Substitute for 2SJ55/2SK175 VMOS FET`s? - diyAudio
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Old 11th December 2004, 11:55 AM   #1
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Default Substitute for 2SJ55/2SK175 VMOS FET`s?

Hello;

I have a faulty Swiss Physics Model 6 power amp for repair. This amp is equipped with 2SJ55/2SK175 VMOS devices in the output stage. I have to replace one complete O/P device set, which means 3 N-type and 3 P-type FET`s. Browsing my parts suppliers, I had to recognize that these devices are out of stock, and I wasn`t able to find a TO-3 substitute. Futheron, I strongly believe these have to be matched, do they? What about using 2SK49/2SJ135 (I know they have a lower Vmax, but as far as I see this won`t be a problem).
Anyone else who has experience with these amps? I can`t find much information in the web; I believe Model 5A (pre) and Model 6 A(power) were the only items manufactured by Mr. Mauricio del Nobile? These amps seem to be rather well-done.

Thanks;

audiomatic
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Old 11th December 2004, 11:36 PM   #2
Konrad is offline Konrad  Norway
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Uh ? those outputs is like almost impossible to break. replacing them ? but normally the fuses in the psu blows long before ure able to blow those output's. Thei also has drive limmitters. U better check drivers. AND check for chorted caps ( the blue phillips) as thei somtimes short. Output relays etc also blows after switching with signal. never never so maybee anyway.
I havent yet seen a 6A with blow'n output devices (!).
BUZ 900 series (lateral fets) from magna teck there is some complementary among them.
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Old 12th December 2004, 02:01 AM   #3
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The lower voltage devices will work if the total rail-rail voltage is less than the max of one device... rule of thumb.

They are essentially the same devices... I think the higher voltage ones are merely selected from the batches.

I agree these are difficult devices to blow - but I have seen quite a few amps with blown devices, although not the amp you mention. Don't know that one's internals.

If the devices have source resistors for each device, matching is *less* important - most amps do. The larger the resistor the more the "balancing" effect.

Contact Borbely Audio in Germany, he may have some still left around... mention that I saw your post, might help.

If you were in the USA I could likely provide the parts...

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Old 12th December 2004, 02:02 AM   #4
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All of the Hitachi MOSFETs in TO-3 packages have been out of production for years. Sometimes you can find new old stock (NOS) for silly prices (like $50 each!) A better bet are the Magnatec clones:

http://www.magnatec-uk.co.uk/mosdata.shtml

The ones you need are BUZ901 and BUZ906. These are better than the Hitachi original parts, plus much easier to find and also much cheaper (around $10 each).
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Old 12th December 2004, 11:03 AM   #5
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2SJ55/2SK175 L-mos are rated at 25% more power than the lower current rated 2SJ50/2SK135, use the lower rated part at your own risk.

Exicon anb Semelab (Magnatec-uk) have TO3 replacements.

Designers using these FETs thought they 'could take it' and just didn't use enough parts.

Inadequate gate protection when driving low impedance loads is another frequent cause of fai;ure.
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Old 12th December 2004, 01:32 PM   #6
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Mr del Nobile encountered much tragedy in his life, i believe the 5 and 6A were his second attempt for audio stardom.
The 6A was very lovingly made, one of the few at the time that used a heatpipe for cooling the outputs.

It is 2SJ50/2SK135 and 2SJ49/2SK134, as reffered in another posting.
I built a number of amplifiers based on the 50/135's, in the 80s the favorite Mosfets, both in the commercial and DIY sector.
Favorite because of their sound, and for their ruggedness.
The 55/175 i used once, for a design that needed higher ratings.
Both types used with sufficient cooling survive fuses with easy.
unless the heatpipe in the 6A collapsed, it is unlikely that the fets have been damaged first.

The 6A has a beautifull casing, an admirable build quality.
In high contrast with some other "Swiss made" that were filthy expensive, covered with "mumm is the word" , with an internal view that looked like a first attempt in DIY.
Unfortunately, like anything Swiss, too much for too little.
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Old 13th December 2004, 11:22 PM   #7
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Quote:
Originally posted by djk

Inadequate gate protection when driving low impedance loads is another frequent cause of fai;ure.

djk,

could you explain this, I don't really get it.
What really happens when these FET's are exposed to low impedance load?

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Old 13th December 2004, 11:59 PM   #8
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Think about what happens in an amp with feedback.

When the amplifier output ceases to rise with rising input we say the amplifier is clipping.

Feedback causes the gate drive to go as high as it can possibly go so that the output can follow the input.

When the gate to source voltage exceeds about 15V it perforates and shorts to the source.

An example of this would be the Hafler DH200 which can drive 200W into 2 ohms. The peak supply to the front end is ±63V, 200W/2R is 28.3V peak. If the amp is now driven too hard the gates will never see more than about 35V or so, as it goes into clipping the front end will slam the gates with the total rail voltage to try and raise the speaker output. While there is a 10V 400mW zener to protect the gates it doesn't last long when being hammered by the 10W driver transistor. After the zener goes, the gate follows.

Some big Infinity speakers got down to 2R, and I also saw this in PA/DJ situations (mainly with the DH500).
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Old 14th December 2004, 12:33 AM   #9
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Thanks djk,

I misunderstood your first post in the issue since I thought L-FET's might deal with some "strange natur" in combination with low impedance load not known to me since you didn't mention what was the real deal to protect the gate, eg. not to exceed the max Vgs which can destroy any FET necessary nothing to do really with the load.

But thanks again for explaning!

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Old 14th December 2004, 03:33 PM   #10
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Hello;

OK - thanks for all comments and your advice. I will take the 2SJ49/2SK134 as I have a small number of these in stock. If it doesn`t work I will have a look for these magnatec devices... as far as I saw Farnell sells them...

thanks again;

audiomatic
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