12ax7 and IRF820 CF B+ approximate current consumption

[kagliostro]

My brain refuse to remember the math that is related with

but I can remember that the consumption of a triode anode follower may be near 2-4mA (is this correct ?)

about the CF I remember that the consumption of a 12au7 may be around 10mA

the question is

which can be the approximate consumption in current of a 12ax7 CF and of a IRF820 CF ?

Thanks

K

[HotBluePlates]

... the consumption of a triode anode follower may be near 2-4mA (is this correct ?)

about the CF I remember that the consumption of a 12au7 may be around 10mA

the question is

which can be the approximate consumption in current of a 12ax7 CF and of a IRF820 CF ? ...

Something between 0mA and a current limited by any series resistance in the circuit.

How much plate current a tube draws depends on its bias (and for a pentode, G1 bias and G2 voltage). So it is wrong to say that a "12AX7 draws ___ while a 12AU7 draws ____."

The 12AU7 could have a higher plate current if its bias, supply voltage and series resistance allows it.

Example:
You have a B+ of 300v and a 12AU7 cathode follower with a 100kΩ cathode resistor. Maximum possible plate current, regardless of bias and signal, is 300v / 100kΩ = 3mA.

That said, the 12AU7 has a lower internal plate resistance than a 12AX7, and could have a higher plate current than a 12AX7 if external resistance (plate/cathode load, cathode bias resistor) allows it.

I assume a similar situation exists with the MOSFET, that its drain current depends on what voltage you apply to the gate (and other leads).

[PRR]

You probably have a large resistor, 50K to 500K, in series with a tube (or MOSFET).

What is the MOST the stage could suck? V/R.

How would it do that? Tube (MOSFET) dead-short.

What is the sound of a dead short? Dead!

So the tube (MOSFET) must have some resistance. In fact it will "usually" take 1/3rd to 2/3rd the total voltage, leaving 2/3 to 1/3 across the resistor.

For a quick rough guess, assume "half voltage". 12IRF7 with 100K from 300V is probably 1.5mA. 13SEX7 with 47K is maybe 3mA. Remember it may be a little more and allow some extra (don't use a supply that will blow-up at 1.51mA, allow 2mA). Then build it.

[kagliostro]

MANY THANKS HotBluePlates & PRR

K

[tubeswell]

The current that flows through the mosfet will be the same current that is flowing through the load for the mosfet (which is in series with the mosfet's source or drain - in the same way that a Rk is in series with a tube cathode and a Ra is in series with a tube plate).

[kagliostro]

Many Thanks Tubeswell

K