D/C filament problems

[Atarijetman]

im using a 4007 diode bridge rectifier to convert 6.3vac to D/C.

I was expecting to read approx 7.62v across the output, but its only putting out about 4.5v.

Does anybody have any suggestions of what could be wrong? My tubes (2x 12AX7) are very lukewarm!

Cheers

 

[joaquins]

Are they loaded already? did you added a cap? how big?

When you use a bridge rectifier you are inserting 2 diodes in series, so they drop at most 1V each, maybe less, when you are doing a HV supply as the ~200V for a tube circuit you just forget about this, when you are working on a regulated PS you just take into account the voltage is high enough and probably don't care much about 1V or 2V, when you are using unregulated low voltage PS as yours, at 6V PS, 2V loss is pretty big, if you are only using 12AX7 you could use a voltage doubler and use at ~12V, with 2V loss, you'll probably be fine.

Why are you using DC heaters? if well implemented AC should't bring a big problem, using two resistors to cancel out the hum, and using elevated heaters in more sensitive cases should be just enough, usually DC are used for really high gain as a guitar amp, but with 12V so less current is needed and even then starting with higher voltage and dropping with a regulator or a resistor so you can accommodate the needed voltage.

JS

 

[Atarijetman]

Thanks for the reply,

Yeah I was expecting to see 6.3 x 1.41 - 2 x 0.7 diode drop = 7.4v so 4v is a big loss.

I was advised to use D/C filaments, so I just went with it!

 

[SSLtech]

did you added a cap? how big?

Inquiring minds would like to know.

 

[joaquins]

If you are not using caps, or caps too small for the load, then you shouldn't multiply by 1.4 which is peak for your voltage, and will make it follow the input voltage less the rectifier drop, and you measure the mid value instead of the peak, or the filtered mid value, which makes 6.3V-2V(from diodes)~4V I'm using 1V per diode just because is not strange to get up to there with relatively high currents in rectifying diodes.

JS

 

[Matador]

With no cap, the RMS voltage after rectification would be about 4.8V : (6.3VAC * SQRT(2) - 2V) * 1/SQRT(2).  This assumes that the drop across the rectifier diodes is 1V each (it's probably a bit less at 700mA heater current).

 

[PRR]

> expecting ... 7.62v ...only putting out about 4.5v

1.414 is for a BIG cap.

With NO cap and an average-reading DC meter, the factor is 0.9.

Minus diode losses, of course.

6.3V*0.9 is 5.67V

4.5V is 1.17V shy of calculation.

Two diodes. And 1.17V/2 is 0.585V. Which is spot-on 0.6V for practical purpose.

IMHO, 1N400x at 0.7 Amp DC current is under-sized.

 

[Matador]

1.414 is for a BIG cap.

Ahh yes, of course.  Thanks for the correction.