It is nice to see used gear moving in this horrible "market".
I am sure the new owner will get years of quality service and sound from the amp.
Do you have plans to replace it with something different...?
Best

You are too worried about power up. Unless you have done something REAL STUPID, the worst that will happen is you will blow a fuse. I seriously doubt this applies to you.
Enter the bulb limiter. It keeps your fuse from blowing so you can look for problems without repeatedly replacing a 2 dollar fuse. They are easy to build, as in the link below.
Best
http://tubenewbie.com/light_bulb_current_limiter.html

I would not use a Variac to power up your build.
If you are worried/cautious, use a light bulb limiter.
Best

Hiss always gets worse with more presence.
Can you get by with just a set resistor for negative feedback.?
If not, you may want to try a 2 Watt Pot made from Conductive Plastic instead of the Two Dollar Carbon Comp pots that we use most of the time.
But first try all the easy stuff like lead dress and preamp tubes.
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DO NOT use the Ground Switch.
Do not use the layout on this forum. Grab the layout from The Ceriatone web-site. That ground switch is old, left-over technology. Keep in mind, HAD started out by moddding Fender Twins/Fender Amps and that was just part of the game back then. Do not wire the ground switch. AND.....a "Ground Lift" is NEVER the mains ground. It is signal ground lift between 2 daisy chained amps.
if you are "star grounding" you may want to consider TWO. One for the power supply/output, and one for the preamp.
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It would have been hard to pick a more difficult "first" build. Dumble was/is a very esoteric individual. His amps morphed and varied a lot over the years.
2nd, 3rd and 4th generation layouts. Skyliner Preamp is a big deal for some guys. The list goes on and on. I have a few different schematics and layouts I could send you, but I am not sure if it would not just serve to confuse matters even more. If I were you, I would just build to Ceriatone's print, do the best/cleanest job you can, and get help from the guys on this forum that have made these (Ceriatone) amps their personal hobby.
Good Luck

Who built the amp.?

Nice Job!
Thanks for taking the time to document all of this.
I wonder where Nik gets those 600 Volt caps from.?

I also date my cap jobs. Good for everybody concerned. 27 years from now, when the new owner of that amp takes it to a tech, the guy will be appreciative for that info......
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Well Done Lad.....
We await your progress.

Wow!!!!!
6xEL34......!
Is this the amp with multi colored knobs on the front panel.?
This kind of job is pretty big, and not my forte by a long shot. But I do not see why it cannot be done. I am sure you will have some set-backs along the way. Only thing that comes to my mind at the moment is filtering. Does the L120 have some pretty big plate voltages.? If so, make sure your caps are plenty robust.
Do you know anything about those Partridge trannies.? Were they a special deal for this amp, or were they a common 100 watt item for both PT and OT.
Good Luck with this my friend. Take lots of pictures.......

A lot of tubes will "fit the socket", but that does not mean you can plug them in and play. Most tube changes will require component/wiring change to the bias circuit and/or tube sockets.
What is it that you do not like about EL34's.? They are on just about every major recording of a Plexi since the late 1960's. Are your power tubes getting tired/old.? have you tried a few different preamp tubes.?
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http://homeoftone.co.uk/
Isn't that the UK dealer that advertises on this forum.?

30 and 60 ?

Is your PT tight to the chassis.?
518 on the plates is really getting up there. It is possible your tubes do not like that much voltage. What tubes are you using and what numbers are you seeing for the current in your power tubes.?
Best