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Old 26th February 2006, 02:45 AM   #1
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Question 2sk77

Hello,

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

ckt
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Old 26th February 2006, 07:54 AM   #2
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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.
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Old 26th February 2006, 09:53 AM   #3
ilimzn is offline ilimzn  Croatia
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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.
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Old 26th February 2006, 11:56 AM   #4
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Quote:
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.
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Old 26th February 2006, 02:52 PM   #5
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Quote:
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???
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Old 26th February 2006, 07:42 PM   #6
ilimzn is offline ilimzn  Croatia
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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.
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Old 26th February 2006, 08:45 PM   #7
dimitri is offline dimitri  United States
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Hey diyman,
do you have the whole schematic, not the simplified version?
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Old 26th February 2006, 09:16 PM   #8
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Quote:
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).
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Old 26th February 2006, 11:21 PM   #9
ilimzn is offline ilimzn  Croatia
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Quote:
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.
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Old 27th February 2006, 01:15 AM   #10
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Quote:
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.
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