BUZ901P BUZ906P replacements

[Ted]

I just brought a scrap Trace AH300-7 back to life by replacing the BUZ901/BUZ906s with IRFP240 and IRFP9240. (Around £10 a set rather than £40)

The Drain and Source of these parts are reversed in relation to the original parts so i had to hack at the pcb tracks..

I had to add 0.47R balancing resistors in the drains. The higher Vt of these MOSFETs meant I had to change the bias pot from 220R to 2K2. Set the bias to 100mA and everything seems fine. Ran it at 300W into a dummy load for half an hour. Checked the bias - it had drifted up a touch. Played through it at decent volume for 2 hours. It doesn't sound bad at all.

[Steve Conner]

Cool, let us know how it gets on long term...

BTW the balancing resistors go in the source, not the drain. But the pins being reversed, it's the pin that would have been the drain before. Just saying this for the benefit of anyone who might google this thread up in future.

Another thing to watch out for is that lateral mosfets have the case/tab connected to the source, vertical mosfets have the tab connected to the drain. On a lateral FET amp you can stick all the devices straight to the heatsink without insulating washers, insulate the heatsink from the chassis and take the audio output from it.

Alex: Don't see any reason why that "upgrade" wouldn't work, though you might have to modify the range of the bias pot as Ted mentioned. The lateral FETs have a lower threshold voltage, about 1.5V if I remember right.

The main problem with MOSFET amps is that designers believed Hitachi's claims that the 2SJ50/2SK135 were short-circuit proof and needed no protection. There's no electronic component on earth that doesn't need protection from musicians.

[Ted]

BTW the balancing resistors go in the source, not the drain.

Ah yes - finger trouble.

Another thing to watch out for is that lateral mosfets have the case/tab connected to the source, vertical mosfets have the tab connected to the drain. On a lateral FET amp you can stick all the devices straight to the heatsink without insulating washers, insulate the heatsink from the chassis and take the audio output from it.

Forgot to mention the addition of mica insulators...

[J M Fahey]

I only use IRFP250, plus .1 ohm ballast resistors, with good results, but the most important thing is I use a Vbe multiplier properly mounted to the heatsink that tracks thermally very well.
Relying on high value ballast resistors yet keeping the simple "pot only" , non thermal tracking bias typical of Hitachi or Toshiba suggested designs is like lighting a pipe or cigarette while dressed in gasoline drenched clothes.

[Ted]

In the interests of tidiness - I was just discussing the use of IRFP240s and 9240son another thread. The amp mentioned above seemd fine for about a week. Then I left it switched on overnight and came back to find it the MOSFETS blown. I didn't investigate further.

[MarkusBass]

I wonder who advised you to replace lateral MOSFETs with vertical ones. Everyone knows that this will not work. This is mainly due to thermal runaway of vertical MOSFETs. You need a power amp specially designed for this type on MOSFETs. Otherwise the amp will work (maybe) 1 minute, 1 hour, or 1 day. Just the next post says that this is: "like lighting a pipe or cigarette while dressed in gasoline drenched clothes". You learned something but it is usually better (cheaper) to learn from other people mistakes :-).

Mark

[Ted]

Nobody advised me to do it, as I said, the amp was scrap, and I had the devices lying around so I tried it. It cost me nothing but my time.

Not everyone knows that it won't work, which is why I posted my experience here.

[nigelwright7557]

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

The 240/9240 tend to oscillate if the VAS is too fast.
I would tend to slow down the VAS rather than try to kill oscillation at the 240/9240.
Its vital you use source resistors on the 240/9240 to share the current better.
A Vbe multiplier is also vital.

[J M Fahey]

I forgot to add in an earlier post that while I use Switching MosFets out of sheer necessity (laterals are not available in Argentina at any price) I noticed something similar to what you say: Switchers are very fast (excellent for that use) and so very easy to oscillate to death even because of the incredibly low inductance of a couple inches straight wire !!!!!!

I have to add a "grid resistor" at *each* gate, so I cut a 10mm pigtail and wrap it around the gate leg (bent upwards 90 degrees) so the resistor body is 1 or 2 mm away from the Mos leg and solder the original gate wire to the other end of the resistor.
I glue the resistor to the Mosfet body.

In this case (heatsink or chassis mounted MosFets) I have not had instability problems even with 10 or 20 cm long wires from PCB to actual Mosfet on heatsink.

If the Mosfet is mounted straight to the PCB, using, say, an L shaped aluminum piece to carry heat away, I cut the track going to the gate, scratch solder mask if necessary and solder a small resistor there, again so that its body is only a few mm from the Mos gate pad.

Maybe overkill, but I never ever have instability problems, so it´s worth it.

And I also use a relatively large series resistor to slow it down, something around 1K .

*Of course* you also need to add a thermally tracking Vbe multiplier bias transistor, I usually clamp or epoxy it to the heatsink.

Remember that switching Mosfets (IRFPxxx and similar) have a turn on voltage usually around 3500 to 3800mV (measure them one by one and label devices with stickies) so design your Vbe multiplier accordingly.

Here´s my simple MosFet Matcher which directly gives you the bias voltage nedeeded to pass 11 mA, a sensible idle bias current

You just buy a lot of MosFets, always a few more than you actually need and match them.

MUCH cheaper anyway than trying to get hard to find laterals.

[alansom]

Buz901p/906p for irfp 240/9240

an optimal afternoon everyone

need a major Information

I have 1 table boogie m pulse asking 6 pairs of buz901p / 906p

but here in Brazil never find

I just find the IRFP240 / 9240

WOULD LIKE TO KNOW IF I CAN SUBTITLE.
I Have Been LOOKING THE BALLAST RESISTORS AND
MEET THE CAN THEY LEAVE FROM 0.22
OR BETTER BY THE REPLACE 0.47?

[Jazz P Bass]

Schematic.

mesaboogie mpulse-360-600 output sectionj.pdf

[alansom]

ok thank you
I switched and put the IFPR in place of BUZ
the sound came out perfect
I get the sound out for 30 minutes
after the relay tripped and the sound stop leaving
I looked all 7815,7915,7805
the energy passes but still not turn on the reley

[Jazz P Bass]

Check the contacts of the relay.
They may be damaged.


Here is the Protect Circuit.

mesaboogie mpulse-360-600 Protect.pdf

[alansom]

okay
Many Thanks
I realized that this amplifier is without THAT 4301

you tell me if they come without?

[Jazz P Bass]

I do not know if all of the amps came with the THAT 4301 compressor ic.
http://www.thatcorp.com/datashts/THA..._Datasheet.pdf

Maybe ask Mesa Boogie?