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How about using the new KT120's?
Can we just plug and play?

KT120 might eat up to 1.95A heater current per tube. That's more than the 1.6A of the KT88. The P782 runs pretty hot already. I don't think I need to put any more load on the heater winding.

KT120 also specifies no more than 50k Ohms from G1 to ground in fixed bias applications. The stock Poseidon schematic uses 100k Ohm parts (R12, R13) which might be more than the KT120 will tolerate.

I'm not gonna be the guinea pig. Let us know if it works for you.

I'm running hammond 300BX 800V C.T. @ 288ma. 5V C.T. @ 1.2A 5V C.T. @ 3A 6.3V C.T. @ 6A
I could use the hammond 378CX 800V C.T. @ 534ma. 6.3V C.T. @ 6A with a 185C10 5V. @ 5.0A or a 185D10 5V. @ 8.6A
Do you think that will work?

Is this a proposed build, or an amp you've already constructed? Are we talking two monoblocks, or a single stereo amp?

Each pair of KT120 will require 3.9A of 6.3V heater.
Each Poseidon board will require 0.6A of 6.3V heater (12AX7 and 12AU7).
I'd say as long as you have 5A of heater per channel, you'll be fine.
At idle, I would provide for 210mA of B+ per pair of KT120 (one channel). Expect you will need more current if the amp is driven hard.

With this in mind, either of the transformers you've suggested should work for a monoblock. Neither would provide enough heater current for a stereo amp. You didn't mention rectification, but I'd be cautious running a pair of KT120 with a single 5AR4 as the rectifier. I'd recommend either using two rectifier tubes, or go with solid state rectification.

6B4G wrote:How about using the new KT120's?
Can we just plug and play?

Sometimes it takes the less experienced to jump in where others might not...

I've been running kt120's in my poseidon amps for 6 month's now, my first tube amp build. They sound great and no magic smoke has been released The only potentially signinficant deviation from Shannon's part list is that R10,11 are 5W instead of 2W.

Power supply is an SDS cap board, and with the bias pot turned all the way down the kt120's are drawing 74mA each. They're driving 4-way speakers with ribbon tweeters, with a pas-3x preamp. (working on a budgie preamp right now!)