BUZ901P BUZ906P replacements

[Alex R]

Hi there helpful people.

The older Trace Elliot mosfet output stage often uses BUZ901P and BUZ906P. Two of each, They are expensive. Can anyone recommend a cheaper alternative?

IRFP250?

Please feel free to eddicate me on the characteristics and use of power mosfet devices should you so wish as I am quite ignorant and thus very hard to patronise.

- with thanks in advance (and in retrospect)

[Steve Conner]

Hi Alex

The BUZ901/906 are a complementary pair of lateral MOSFETs (200V, 8A, 125W) designed specially for linear use. They're worlds better than trying to make vertical MOSFETs like the IRFP250, designed for switching power supplies, work in a job they're not meant for.

You might be able to make them work, but you'd need to add ballast resistors to keep them thermally stable. (Ampeg used 0.47 ohm and their MOSFET amps still seem to end up in the shop a lot.) You also have the issue of finding a complement: you don't get many P-channel vertical FETs. This thread on diyaudio.com:

http://www.diyaudio.com/forums/showt...threadid=20646

recommends IRFP240 and IRFP9240.

The only alternatives I can think of to the Magnatec devices are the Exicon ECF10N20 and ECF10P20, and I bet they're no cheaper.

Can you guess why Trace redesigned their output stages with BJTs? Usually when a MOSFET Trace blows up, you just replace the whole power board with a newer "Bipolar bear". I don't know how the price compares with four MOSFETs at £10 each.

[MarkusBass]

You could consider 2SJ162 and 2SK1058 lateral MOSFETs. They have slightly worse parameters (160V 7A 100W) and a different package - TO-3P. I used them (2 pairs) in 200W bass amp. And the price is low - about 2 Pounds. Which Trace Elliot are talking about?

Mark

[Alex R]

Thanks Steve, that helps my understanding.

We're talking about a GP12 but these mosfet power stages appear in various Trace bass amps, the boards are labelled 150/300 watt bass.

So the witty-named Bipolar Bear boards came later? Hadn't cottoned on to that. They run HOT!

[MarkusBass]

Alex,

There were several versions of GP12.
In "GP12 250W SMX series" the output transistors specified on the schematic are: 2SK135, or 2SK1058, or BUZ906 and 2SJ50, or 2SJ162, or BUZ901. As you can see 2SK1058 and 2SJ162 appear on the list. And two pairs of 2SK1058/2SJ162 can deliver 250W to the output.
However, the schematic of "GP12 300W 12 band series" lists only BUZ... transistors. I assume you are talking about 300W. What is the power supply voltage it this case?

Mark

[drewl]

I went through this while repairing some Tubeworks amps recently.
the sk1058's and j162 are fairly cheap and available.
I tried to get the IRFP240, 250 9240 etc. to work because we have hundreds at work (for switching power supplies!) but they had nasty crossover distortion unless the ballast resistors were changed as stated above.

[Mars Amp Repair]

I know this thread is a bit old, but I also found that the mosfets that marshall uses in the mosfet heads worked well a trace bass rack head...I can't recall the model number right now.
I used the 2SJ50 & the 2SK35. I also added 0.33ohm ballast resistors as well as a couple of 22pf disc caps across the source drain & everything worked fine.
Without the ballast resistors the mosfets did not current share well. Also I had a weird oscillation on top of the waveform without the 22pf caps to snub it.
glen

[Mars Amp Repair]

oops, I meand 2SK135.

[Alex R]

Lots of helpful info in a small space here. I have the thread bookmarked - thanks to all of you.

[Alex R]

OK then, further to this discussion about switching mosfets vs types designed for audio - the Ampeg SVTIII Pro uses IRFP switching mosfets, with 0.45 ohm ballast resistors. Would it be a useful upgrade to fit the linear types instead?

I have one in at the moment that has had just one mosfet replaced in the past (I would of course not do this) , and as it does not match the others it draws too much current for me to be able to settle the amp down - it's either crossover distortion or way too much current through that one device.

I understand there should average 20-25mV across the resistors.

[Mars Amp Repair]

I doubt it would be feasible or really even possible to change to linear devices as the entire power stage has been designed about the Mosfet xistors.
you just have to order the devices from Loud & they'll send the ones rated for that amp. usually an 'orange' or 'blue' dot indicates the ranking.

I do find once you get the bugs worked out, they are relatively reliable amps. glen

[MarkusBass]

you just have to order the devices from Loud & they'll send the ones rated for that amp. usually an 'orange' or 'blue' dot indicates the ranking.

Of course, you can also do the MOSFET matching yourself. Search google for the "MOSFET matching" phrase. May be cheaper than from Loud Tech .

Mark

[guitarstrummer36d]

Hi, this is my first post so please bear with me! I'm interested to find out if anyone has had trouble with the thermal switch on the heatsink of the Trace Elliot GP12. I can't find a spec for it, all I know is it measures 0.4 ohms and it doesn't do anything if you warm it up. I have only metered it switched off as the amp has 4 very dead mosfets ( shorted out cracked cases, accompanied by blackened 5 amp fuses.

Thanks for any help you can give

[Alex R]

If you take it off and clean up the face that bolts to the heatsink you should find the switching temperature engraved there. The part may be listed as a thermostat switch. It's fairly standard, just measure the distance between the bolt hole centres. The temperature isn't too crucial either so long as it's in the ballpark, as generally the mosfets blow ages before the heatsink gets hot enough to switch the thermostat off! I wouldn't go below the rated temperature though as have known these turn themselves off at hot gigs.

It might be better to start a new thread on the forum if this doesn't answer your question.

[oc disorder]

Hi guitarstrummer36d

There are a few "GP12's" but this one seems to have a 100deg C switch.
The others use a relay and so far haven't spotted a thermal switch,
GP12 250Watt SMX series.pdf
Trace Elliot Schematics
I found British Audio Service to be very helpful in obtaining parts for Trace Elliot and no I don't have shares in them !
Some of the "newer?" switches have the temperature stamped into the top terminal so if it has been soldered to it takes a bit of effort to clean off the solder to make it legible.Also have seen it stamped around the edge.

Like fuses , sadly sometimes they are the last to go open circuit