4x jj kt77 in jcm800-type circuit

[thetragichero]

i may be overthinking all this...
gutted a PITA jcm2000 dsl to build a jcm800-type single channel amp. I've got a beefier power supply that will put out about 600vdc unloaded, a matched quad of jj kt77 for it, and a few things I've been pondering before i heat up the iron:
1. how concerned about screen voltage should i be? will be CLC filter and i purchased some 5w 2k2 to use instead of the standard 1k/1k5 Marshall values
2. ot primary is 1.7k standard Marshall 4x el34. I've read that kt77 like a higher impedance, similar to a 6l6. should i run one number up on the secondary (so 32 16 8 instead of 16 8 4) in order to achieve a 3.4k plate load? am i thinking too much? (obviously if higher impedance is preferable a suitable transformer is the proper answer but for now I'd like to use what i have)

anyway school me. this is what happens when i have too much time away from the bench - i get to thinkin instead of doin

[martin manning]

With 600V on the plates a 3k4 primary looks reasonable; 4k might be a bit better. You'll need to get down to about 300V on the screens, though. What's the PT secondary look like? CT or no?

[thetragichero]

435-0-435 is the high voltage secondary according to the schematic from the 50s/60s so a bit higher with the 122v mains at my house.
you've got the wheels turning

[thetragichero]

is it as simple as taking one 435v tap and bridge rectifying it and taking the other and half wave rectifying it?
that would give unloaded 600v and 300v (roughly)
only thing on the 600v tap would be just the plates. 300v screens and the preamp
am i picking up what you're throwing down?

[martin manning]

That won't do it. The half wave side would be stacked on top of the FWB side like a doubler. I was thinking if you had a FWB using the outer legs and a CT (like some Marshalls), you could do a stacked reservoir and take the screen voltage from the middle.

[thetragichero]

oh so like this weird thing that had me scratching my head?

[martin manning]

Exactly.

[thetragichero]

okay I'll have my hand at redrawing it (the Marshall schematics make my brain hurt) so i have a better understanding of how it all connects when i have about 6 hours of dead time late tomorrow night and confirm what i think it means and post for confirmation
thanks for your help!

[martin manning]

But you need a 217-0-217 PT (435 across the outer legs) to get 600V plate, 300V screen with that configuration.

[sluckey]

oh so like this weird thing that had me scratching my head?

No. That schematic will have the screens working at about the same voltage as the plates. You need to take the screen supply from the PT center tap to do what martin is suggesting.

[thetragichero]

makes sense. voltage divider and all. no parallel resistors like in the big fenders?

[Stevem]

One things for certain, that running a original Marshall OT on its highest impeadance tap with its extra windings and current capabilities will help it to live longer!

[pdf64]

Parallel balancing resistors would be superfluous, as the winding CT is doing that function.

[Phil_S]

Full wave, cap input will get you about 600V on the plates. If you want to use that schematic, as noted above, you've got the wrong transformer.

[thetragichero]

thank you everyone for the replies!
some googling has lead me to some old dynacord hifii amp schematics that actually stack two rectified secondaries in top of each other for 750v plates 425v screens and then biases relatively cold (50÷)
towards the upper limits of the jj kt77 specs? you betcha. but I'ma try it gorammit
I'll still have a power supply schematic for y'all to vet in the am