ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

previous: Mike It's 5V before the resistor! About ... -- 9/15/1999 12:03 PM

Re: DC Filament voltage low

9/15/1999 1:45 PM
kg	Re: DC Filament voltage low If it's that low before the resistor, then it's certainly the diodes. Most diodes have about a .6 to .7 V drop across them, due to the energy level required to cause the junction current to flow. If you've got a full wave bridge, that's two diode drops that must be accounted for, a full 1.2 to 1.4 V that comes right off the top of the AC voltage. So a 6V3AC secondary has a peak of 1.41 times Vsec, which is 8V8AC. Subtract 1V3, and you're left with 7V5. No matter how lightly you load your winding, therefore, you CANNOT get more than 7V5DC. Now, as soon as you start to draw current from your heater winding, that voltage will drop. The reason it does is due to the DCR of the winding itself, as well as the DCR of the primary winding (believe it or not). This all comes into play as the regulation of the transformer, which is essentially the difference in output voltage from no load to full load. What kind of heater load are you placing across the supply? You might also try replacing the 1R resistor with a choke, like a cheapy job from radio shack for 12V car stereo use. This will have a lower DCR than 1 ohm, and a higher AC impedance to hum/ripple/noise. This will not change the voltage right out of the rectifier, though, and that is what seems to be low. Ken

9/15/1999 1:45 PM

Re: DC Filament voltage low

If it's that low before the resistor, then it's certainly the diodes. Most diodes have about a .6 to .7 V drop across them, due to the energy level required to cause the junction current to flow. If you've got a full wave bridge, that's two diode drops that must be accounted for, a full 1.2 to 1.4 V that comes right off the top of the AC voltage.

So a 6V3AC secondary has a peak of 1.41 times Vsec, which is 8V8AC. Subtract 1V3, and you're left with 7V5. No matter how lightly you load your winding, therefore, you CANNOT get more than 7V5DC.

Now, as soon as you start to draw current from your heater winding, that voltage will drop. The reason it does is due to the DCR of the winding itself, as well as the DCR of the primary winding (believe it or not). This all comes into play as the regulation of the transformer, which is essentially the difference in output voltage from no load to full load.

What kind of heater load are you placing across the supply?

You might also try replacing the 1R resistor with a choke, like a cheapy job from radio shack for 12V car stereo use. This will have a lower DCR than 1 ohm, and a higher AC impedance to hum/ripple/noise. This will not change the voltage right out of the rectifier, though, and that is what seems to be low.

Ken

Replies:

Mike I'm running 3 12ax7's only. I did p... -- 9/15/1999 2:19 PM