Sony vFET Amplifier Part 2

[propitious]

Hi,

I just recently unwrapped my kit and opened up the output transistors bag. Found out that the measured values are almost impossible to read, like it was erased or faded... what can I do with this?

Thanks
Do

A couple of mine were somewhat faded also. However, I believe both J's will be matched with each other having the same value. So if you can read the value on one J, it will be the same for the other J (likewise for the K's).

 

[MASantos]

Is there anyone knowing if there will be just PCB boards available again in the store for the Sony Vfet amp? I happen to have som vfet's that i would like to use. Or maybe gerber files to make your own.

Regards
Anders

I might have a pair of extra boards. I bought 2 kits to make the original version so I wont need a pair of boards. I am however in between houses and I'm not sure where the boards are stored. If you are willing to wait a couple of months I might be able to help you.

Manuel

 

[Nelson Pass]

I just recently unwrapped my kit and opened up the output transistors bag. Found out that the measured values are almost impossible to read, like it was erased or faded.

Can you show us a photo? If we still can't make it out, then you would set the
bias voltages at 12 volts, which is about as high as they go, where there
will be little or no bias current and then adjust them down, watching the
current draw and the output DC offset, just like the normal procedure.

 

[JPS64]

Are they reliable?

VFET Sony's Rare 2SK82 and 2SJ28 (KE-33) VFET (1 Pair)

JP

 

[JPS64]

Are they reliable?

VFET Sony's Rare 2SK82 and 2SJ28 (KE-33) VFET (1 Pair)

JP

 

[2 picoDumbs]

Yes. That's where I got mine from, but thought he had only p-channel parts left.

 

[secanbj]

They are reliable. I guess most of us who did not buy the kit from diyaudiostore bought their vfets from circuit.diy. And if i am not mistaken Nelson recommended them(can be wrong here).

Regards
Anders

 

[2 picoDumbs]

Has the store run out of parts?

 

[Soundhappy]

Has the store run out of parts?

Yes after e-mail response from Acronman he don't get more Sony vfet's
All store stock was sold out.

 

[Soundhappy]

Papa keep vintage Sony's for Diyers SIT-3 amp kit in near future.
Have a nice day 😀

 

[Nelson Pass]

Yes, I allowed for a large failure rate that did not occur, so we are planning
a different DIY amp in the near future with the remaining parts.

 

[ernesternest]

There is no such thing as too many amps [emoji39]

 

[WBS]

Yes, I allowed for a large failure rate that did not occur...

It's probably a testament to the solid design and concise instructions!

 

[dbis]

Another front end bias question, continued

Hi everybody,

I was able to build and bias front end and vfets on the 1st channel with no problems but the second channel seems harder to me. Resistor check was OK. Voltages good as well except for T6-T11 and T8-T13: +8.2V and -7.9V instead of ~5V. When I finally get to about 1.5V on R5 and R6 with as close to zeroV offset(T18-G) as possible, the DC offset doesn't seem that stable. After more than 30mn at 1.5V the DC offset runs up and down between about +120mV and -140mV. I can just sit and watch it go slowly back and forth over a span of about 15 minutes. Is that amount of fluctuation normal?

Another thing that seems funny to me, but I didn't really notice while biasing the first channel, is that the heatsink and the MOSFETs don't get warm at all. The only difference, build-wise between the two channels is that I used some keratherm pads instead the mica ones since I had cracked one of the mica ones. I checked and there's no continuity between MOSFETs and chassis. So don't think the insulator difference is a problem.

Also I should mention that at first I thought that fluctuation was caused by jfets, so, since I had an extra Diyaudio store matched B grade pair I switched them. Didn't make any difference.

Any help would be greatly appreciated.

Thanks

Me again, dbis,

So I finally got to try again last night and I'm getting the same thing. My T18 DC offset isn't very stable. I can get it to around zero with 1.5V on R5,6 but it can move over 130 mV +/- back and forth, even after running 30mn. Perhaps that's normal and I can move on to installing the VFETS. I am attaching a zipped 20 sec movie showing how it changes. I just want to be careful everything is OK before installing the VFETs. The other question I had is if the Mosfets are supposed to get warm at all, because mine don't get warm and I can't remember if they did while I was biasing the FE of the first channel. That channel works fine.

Nelson suggested measuring current draw across the 0.1 ohm resistor by measuring voltage. I got 1V but I'm not sure how to calculate the current draw (Sorry, still a newbie but getting there slowly).

Thanks again,

Dan

 

[dbis]

Oops, I forgot to attach photo of the board in question. Thanks.

 

[Nelson Pass]

I think you're OK. If the VFETs aren't installed, then there is no feedback
to the front end - you are seeing the offset "open loop", and the level
of accuracy on DC is as good or better than you should expect.

This should clear up after the VFETs are installed.

Nice job stuffing the board, I might add.

 

[dbis]

Cool! Thanks so much Nelson.

 

[Gyuri]

Thanks Papa, it was a nice explanation, what happens in circuit without VFETs.

dbis:
That 1V across R32 seems a little bit huge. It means 10A, if R32 is 0.1Ohm as in schematic.
I=U/R

 

[dbis]

Hi Gyuri, Yeah seemed a little funky to me so I think I may have measured it wrong. All my resistor and voltage checks before applying power to the front end were ok. I always use a dim bulb to power on for the first time and that was fine too. Thanks.

 

[Gyuri]

Double check it again, then go on with VFETs.
Good luck!