Harthke 7000 mosfet amp schematic need

[MarkusBass]

What WERE the stock output MOSFETs in those, I sure don't remember?

Since the amp has no thermal tracking it must have been using lateral MOSFETs, for example 2SK135/2SJ50. And I don't see a reason why such a power amp would be prone to failure. I think that the main reason to replace the output transistors with bipolar ones is that 2SK135 is not manufacured any more. But it is still avaliable on ebay, or can be easily replaced with any lateral MOSFETs available here: Lateral Mosfet
And sending IFR... by Samson service was just a mistake - this transistor cannot work in this amp without modification of the amp.

Mark

[Enzo]

As far as I know the 2SJ50 pair are in TO3, the Hartke amp has two rows of TO247.

I have been a Samson service center close to 30 years, and that MOSFET amp definitely was unreliable, and they do recommend replacing any that fail with the bipolar amp board.

[J M Fahey]

Well, they *did* go up in flames, and we are talking about many amps straight out of the factory, not an odd one which was improperly repaired in a small shop.
Maybe the originals or some prototype did indeed use 2SK/2SJ and sometime later were replaced by IRF or they were like that straight from the beginning.
And they did send IRFP240 after all.
Parts handling at a Factory is not done by an Engineer or Tech but by a clerk who gets an order, searches it in the database, goes to the proper box in the parts storage room and grabs what's inside it.
He has no clue as to part type or specs, so if he sent IRF240 is because that's what they have in that box.
FWIW I have seen other amps improperly using IRF, most notably the bigger Dean Markleys ... but they were usually underbiased, so they survived ... although they made terrible bedroom amps

[MarkusBass]

So Hartke was not aware at that time that vertical MOSFETs cannot be used in the circuit they designed. Similar power amps were build in eighties by Marshall and TraceElliot and the amps worked very good. But they used lateral MOSFETs: either 2SK135, or BUZ901. I think that BUZ901 was available in TO274.

Mark

[Enzo]

The 2SK134/2SJ49 and similar pairs of TO3 outputs worked great. I loved the effortless sound of the Trace Elliot amps of that era. I was not a fan of the Marshall, but they worked. Then those TO3s became unavailable. Companies scrambled to make something similar.

Various folks have made MOSFET amps with dubious results. I have a Crate BX100 next to me with IRFP140/9140 outputs. The SVT3Pro uses the IRFP240/9240.

Those TO3 types were going for $35 each 20 years ago, if you could find someone selling them.

I have a four channel power amp here in the shop. Soundcraftsmen I think, it uses those TO3s as outputs in each channel. I won't be fixing the amp, but I am saving it for those parts. Each one represents a repair i can do that would otherwise require me fudging something different into the circuit. That amp carcass is worth hundreds of dollars to me.