Denon amp 2SK851 substitute

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#3
edit: 2SK856 is a recognized sub.
Generally, I think IRFP240 is preferred for audio power applications. IRFP250 and IRFP250N are widely sold as a cheap, generic part by platform sellers but I notice that alleged original Toshiba 2SK851 parts are widely available anyway. If you risk buying fakes regardless, buy those alleged original spec. Toshiba types with a Painted "T" logo and marks at least. Laser etched marks alone, are most likely an indication of fakes.
 
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#4
Thanks!
Did a search for 2SK856 in Mouser, Farnell, Digikey, Elfa Distrelec etc but no hit.
I am asking for advice because I do not want to risk getting fakes.
I did find some places that have the 2SK851, but I dont know if they are reliable sources.
Any suggestion to shops? or other subs?
 
#5
You won't find obsolete semis or other components at new parts distributors like those you checked. These semis are all old, obsolete parts. Inchange in China actually offer genuine spec. copies and many other obsolete semis but only in commercial quantities.

There are a couple of Ebay sellers but don't expect cheap prices for "pulls" or genuine original parts removed from scrapped amplifiers. Here's the lowest price I saw at a glance : https://www.ebay.co.uk/itm/202435237416?epid=1528680162&hash=item2f22149628:g:58gAAOxy-W9SQqK3

Take look at Ebay UK for yourself - they sure are expensive offers but if you want to minimise risks, that's often the cost, wherever you shop for anything obsolete or considered to be rare and nobody seriously values used parts higher than those guys.
 
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#6
Having taken a closer look at 2SK856, I don't think that's going to be a wise choice anyway, given the low Vds rating of only 60V (apologies for what must have been inappropriate advice) So, I would replace all 851 semis in the set with new, genuine IRFP240 and readjust the bias current to original settings but note; that may require some expertise from a repair tech.

So I suggest that you don't take this course alone, unless you are confident with audio repairs and already have the proper instruments and tools.
 
#8
Thank you for replying!
Yes I saw these ads from UK. Quite expensive but at least safe.
I always use "alltransistors" to see if I might have some subs laying about. No luck this time.
TO-3 and TO247 aren't all that different, so it made the possibilities a lot bigger. Hence IRFP240.
Think I'll just try that.

Again; thank you!
Jørgen.
 
#9
According to Bob Cordell, the ideal audio mosfet (i.e. for linear operation) should have a relatively high Rds compared to an efficient switching type for power supplies etc. and the vast majority of power mosfet applications where ideally, you would want the lowest Rds possible. The higher internal resistance apparently provides a somewhat softer switching action with less spurious artefacts than hard switching types, at the cost of some extra heat dissipation. That's likely why IRFP240 is almost universal in Nelson Pass's DIY suggestions for audio output power too.

However, I've only repaired a few amplifiers with blown output mosfets and never tested at full power for long so I can't claim to have proven this will work 100% safe for your amplifier and that's why I suggest you get experienced help rather than risk further damage. Another reason is that we don't know the cause or mode of failure here so the original problem may still be there, waiting to blow the replacement parts too. This may only occur when your amp. is used near its maximum rating but I find that most amplifiers never see more than actual 5-10W peak in daily use anyway, so the issue may not be apparent.
 
#10
This amp have taken a severe beating at a party, I suspect. Both transformers had open thermal fuses. They are replaced.
There must be a fault somewhere else in the circuit also, as the mosfets blew immediately after replacement. I put this back on the shelf some years ago due to lack of equipment and experience. Got some more years on my back plus proper gear to do further tests now. Since it is my amp I have time to do a proper job and learn more.
Thanks!
 
#13
korpberget said:
This amp have taken a severe beating at a party, I suspect. Both transformers had open thermal fuses. They are replaced.
There must be a fault somewhere else in the circuit also, as the mosfets blew immediately after replacement. I put this back on the shelf some years ago due to lack of equipment and experience. Got some more years on my back plus proper gear to do further tests now. Since it is my amp I have time to do a proper job and learn more.
Thanks!
Click to expand...
Do you have a dim bulb limiter? Saves nerves and semis...
 
#18
Hi.
It's probably a little late, but I had the same issue on a pma-2000 ( first version )
I ended up using TK30J25D. Please note these have much lower Vgs threshold voltage.
You need to change the bias circuit. I changed the 5K6 resistor to 2k7. Now with the 4k7 trimmer
turned to minimum resistance, I can regulate Vgs from ~1.1 volts to ~ 3.6 volts in mine.

Bias.png


Other things to note.
These mosfets have much lower RdsOn than 2SK851 and you need a source resistor on both output transistors
to help stabilize bias current and avoid termal runaway. The gate resistors should probably be 100 ohms.

I took the inspiration from the pma-1520AE

/Jørgen
 

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#19
Hi,
first time for me here!
I'm probably in late but I have a Denon PMA-1500RII with a blown channel too. I bought some 2SK851 designed as "generic" so even if they have a "TOSHIBA" mark on I suppose they are clones (fake...).
I will probably try these but even if the replacement will goes well I know that I won't be happy until I will not try the amp with genuine mosfet.
Did you were able to find a good repalcement? Did you try the TK70J20D?
Thanks!
 
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