My favorite part(s) of the entire build! There is just something about using batteries within these types of devices .. when I first used batteries to hear what fixed bias sounded like compared to cathode bias in my amps circuit, firstly the sound quality amazed me but IMHO there is elegance in its simplicity, especially when it does such a good job.
When it can be used, battery bias is great. Not really practical for power stages however as the bias currents will be all over the place. Not a problem with low power gain stages however since the tubes are not typically biased near their maximum plate dissipation. A conventional negative bias supply with adjustable bias preferred for power stages.
I love to make my dry joke about changing the batteries so the amplifier doesn't lose power, but I controlled myself when I posted that pic. Every once in a while someone falls for it