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

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

Postby Krinkle on Tue Sep 13, 2016 1:11 pm

Hi everybody. I've lurked on this site for years, but I don't think I've ever posted. I usually hangout at the MA forum, if I do post online at all, I mostly just read. Anyways, here is a copy of my post over there, I'm trying to get some input on a new build of mine.

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 Merren Audio. 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? Thanks in advance for any replies.

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 :D

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
Krinkle
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Re: DIY Marshall Major Build - 2 KT88's - Screen Resistor Va

Postby Dr Tony Balls on Tue Sep 13, 2016 2:26 pm

485V is just fine on a KT88. JJs are rated for 600 on the screens!
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Re: DIY Marshall Major Build - 2 KT88's - Screen Resistor Va

Postby Krinkle on Tue Sep 13, 2016 8:21 pm

Thanks, that's good to hear.

You don't happen to know what voltage would be across the screen resistor, do you? I may just cave in and put a 25W 1k there, but I'll wait and see if anyone else chimes in first. I already have 7W Welwyn 470R's, I was hoping I would be able to safely use those, but if not, I'll put the big ugly 25W aluminum 1k in there.
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Re: DIY Marshall Major Build - 2 KT88's - Screen Resistor Va

Postby Gooch on Tue Sep 13, 2016 8:47 pm

Krinkle wrote:Thanks, that's good to hear.

You don't happen to know what voltage would be across the screen resistor, do you? I may just cave in and put a 25W 1k there, but I'll wait and see if anyone else chimes in first. I already have 7W Welwyn 470R's, I was hoping I would be able to safely use those, but if not, I'll put the big ugly 25W aluminum 1k in there.



Your 470-Ohm will be fine at that voltage. Early Majors used a 250-Ohm screen resistor. That was with GEC tubes of course, but with 600+ Volts B+.
Dying ain't much of a living.
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Re: DIY Marshall Major Build - 2 KT88's - Screen Resistor Va

Postby Krinkle on Tue Sep 13, 2016 10:45 pm

Thanks Gooch. I'll put it in and keep tabs on it for a while to see if it shows any sign of overheating. It'll be a week or so before that happens, OT is not here yet.
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