dc filament design question

[gary h]

Hi all,

I'm trying to design a PS for my Aikido line stage. At this point I'm wondering how much of a voltage drop I should expect from running a 6.3 V filament through a diode bridge rectifier. I'm under the impression that most of the transformers I have speced design their filaments to supply AC directly (12.6V at 0.6mA for example.)

What's the conventional wisdom in this situation?

Thanks,

gary

[duderduderini]

Hey Gary. Generally with a diode bridge and cap(s) you get a 1.4 increasing factor to your output voltage from your filament transformer and then depending on the current the trans can deliver vs what your filaments need will determine your voltage "drop" Have you thought about using a regulated heater supply using say an LM338K adjustable regulator?
Generally I tend to run my filaments at 6 volts (if the spec is 6.3) as the worst thing you can do to a tube is run over voltage even a bit..
Hope this helps
Nick

[Tom Bavis]

The bridge will drop about 2V, and the voltage drop due to winding resistance will be a bit more due to the reduced conduction angle. A 6.3V winding should be able to provide 6.3VDC, but you may have to use Schottky diodes for their lower voltage drop. 12V winding will be easier, since current is half, and diode drop is proportionally smaller. Jut pair tube heaters in series.

[woody]

I had a similar case where I had a ~ 6v transformer and had ~ 6.75v after I rectified it. My curent draw is .9A so I put
a .47ohm resistor in seriese to drop it down to 6.3v.

[jazzbo]

General rule of thumb states that your transformer should be AC current rated for ~1.8 times the required DC current, e.g. for a 1A DC draw, use a transformer with ~ 2A AC rating.

[lochness]

I'm not sure if I would bother with DC heaters. Unless I'm using DH tubes I don't bother. Never had a problem if the design of the amp/preamp was good. Even with DH tubes I rarely find the hum of any annoyance assuming I even notice it.

[SHiFTY]

6.3VAC turns into about 8 volts or more.

If they are just normal dual triode preamp tubes you can probably get away with tightly twisted heater wires run close to the chassis and away from signal wires.

If you really want DC heaters use something like an LM317 to regulate it down to 6.3V or just below. Easy and quiet.

[gary h]

Thank you all for your responses,

duderduderini, some of the trannies I've looked at are in the Hammond 270-272 series, giving 275-0-275 or 300-0-300, with a 6.3V filament supply between 3-6A.

jazzbo, the Aikido design I'm using employs 4 6SN7s which each require 6.3V and 600mA current. That requires 2.4 A of power and a 5A filament supply by your calculation.

I'm trying to design a tube rectifier circuit for the B+ using psud2, I have yet to see a tube heater used to rectify the low voltage secondary, is this a recommended alternative to using the Shottky diode bridge? In other words, could I run 2 12X4 heater tubes, one to rectify the B+ voltage (300V) and one for the filament supply (6.3V)?

thanks,

gary

[Eli Duttman]

Gary,

It's MUCH simpler to build 12 VDC heater supplies than it is to build "6" V. supplies.

You need 1.2 A. at 12 VDC. Greinacher voltage double a 5 A./6.3 VAC winding using a pair of Schottky diodes. Use 15000 μF./15 WVDC 'lytics in the doubler cap. stack. Follow the doubler with a pair of well heatsinked 7812 3 terminal regulators. Each regulator IC feeds a series connected pair of 6SN7 heaters.

[gary h]

Thanks Eli,

could you point me to a schematic?

thanks,

gary