help with heater supply

[jarthel]

Here's the schematics (http://home.swbell.net/acavalli/images/MJ_Power_Supply_2.gif).

Well my Hammond xformer is giving out 7+V on the heater secondary. Looking at the schematics, the design states that I place the 6X4 before the diode bridge and the 7+V going to the 6X4 seems alot.

Am I better off connecting the 6X4 after the diode bridge?

Thanks

Jayel

 

[SY]

Connecting the 6X4 filaments to the same winding that feeds the diode bridge can a potential problem for reliability because of the average cathode-filament voltage ratings. You'd really want a separate secondary to power the 6X4. And AC for its filament should be fine as long as you have decent filtering/bypassing. The highish voltage for that filament probably won't be a problem.

The usual high voltage warnings apply. Discharge caps, Poke around with great care. Wear eye protection around new circuits with large electrolytics.

 

[HDTVman]

I don't think I'd worry. If you measured the transformer voltage unloaded it will most likely drop down to very near 6.3 VAC when loaded by all of the tube heaters. The heater voltage for most tubes is +/- 10% anyway, and the AC power to you house will vary that much.

Later BZ

 

[SHiFTY]

Remember that valves can easily tolerate +/- 10% on their filaments with little or no impact on life. 7V is (just) outside that limit, so no worries. Use a 0.25ohm 5W resistor in series with it if you are worried.

More important IMHO is to get decent low voltage caps for that DC heater supply... all my caps seem to die after a while on my 6.3V 1A supply...

BTW I have been using a 6X4 with a 40uF cap input, it seems to be just fine!

 

[dhaen]

Hi,

Even the heater wiring and valve socket drop some voltage.
You'll only know the true voltage when you've wired up and measured actually on the valve pins.

Another small point is that with the bridge and valves connected, your meter will not measure the true AC RMS voltage. This is because of the distortion that the bridge/cap cause to the AC waveform due to voltage drop in the power transformer's heater winding. This could make a couple of % difference in the measurement. So don't worry

I second SY's H-K insulation comments, and would always use a separate winding for rectifier heaters if at all possible.

A small 6v toroid transformer, when partially loaded, is great for powering that "odd valve" that needs it's own supply.

Cheers,

 

[SY]

Though shifty, dhaen, and HD are correct that the winding voltage will drop under load and that ten percent variations aren't enormous, you should also note that according to the RCA tube manual, the 6X4 is particularly tolerant of heater voltage variation. That's the good news.