[R-390] OT: Pass Transistor Question

[Bob Camp]

Hi

Power mosfets are often recommended because they "equalize" the power  
load. More heat = less current. The problem is that this only occurs  
above a specific current level. Below that current level (actually a  
Vg level) then work the other way around.  You need to size the parts  
carefully to get the balancing to work.

Bob


On Oct 17, 2008, at 7:32 PM, Ian Gallimore wrote:

> Has anyone had experience using power mosfets as pass elements? From  
> what I've read, equalizing resistors are not needed, but I wonder,  
> being REALLY CONSERVATIVE, if a small resistor, say .1 ohm, in each  
> source lead might be a good idea. I have a plan germinating to re-do  
> a big old GRC PSU putting out 28V at 50 A as a variable voltage PSU  
> using a multiturn pot, a three-terminal Voltage regulator and mANY  
> parallelled power fets as an improvement on an SCR pass element.
>
> Ian VA3ODA
> ----- Original Message ----- From: "ews265" <ews265 at rochester.rr.com>
> To: <r-390 at mailman.qth.net>
> Sent: Friday, October 17, 2008 7:00 PM
> Subject: Re: [R-390] OT: Pass Transistor Question
>
>
>> Don,
>>
>> Thanks for the info on the MJ15003s.  I've already purchased a  
>> handful or 2N3773s since they were cheap and available so I'll give  
>> them a shot first.  They are Brand X by the way.  Could be one of  
>> the thousands of processes Motorala must of spun off to the 2nd  
>> line fab houses.  If I end up with flight at/near current limit I  
>> won't be too concerned since I don't plan to run the amp anywhere  
>> near there anyway.  And yes I am the curious type.  Sounds like  
>> this could turn into an interesting exercise.
>>
>> So how did Crown Audio get the kind of recognition they received?   
>> Given the parts available at the time, dealing with basic design  
>> problems is one thing.  The packaging  however is such a  
>> disappointment.  I think I could have done better in my garage.
>>
>> Regards,
>> Jon  WA3MVM
>>
>>
>>
>> 2002tii wrote:
>>> Jon wrote:
>>>
>>>> This is an old beast that I just happened upon that has one bad  
>>>> channel. Right now I'm resisting working on it just for the  
>>>> mechanical reasons you describe.  Sounds like I don't need to  
>>>> worry about matched/selected parts and your comments do explain  
>>>> the parasitic suppression components. Also sounds like the older  
>>>> transistors may have had a lot of phase shift at their upper  
>>>> limits with the potential for creating havoc with the amp's  
>>>> feedback loop.
>>>>
>>>> The mechanical packaging really is pretty bad.  I was  
>>>> unpleasantly surprised.  Documentation's nothing to write home  
>>>> about either.  Digging in to the docs still leaves ???  Anyway  
>>>> I'll be sure to sweep the output for any signs of life after I  
>>>> get it back together.  Any special conditions that's apt to make  
>>>> it go into flight; level, freq etc?
>>>
>>> In all honesty, unless you are just obstinately curious, if you  
>>> haven't put the effort into it yet -- don't bother.  After  
>>> hassling with the worst mechanical design you've ever experienced  
>>> in an electronic product, you will have a thoroughly mediocre  
>>> amplifier (at best).  Sell it on eBay and buy an Ashly  
>>> FET-200/2000 or a Bryston 4B.
>>>
>>> If you insist on repairing it, use MJ15003 output transistors  
>>> (real Motorola ones) instead of the original 2N5631s.  If the  
>>> TO-66 drivers are broken, good luck -- very few manufacturers make  
>>> TO-66 packages these days.  You can make a TO-220 transistor fit,  
>>> but being plastic-cased transistors, they don't last.  Whatever  
>>> you do (including using stock parts), be prepared to adjust the  
>>> compensation to maintain stability. (Note that Crown provides two  
>>> schematics, one for each brand of output transistors they used.)
>>>
>>> Run it up to clipping without a load, with an 8 ohm load, and with  
>>> a 4 ohm load, using any convenient sine wave in the 500 Hz to 2  
>>> kHz range. Look for bursts of oscillation as you do this (it will  
>>> appear at the same spot each cycle -- check particularly as the  
>>> amp enters and leaves clipping).  Don't push it very far into  
>>> clipping without a load.
>>>
>>> Do not try to reach full power above 10 kHz unless you are willing  
>>> to fix it again.  I generally ask power amplifiers to drive 2 kHz  
>>> square waves into an 8 ohm load resistor paralleled by a variety  
>>> of capacitors from 0.001 uF to 1 uF, but Crowns will not tolerate  
>>> this, either.
>>>
>>> Good luck!
>>>
>>> Don
>>>
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