Tube Mini-Booster

[Dart]

Is it possible to use two halfs of a 12AX7 in Jack's mini-booster instead of FETs (with higher voltage) ? I think I can make a nice simple tube booster
in a box or an additional preamp boost stage too from it
Is it better compared to cascoded boost stage (Firefly) ?

[Gringo]

Check this out:

http://laplaza.org/~fvuotto/mustage.html

There is a lot of info if you google 'tube "mu stage" triode' (without the 's). Old electronics textbooks are a good source too.

[R.G.]

The so-called minibooster is in fact a JFET reproduction of the much earlier vacuum tube "mu-amp", so named because the gain of the stage is almost the "mu" of the tube. So yes, it works with tubes.

It's even better if you put a resistor between the source/cathode of the upper FET/tube and the drain/plate of the lower FET/tube and take the signal off the source/cathode. That gives you a low output impedance and little or no sensitivity of gain to loading. See the article "Mod your Mu-amp" at GEO for how and why this works.

[Ben N]

How about a tube/FET mu-amp?

Ben

[Tom Sawyer]

Hi, how much is the usual Z out for a cathode follower stage?

Regards.

[puretube]

roughly spoken, it`s 1/S;
for a 12AX7, with an S of 1.6mA/V, that`s 625Ohm;
(or: Ri of the 12AX7 = 62.5k, divided by its µ of 100);