lm317 and heater supply.

[pbkrag]

Working on a Power Supply (my first) for a headphone tube amp, And having a problem with the Heater supply. Thinking of doing something with a pair of 5842s. The Transformer was taken out of a SS amp and does not have a 6.3v winding. I have a 5 or 10. So I used some uf4007 and built a bridge rectifier on the 10v and comes out at 13v. then added a lm317 and getting 6.8v when I hook it up to my meter. But when I connect it to the heater pins on a tube, the voltage drops to about 0.4v. What am I missing?

 

[nigelwright7557]

Have you used a smoothing capacitor ?

 

[pbkrag]

After the bridge I have an CRC filter and then the LM317 using 220r and 980r to program the 317. followed by a 1uf cap from the Vout to ground.

 

[tubesteve]

LN317

the heater cold resistance is much lower than under working conditions. Therefore it can be, that this lower resistance is causing a current limit / short circuit protection of the LM317. Start up currents of tube heaters are much higher than the rated ( operation) currents.

 

[pbkrag]

LM317

Here's a quick sketch of what I have going on.

 

[Vincent77]

Take out the 100R resistor, you don't need it. Or replace it with a proper value, to drop some volts, leaving at least 3v headroom for the LM317.

If your tube draws 0.3A of current, you'll get a 0.3*100 = 30v drop over the resistor. It can't work.

 

[funk1980]

Vincent77's answer is right on the money. This kind of CRC filtering is unnecessary when using a LM317 regulator. Specially for heater purposes.

And AFAIK, the 5842's heater should be at 6.3V, so your adjustment network needs some value tweeking.

 

[yagoolar]

As tubesteve mentioned - cold filament resistance is much lower than a hot one, and you may consider slow turn-on circuit for LM317. Another approach is to combine voltage regulator followed by current regulator based on two LM317s. It limits filament current to a defined value, ergo for lower resistance, voltage drop is also lower. Voltage regulator + lower resistance equals higher currrent.
I use the pre-regulator and current regulator to supply DHT filaments in a power amp.
http://www.diyaudio.com/forums/powe...tracking-pre-regulator-current-regulator.html

 

[pbkrag]

Thank You Vincent77 and everyone who answered. I removed the 100r and the heaters light up. With two connected they settle in at 6.2v.