• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

VFETs for Sony TA-5650

grandpaslab

grandpaslab

Active Member
I'm sure this has been asked many times before, but does anyone have a line on real VFETs? Specifically 2sk60 / 2aj18, for a TA-5650. I've got a unit with 2 bad VFETs and I'm on the fence about whether to hang on and try to find replacements, or just part the thing out. eBay seems to be chock full of fakes....any advice appreciated, this is the first one of these I've worked on.
 
Register to hide this ad
I don't know about the transistors but those particular amps - under no circumstances should they be powered up on a variac or a DBT. It toasts the outputs.

Someone around here probably knows of modern replacements. There are depletion mode MOSFETs being made. If nobody comes up with it then we'll have to find the datasheet and probably go to Digikey's website and use their selector.
 
JURB said:
Someone around here probably knows of modern replacements. There are depletion mode MOSFETs being made. If nobody comes up with it then we'll have to find the datasheet and probably go to Digikey's website and use their selector.
Click to expand...

:lurk:
 
Everything I've read says there are no modern replacements. I'd love to be wrong about that!

Also I just read on another thread that VFETs measure short drain-to-source when they're good, so maybe the ones I though were bad are good, and vice-versa. Either way, one pair is bad.
 
As far as I know VFET's are not being made any more, certainly not the required ones for this amplifier. VFET's are effectively power JFET's - thus they are quite different to MOSFET's which are unsuitable for the amplifier.

It's the one thing you want to be careful of when buying one of these amplifiers, "are the VFET's good"? - used examples of the transistors turn up now and again, but you will have a hard time determining if they are originals or not, and if they are original, are they undamaged? otherwise you have yourself a doorstop.
 
Hyperion said:
As far as I know VFET's are not being made any more, certainly not the required ones for this amplifier. VFET's are effectively power JFET's - thus they are quite different to MOSFET's which are unsuitable for the amplifier.

snip/.
Click to expand...

There ARE depletion mode FETs being made. However they are not equivalent to JFETs. They are not reversible and polarity tolerant. With the old JFETs you could make a variable gain control that would not distort. Doesn't quite work with the depletion mode MOSFETs because they are polarity sensitive.

Even with JFETs you had to make sure the wrong polarity source voltage did not exceed the gate voltage. With the modern ones even that does not work.

However, if you find them with a close Gm and pinchoff voltage they will work in that circuit. Is it worth it for me to actually find out ? This is not something you figure out in a half hour. In fact to do it I might be tempted to actually get on the phone with Vishay or someone, and I do not have free longdistance.

The other thing, are those in TO-3 cases ? If so the plot thickens because those cases are getting rare. If they are TO-3, you might have to do a little bit of adapting.

It really will be a PITA to fix, but what else you gonna do, kick the dog ? We like dogs and don't want to kick them.

And BTW, some are so static sensitive that if you think it might rain and look at them too long they go bad. Just like that.
 
Next question, I guess, is: do I need to worry about ranking? Does each quad need to be close in rank? Or just per pair?
 
Not familiar with that rank of '-25'. The only usable rank in the 5650 is -54 to -57. That rank of -25 is so far off the scale, that I'd suspect that those are remarked parts (counterfeit), or if original, perhaps special ranked parts used in the Sony Class D amp of the period (the Sony TA-N88 and 88B).
 
Well, all the VFET's in the unit should all be the same rank. In a pinch, you can use a part with a rank of one above or below the rest of the devices, but it's less than desirable.
 
Stoffie said:
Aren't the V-FETs in the TA-N7B and TA-F7B ranks 25-26 ? At least the ones in my N7 are.
Click to expand...
I believe you are right...I looked back to my notes and saw that the N7B commonly has the -25 and -26 rank parts. These are unusable in the other VFET amps.
 
EchoWars said:
I believe you are right...I looked back to my notes and saw that the N7B commonly has the -25 and -26 rank parts. These are unusable in the other VFET amps.
Click to expand...

I'd have to check my TA-F7B but this is a picture of one of the channels of VFETs on my TA-N7B during the overhaul.

dsc00104_rs.jpg
 
It appears I was wrong. Well close to wrong at least. I DO remember finding depletion mode FETs not long ago, but not in the TO-3 case. Technically you could find some small FETs for drivers and use bipolar for outputs, but that is a hell of alot of mod so I am not sure you can still call it a Sony.

How good of a preamp does it have ?
 
My TA-F7B has a full set of -35s, FWIW. I thought 54-57 were the only usable ranks too, but I guess not.
 
You must log in or register to reply here.
Back
Top Bottom