Confused at the "gain" rating of tubes/transistors.

Thread Starter


Joined Jun 4, 2018
Hi all,
Upon multiple occasions while designing circuits (mostly for audio use), I run into the problem of not being able to boost my signal enough before buffering/transforming it to speaker levels. That got me thinking, "if the <insert tube/transistor here> has a gain of <x>, then why isn't my signal boosting according to this?? In the case of a 12AT7, how come no matter what configuration I try, why can't I boost a 1v signal into a 60v signal? Is output impedance a factor as well? Any enlightenment would be greatly appreciated.



Joined May 3, 2013
12AT7 is basically a Driver with a good current gain, used as a Phase Invertor to drive Power stages. Replace with 12AX7 which is a Voltage amplifier.


Joined Mar 14, 2008
Even with a tube with a gain of 100, I get nothing more than an amplification factor of of 40.
The voltage gain (mu) of a tube is with a load impedance of infinity.
In real circuits the gain is reduced by the voltage drop between the tube internal plate equivalent resistance and the plate load resistance.
Thus if the tube plate resistance is 68k and the load resistance is also 68k, the voltage gain will be reduced to 1/2 mu..


Joined Aug 21, 2008
Get a higher mu (or higher Gm) tube or increase the plate resistance. This will give you more voltage swing but at a higher output impedance.

If you want to use this to drive a speaker you'll need to use a buffer amplifier to get a low output impedance or more traditionally, use an output transformer designed to improve the impedance match between the tube plate the speaker -those used to be very common a very long time ago, so you can probably still find one in an old tube radio receiver if you can find radios from the 1960's or earlier.

Even an old transistor output transformer will be much better than trying to drive the speaker from the plate, but be careful about the interwinding breakdown voltage.


Joined May 20, 2015
I significantly increased the dynamic load of the triode with the help of follow-up feedback. The emitter follower also lowered the output impedance of the amplifier:Draft6.png Draft7.png
Last edited: