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VFET revisited

so, I assume the original VFETs did not employ Metal Oxide Semiconductor technology, as in modern MOSFETs, correct?
 
It seems by the very nature of the device that failure would be inevitable. Probably why so few VFET's exist today.
 
Sorry, didn't get back sooner gpdavis2.

Echo had a nice post, but you were talking Sony? I don't know about the other output modules they use, but the 2SJ18 is a pMOS, not a pFET device.

Regardless, Spectral adverts their amp as VFET devices, which is what my original post was about. Hafler also uses VFETs (or VMOSFETs, not important enough for me to actually look, but I would assume that the large advantages MOSFETs have would lead to their use).

The real question is can they be substituted for, and I don't believe so.
 
The real question is can they be substituted for, and I don't believe so.
Click to expand...
That is correct...one is depletion-mode, the other is enhancement-mode...no substitutions allowed.:no:
 
Originally posted by DanTana
It seems by the very nature of the device that failure would be inevitable. Probably why so few VFET's exist today.
Click to expand...

Nothing I've read or heard from anyone leads me to believe that VFETs are/were unreliable. In fact, I've read/heard just the opposite. The reason they are so few is that they were only used in the TOTL products and then only for about 3-4 years some 25 years ago. And folks tend to hang on to them once they have.

VFETs had the bad misfortune of coming into the market-place at the same time as the MOS technology started coming to the fore. MOS simply swamped the alternative technology as it was used up and down range of semiconductor applications (from the smallest to the largest). So am guessing VFETs simply were overwhelmed by economics.
 
Well, I'm definitely no electrical engineer, just sounded by the description with the very thin wafer/silicone areas and the expansion/contraction effect that they were doomed to eventual failure. I'm sure this would take years. The overall designed sounds great, tube like operation, I'm all for that, I would of loved to see them bring back VFET technology, but I guess that will never happen, except maybe when the patent expires and it becomes free to use. Anytime it seems Sony designs something, you have to pay them through the nose for the rights, look at Betamax, and this sounds similar.
 
Worked on several VFET Sony amps about 20 years ago, very prone to failure - forget to hook up speakers and outputs went buy-buy if you turned the volume up. The price then was $32 each for the VFET outputs, only availible from Sony !! Bet this is why you don't see them any more.
 
Have been talking with Yamaha about finding a set or two of the VFETs. They have been very helpful, but long out of production and out of stock. The only set (two pairs, not four, as needed for the full amp) I've found has a set price of $600. This is the used price from an existing amp that was damaged by being dropped. That is twice what I paid for the entire amp, so think I'll keep looking. BTW, Yamaha's price for a matched set of four transistors (1 channel) like that being offered at $600, was $418.72 in 1981. So the Sony was very inexpensive by comparison.
 
Info on MOSFET Devices

I found this PDF file on Perreaux's website, found it a quick and easy read - very informative. Not V-FET per se, but they give it a nod. Worthwhile, though.

Best,

Wiggy

PS - You need Adobe to read it - PDF file.

Short Tech Paper on MOSFET tech
 
My apologies.

It is indeed correct that the Yamaha C1 and C2 series did NOT CONTAIN V-FETs,and only contained FETs as was previously stated in this thread.
My mistake.
 
How important are matched ranks?

I have a contact in the Netherlands who has bought new 2sj18 and 2sk60 VFET devices as used in the Sony gear. The only problem is they are not of matching ranks, e.g. the 2sj18 may be rank 54 and the 2sk60 may be rank 56.
How important (especially in this age of scarcity) is it to use matched ranks? I know the Sony service manual specifies matched and specific ranks for my 5550.

Scott
 
I don't think you could mis-match ranks and have a properly running amplifier. While you could have different ranks on different channels, on the same channel you need them matched. I know of no way around this.

Regarding reliability, JFET's (including VFET's) are about the least likely to fail of any solid-state output device. If you have a working amplifier with JFET outputs, it is going to take a catastrophic failure to kill an output transistor. Just to put anyones mind at ease...
 
Ok, I'm looking at the TA5650 schematic and now I remember what I was thinking; it is possible to convert it to Mosfet output stage by a couple of hacks in the driver stage. I was going to write more but I have to go fix our front door, it's stuck from the darn humidity...
 
Originally posted by dr*audio
Ok, I'm looking at the TA5650 schematic and now I remember what I was thinking; it is possible to convert it to Mosfet output stage by a couple of hacks in the driver stage. I was going to write more but I have to go fix our front door, it's stuck from the darn humidity...
Click to expand...

I'll bet you 50 cents you can't do it without resorting to dubya's statement about chaos and order. If it turns out to be a simple hack I'll send you the schematic for my B2 to figure out.
 
Tentatively, here it is: positive side driver Q312 changes from a PNP to NPN, and Negative side driver Q313 changes from a NPN to PNP. (should be able to just swap the 2 transistors.) Also Emitter and collector connections must be swapped on those drivers. Base connections stay as is. Now you connect the gates of the mosfet outputs to the emitters of the respective drivers. Also the drains and source connections of the Mosfets have to be swapped. Sounds like a lot of work, but it shouldn't be that bad. In this amp I think the outputs were connected by wires so all you have to do is switch the wires around. With an amp that has everything mounted on a pc board, it's alot of cuts and jumps.
 
How are you going to get the voltages set to the new values? If I recall correctly, the VFET uses considerably different voltages than would a MOSFET. What the MOSFET needs may not be available.
EchoWars may have some thoughts on this, as well.
I remain sceptical, but wish you luck. Make sure you take pictures to post. Including the ones with the smoke.
BTW, are you doing this because the outputs on your Sony have failed? If the VFETs went, am wondering if they failed due to some upstream component failure or did they die on their own? There are some fairly tricky (high) voltage/bias levels required within my B2 on the VFETs. If something gave way upstream it could well take them out.
 
No, I don't have the TA-5650 anymore. BTW, mine didn't have blown outputs, the bias setting diodes went intermittantly open. After replacing them it worked fine. I am merely posing a theoretical solution for those with blown outputs. As to the new voltages, I assume you mean bias voltage. With the circuit changes I described the addition of an adjustment pot in series with the bias control diodes should be the only needed change. There may be couple of resistor value changes, but that should be it. If anyone ever sends me one of these VFET units with bad outputs, I'll post the results and necessary mods.
 
More "VFET revisited"

Although obviously not vintage, the review of the Halcro stereo amp in the most recent 'Stereophile' mentions that it uses VFETs in its output stage. I don't recall this from the earlier review of the Halcro monobloc amps. If so, then the VFET is not dead. I will be contacting Halcro to see what I can find out. But, there is hope.
 
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