Class AB you can usually go with about 70% of the rated plate dissipation (e.g. for a 15W tube and 300 volts on the plate, 100% of the rating = 50mA and 70% = 35mA).  Pre-amp and PI tubes are much more personal.  I like tubes that are pushed to one "side" or other.  The Real McTube is a classic circuit that got a great sound out of tubes "misbiassed" cold and hot for lots of clipping.
cheers