2sk77

[diyman]

Hello,

I'm trying to fix a Yamaha B1 amp for a friend with bombed out 2SK77s. Any substitutes possible??

ckt

[Charles Hansen]

I assume these are the output devices in the funny cases. (I don't have a schematic in front of me.) If so, there is no substitute whatsoever. These devices are completely different to any other part ever made.

[ilimzn]

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.

[jacco vermeulen]

Originally posted by Charles Hansen
I assume these are the output devices in the funny cases.

Schwarzenegger TO3, right ? If only they had read the same script.

[Leolabs]

Originally posted by ilimzn
the only way to get a replacement is out of another B-1 amp. This transistor is completely unique.

Why can't be replaced by modern Hitachi(Renesas) or Toshiba MOSFET???

[ilimzn]

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.

[dimitri]

Hey diyman,
do you have the whole schematic, not the simplified version?

[Miles Prower]

Originally posted by ilimzn
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.

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).

[ilimzn]

Originally posted by Miles Prower


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).

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.

[Miles Prower]

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.

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.