Neg feedback switch

I have a Triode of Japan 300B Amp. It has a on and off negative feed back switch. What purpose does it serve. When it is engaged it seems to lower the volume.
Here is a cursory answer.
Adding NFB will lower the amplifier’s output impedance and also increase the damping factor(DF). This can be advantageous for some(but not all) speakers. Adding this NFB will lower the gain of the amplifier thus the lower volume you note. Some speakers will sound better will zero NFB, it just depends on the particular speaker in use. For example my speakers sound their best with zero NFB amplifiers. Listen to both settings and choose what sounds best to you.
It decreases amplifier gain to bring more dynamic headroom.  
Thanks guys

All tube amp should have this option, as it’s a few dollars in parts and easy to do.

I used to put them on my tube amps, on or off. And yes no/less feedback = more amplifier gain.

When the feedback was switched on, I also had a dial you could turn to vary the amount of feedback from almost zero to 35db of feedback. This also lets you tune the amps damping factor to the speaker, depending if the speaker was an under or over damped design.

Cheers George

It decreases amplifier gain to bring more dynamic headroom.  
This statement is false. 'Dynamic headroom' is a function of the class of operation (class A, class AB, etc.) and is unaffected by feedback.

The amplifier gain is used to reduce the 'output impedance' by using the gain as a servo mechanism to correct for errors in the output signal.