PT Clarifications

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You are very unlikely to draw 10mA per plate from a 12AX7. Its normal operating current is in the 1mA to 2mA region. If you look at its datasheet you will see a typical operating current of 1.2mA listed. Even if you assumed 2mA per plate you would need no more than 8mA for a pair of 12AX7.

Your 6X4 looks OK. Sometimes you need series resistors in the plates but your load current is so low this should not be necessary. If you want to use a single 6.3VAC supply for the 6X4 and the 12AX7, make sure one side of the 6X4 cathode is connected to HT 0V to ensure the tubes specified heater/cathode voltage is not exceeded.

Cheers

Ian

our arithmetic looks OK to me. I am not sure why you have one side of the heater connected to 0V via a resistor. Better to eleiminate this and just connect the heater winding centre tap direct to 0V.

Cheers

Ian

80hinhiding said:

Good to hear. 

Oh okay, I misunderstood.  Is this so they have no possible difference in ... difference?

Click to expand...

One side of the 6X6 heater needs to be at or near 0V. One way to do this is to connect one of the heater pins straight to 0V. Another way to do it, if you have a heater winding with a centre tap, is to connect the heater centre tap to 0V. The 6X4 heaters are then 3.145VAC above 0V which is close enough. This method has the additional advantage of reducing hum pick up in the audio tubes.

Cheers

Ian

80hinhiding said:

That's interesting.  Hum pick up is reduced, because it's not right at 0 all the time? I'm here tracing the path of the electricity with my finger and think I've got it.

I haven't shown any of the filtering but I was thinking of using a DC on the heater.  Should I just use AC for the heater?

Sketch attached.

Adam

Click to expand...

Hum pick up is reduced because using the centre tap means the heater voltage is balanced. You could use DC for the heaters but then you cannot use the centre tap. You also need to smooth the recitified heater voltage otherwise it is little different to AC.

Cheers

Ian

There's loads of stuff about this on the web. Check this out:

http://www.valvewizard.co.uk/

Cheers

ian