Can someone explain Ultra Linear operation in guitar amps?

[Hiwatts-n-Gibsons]

Fender’s ultralinear amps were introduced in 1977 in all of the 6L6 amps except for the Vibrolux Reverb, which is the only 6L6 Fender amp not to get the Master volume thing, either.
There are other guitar amps that utilize UL output transformers..,notably the Dr. Z Carmen Ghia and the Rte. 66. Dr. z came from the Hammond organ world and carried that use of UL outputs from the Hammonds into some of his amps. The first Carmen Ghia Amal’s were built directly out of the AO35 reverb amp from Hammond organs.

I had a Route 66 for a little while. It really opened my eyes about ultra linear designs. I always associated them with the very bright and immediate sounds from SF Fenders, and the 66 was the polar opposite. It was milkshake thick, warm, and just gorgeous sounding. I think it had a lot to do with the EF86 and GZ34 rectifier tube. It had punch, but it was soooo smooth.

It had teeth, but was never strident or piercing. Just a beautiful and warm amp, with a big sound stage even at low volumes. I ran it with a GZ34 nstead of the 5U4 rectifier. What a wonderful.amp for blues, jazz, and rootsy rock-n-roll tones when hit with good Filtertrons, P90's, Dearmond, or Staple type pickups.