DIY Marshall Major Build - 2 KT88's - Screen Resistor Value/Voltage Questions

[Krinkle]

There are a few guys out there who have built "Mini Majors", mAx being one of them that you may be familiar with. I have my own build started and I'm trying to make sure it doesn't experience the "Smoke on the Water" effect, as Dawk calls it.

I have a killer OT coming from M****N, who may be persona non grata here, not sure exactly what the story is. It is a copy of the original OT when it comes to materials and build style but with the power and primary impedance matched for 2 KT88's instead of 4, and the UL taps at the correct percentage as the original (33%), as opposed to the Mercury Magnetics OT (12%) some people have used for their mini major (not the 200W MM replacement). Also, from what he tells me he has determined how to build it so it will not fail. I am assuming he means as long as I don't do anything stupid.

Which brings me here;

I will be using a Metroamp laydown PT that I pulled from my Metroamp build. This will put my B+ at about 485VDC. From poking around the net I see a bunch of info/opinions. I'd like to determine the best possible value for the screen resistors, not just default and put one as big as a truck in there. So I'm asking for some help in figuring this out.

If my plate/B+ voltage is at 485VDC, will there be times when the screen voltage is very close in value. If so, is this too high for modern KT88's (I have a matched set of JJ's)?

Does anybody here know the application and math well enough to just give me a simple answer?

I'll leave it at that for now as I've already been too long winded. Here are some shots of the build so far for anybody who scrolled down this far

http://s793.photobucket.com/user/Darrin ... sort=3&o=3

http://s793.photobucket.com/user/Darrin ... sort=3&o=2

http://s793.photobucket.com/user/Darrin ... sort=3&o=1

http://s793.photobucket.com/user/Darrin ... sort=3&o=0

[Roe]

screens and plates should be around 600v. originals used 250R screen resistors connected to the ultralinear taps. Many would prefer 470R for safety reasons. I recently tried an original major running with 470s and it worked just fine

[Krinkle]

My PT will give me about 485VDC maximum so I'm going to have to live with that. I'm not looking to get max power out of this amp, just trying to use 2 KT88's and take what I get.

What is the wattage of your 470R resistors?

[Roe]

5w should suffice

[Krinkle]

5w should suffice

Is that what you're using?

[Roe]

no I don't have a major

[Krinkle]

Sorry Roe, I should have been more clear. Do you know the wattage of the 470R resistor in the Major that you tried?

[Roe]

probably 5w. At 485v 5w should be enough anyway