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Old 11th December 2012, 04:14 PM   #651
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Originally Posted by 6L6 View Post
Gentlemen,

I am helping a forum member with his F5T -- I will save you the details and ask;

Would there be any problem building a v3 with ~ 42v rails and only 2 pair of output transistors?

Thank you,

-Jim
it will only be a high voltage V2. with output of 55W class A at 8ohm.
max safe bias will be about 1.9A. that will give you dissipation of 160W over 2 pairs. so 40W pr fet.
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Old 11th December 2012, 04:21 PM   #652
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When I mean v3 it's because of using the cascode input stage to keep from zapping the jfets.
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Old 11th December 2012, 04:22 PM   #653
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yes. cascoding will be a must
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Old 11th December 2012, 04:46 PM   #654
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unwinding few turns form Donut isn't a solution ?
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Old 11th December 2012, 04:47 PM   #655
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Or maybe put a Zen-style regulator in the PSU, referenced to 32v, There is at least a 4v insertion loss, I think... so would have to burn up 6-7v at ?200w?... That would take too many transistors. Nevermind.

Last edited by 6L6; 11th December 2012 at 04:53 PM.
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Old 11th December 2012, 04:49 PM   #656
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me no likee

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Old 11th December 2012, 06:45 PM   #657
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Quote:
Originally Posted by 6L6 View Post
Or maybe put a Zen-style regulator in the PSU, referenced to 32v, There is at least a 4v insertion loss, I think... so would have to burn up 6-7v at ?200w?... That would take too many transistors. Nevermind.
Or use a LCRC Psu to lower voltage... maybe too much voltage drop
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Old 11th December 2012, 07:25 PM   #658
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If each Fet can handle 12 amps full load, then derating to 60% for longevity gives you a max of 7.2 amps/FET. That's and additional 1.2 amps x 4 fets *the other 4 are off*. Which is 4.8 amps or an extra 240 VA.

70% is 8.4 amps/FET => 2.4 * 4 = 9.6 amps or an extra 480 VA. That is a 1080 VA transformer.

Derating to 80 % gives your 9.6 amps/FET, or an extra 4 x 3.6 = 14.4 amps or 720 VA for a total of 1320 VA.

Looking at this backwards, if you have a 1500 va transformer, you've got 900 VA above 150 watt class A. 900 / 4 = 225 VA /FET = 4.5 amps. Which means that each FET can push 10.5 amps steady state, which only derates to 10.5/12 = 87.5 percent.

I'm not so sure this is a good idea. It is looking like a 1KVA transformer is about the max you want to supply to this sort of monoblock.
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Old 11th December 2012, 08:13 PM   #659
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If i had that high a rail voltage, I would b looking at more fets, not less. I would be shooting for fet temp in 35-40C max before heatsink, just my 2C. You never know with all these crazy DIY folks
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Old 11th December 2012, 09:22 PM   #660
BigE is offline BigE  Canada
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I'm struggling with this. Counting is hard....

If each Fet can handle 12 amps full load, then derating to 60% for longevity gives you a max of 7.2 amps/FET.

600 VA is used for 150 watts class A into 8R0. That's 6 amps of pure class A power. That is 1.5 amps over each FET, for a total of 2.25 amps/FET

You've got 5 amps/fet left to go, or 20a amps total. That makes for 1600 KVA total transformer size.

Is that about right?
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