I had a Sunn Sonic 1 head in 1968, and discovered what looked like a 1/4" phone jack between the two power tubes. Using a mirror, I saw a screw slot at the bottom of the hole, so I used a screwdriver to turn it to see what it did. This led to taking it back to the dealer to have him bring the bias back up to spec. If it ain't broke don't fix it., but I still make the same noob mistake in other areas of my life from time to time.