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Old 18th November 2013, 09:13 PM   #1
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Default 400a MJ15003/4

With immense (and sometimes intense) help from this forum, back in 2006 I repaired and refurbed a very early 400a amplifier (threshold). At the time it was recommended to me (not by Mr. Pass) that I go with MJ15003/4 power mosfets. Which I did.. and its played for several years with no issue at all.. in fact, many hours until about a year ago..

Recently one channel hiccuped and I think I have nailed it down to a poor solder joint or possibly bias drifting due to old pots (kept blowing a fuse).. I have it up on a variac now on a puny 3amp fast blow fuse with no issues and decided to add a current limiter per Mr. Pass way back when.. Which I am hopeful will allow me to bring it up without the aid of a variac along with adjusting the bias.

My question though is this.. I have not seen anyone use these particular mosfets but I have seen people who use the MJ series say they are able to pump out more power.. I dont care about power, I care about sound and mostly what the 400a lacks.. an airy characteristic.. so my question (finally) is this: can or more importantly SHOULD I exceed the bias settings recommended? Right now its biased to 45c (or the 5 second touch rule)... would I be wise to go higher? That series per the specs says max operating is a whopping 150c if I recall correctly.. that seems rather insane to me. Maybe its 150f.. I will go have an other look! Perhaps thats just pre Chernobyl capabilities.. kinda like a dragster

The only other upgrade I did was to the two main caps which I went with 75v 33,000 mfd caps.. Other than that all resistors, and caps were just replaces as is.. and the other transistors were left alone (actually, they were removed, the ends of the leads cleaned and resoldered).

IF you guys recommend I go higher, my next logical question will be what to fuse it at.. Although I think with the currently limiter (CL60) I think I could leave the 5 amp fuse in perhaps.

Other than that, I still have fond memories of this repair/rebuild.. what a great learning experience!

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Old 18th November 2013, 09:26 PM   #2
Zen Mod is offline Zen Mod  Serbia
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Location: ancient Batsch , behind Iron Curtain
do not go above 45C
my Papa is smarter than your Nelson !
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Old 18th November 2013, 10:47 PM   #3
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Thanks.. I will leave it as is.
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Old 19th November 2013, 11:29 AM   #4
AndrewT is offline AndrewT  Scotland
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the 250W rating of the MJ15003/4 is available when Tc is 25C
If you raise Tc then the power rating of the BJT transistor comes down.
By the time the Tc has risen to 175C the power rating is Zero Watts.

If the sinks are at 45C (Ts) then Tc is likely to be around 55C to 65C.
At Tc=65C, the power rating of the MJ15003/4 has reduced to [175-65]/[175-25]*Pmax = 183W
The 400a fitted with MJ15003/4 can easily manage with a Ts @ ~45C

What is the worst case operating temperature of the heatsink?
regards Andrew T.
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Old 26th November 2013, 01:57 AM   #5
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The MJ15003/4 are not MOSFETs but good old school bipolar transistors. The NPN MJ15003s are perfect, used in Class A amplifier circuits, due to their robustness and very good sound quality. At the elevated temperatures of Class A operations (case temp 80 deg), maximum dissipation is restricted to 45W per transistor with plenty of heatsinking. I can't speak for the MJ15004 as I have not used it, maybe in future in a PNP configured Class A amp, but as it is, the NPN will keep me busy enough in any case.
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Old 26th November 2013, 03:22 AM   #6
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The original devices are 4 MHz Ft figure, but the MJ15003,4 are about 1 MHz.

They will work OK, but you can get the beefy 4 MHz devices from the later
offerings by Motorola / On Semi.

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