2sk77

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#3
Yes, it's the (in)famous Yamaha-only VFET.
You should, however, be sure you are testing it properly, as a WORKING VFET measures a near SHORT across D-S, and a diode from either D or S to G. If it is open, then it's dead 🙁
The VFET is a depletion device and is normally ON, it is essentially a power JFET.
If it indeed is dead, the only way to get a replacement is out of another B-1 amp. This transistor is completely unique.
 
#6
Because it operates in exactly the oposite way. Practically all MOSFETs used in amps are enhancement mode, i.e. they need a control signal to turn them ON. VFETs are depletion mode (like JFETs, and tubes) and need a signal to turn them OFF. Replacing a VFET with a MOSFET (unless it was a depeltion mode MOSFET, but these are rare and currently not made for the required voltages and currents) would require a different amp topology, so essentially, you would get a different amp. You could hardly call that a replacement, more a redesign.
 
#8
ilimzn said:
Yes, it's the (in)famous Yamaha-only VFET.
You should, however, be sure you are testing it properly, as a WORKING VFET measures a near SHORT across D-S, and a diode from either D or S to G. If it is open, then it's dead 🙁
The VFET is a depletion device and is normally ON, it is essentially a power JFET.
If it indeed is dead, the only way to get a replacement is out of another B-1 amp. This transistor is completely unique.
Click to expand...

What're you talking about?! Every VFET I've ever seen has been an enhancement device that does not turn on until supplied with its specific V(gson) (usually 2 -- 3Vdc).
 
#9
Miles Prower said:


What're you talking about?! Every VFET I've ever seen has been an enhancement device that does not turn on until supplied with its specific V(gson) (usually 2 -- 3Vdc).
Click to expand...

You should alow for the possibility that it exists even if you have not seen it 🙂

VFET is really a misnomer. What you are talking about is a VMOSFET, VMOS for short, unfortunately, many abbreviate this as VFET, not knowing that before there were VMOS devices, several manufacturers experimented with, and then manufactured vertical JFETs, naming them VFET. Like all JFETs, these are depletion devices. Jamaha, Sony and NEC manufactured them starting in 1975, and ending with the last Sony produced units used in their class D amp, in late 1984. For more, see here:

http://www.thevintageknob.org/VFET/VFET-main.html#

I would bet these days many would love for such devices to be available again (Look at Nelson Pass' latest offering, in fact). With current manufacturing technology, they would be practically as difficult to make as current VMOS.
 
#10
VFET is really a misnomer. What you are talking about is a VMOSFET, VMOS for short, unfortunately, many abbreviate this as VFET, not knowing that before there were VMOS devices, several manufacturers experimented with, and then manufactured vertical JFETs, naming them VFET. Like all JFETs, these are depletion devices. Jamaha, Sony and NEC manufactured them starting in 1975, and ending with the last Sony produced units used in their class D amp, in late 1984.
Click to expand...

OK, always heard those things referred to as Static Induction Transistors. Low g(m), small amp factors: no wonder that VMOS knocked 'em off. Too bad: SITs really are better analog devices than switches.
 
#11
Well, the gm is not THAT low, comparable or even slightly higher than their competitor that eventually drove them out of the market, LMOS, i.e. laterals. The main reason was the manufacturing process for laterals is is very similar to what was normally used for most semiconductors at the time, whereas the VFET process was quite different, and MUCH more expensive.
VFET, however, have a very low Cgs compared to modern MOSFETs, about 150pF for the 10A/250V Sony device used in their class D amp.
VFET are true power JFETs, SITs in general have no PN junction between the channel and the gate but use a metal interface. That being said the terminology becomes somewhat blurred as different topologies come about.
 
#16
Vfet are rare today, but 2SK77 is unique, bigger, and very very rare Vfet.

You can try to substitute it only with the TOKIN/NEC 2SK180 I think, but this last is also very rare.

YAMAHA B1 schematic from my site 😀: http://www.amplimos.it/images/Yamaha%20B1.jpg

Francesco
 

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