Replacing VFETs with MOSFETs
. I know it's asking a lot but i fear people will struggle to follow in your footsteps.
zaibatsu
yam
Thankyou for sharing your story from start to finish, a great read (and result), I'm sure the write-up and schematic will be useful to others in future.
There are some who like to tell half the story and then hide the other half (particularly in the v-fet realm) under the guise of "saving people from themselves", which is an antiquated way of thinking. Profit is made by hiding, progress is made by sharing. People like you are the future.
Perhaps of interest, a clever guy (Mirko Essling) on diyaudio did something similar to replace the 2SK-77 (using an IXYS depletion-mode MOSFET) in the Yamaha B-I some time ago, username is Jon Lord:
https://www.diyaudio.com/forums/solid-state/205628-yamaha-b1-amp-2sk77-5.html#post2955149
See post #43. I've attached the transfer curves for those without an account. Apparently the result sounded equal or better, similar to yours.
There are some who like to tell half the story and then hide the other half (particularly in the v-fet realm) under the guise of "saving people from themselves", which is an antiquated way of thinking. Profit is made by hiding, progress is made by sharing. People like you are the future.
Perhaps of interest, a clever guy (Mirko Essling) on diyaudio did something similar to replace the 2SK-77 (using an IXYS depletion-mode MOSFET) in the Yamaha B-I some time ago, username is Jon Lord:
https://www.diyaudio.com/forums/solid-state/205628-yamaha-b1-amp-2sk77-5.html#post2955149
See post #43. I've attached the transfer curves for those without an account. Apparently the result sounded equal or better, similar to yours.
Attachments
smurfer77
Super Member
PE9ZZ thanks for writing this up on your website. I will read this very slowly and many times 
. This is incredibly useful information. I have a few other amps to clear off of my workbench first, but hope to give this a shot with my TA-4650 or TA-5650 during early 2020.
Bravo again!
One interesting point is, how important the preamp section is to the sound of these amps. I think many credit the nice sound to the output transistor type, but your findings imply that these differences are subtle compared to overall circuit, including preamp (?).
. This is incredibly useful information. I have a few other amps to clear off of my workbench first, but hope to give this a shot with my TA-4650 or TA-5650 during early 2020. Bravo again!
One interesting point is, how important the preamp section is to the sound of these amps. I think many credit the nice sound to the output transistor type, but your findings imply that these differences are subtle compared to overall circuit, including preamp (?).
jeromach
With sufficient thrust, ducks fly just fine
Ah ok
I see. Yes, a B-1 I never had on my bench, so that indeed may be quite different from a B-2. Still, I have a (big) weak spot for these vintage Yamaha amps (B-2, CA-1000/1010, A-960 etc.), so a challenge like a B-1 I would not let pass by if it ever would come my way. Call me masochistic if you want... 
I see. Yes, a B-1 I never had on my bench, so that indeed may be quite different from a B-2. Still, I have a (big) weak spot for these vintage Yamaha amps (B-2, CA-1000/1010, A-960 etc.), so a challenge like a B-1 I would not let pass by if it ever would come my way
. Call me masochistic if you want... 
B-1 is quite different from most of any other amp, not just the B-2.Ah ok
I see. Yes, a B-1 I never had on my bench, so that indeed may be quite different from a B-2. Still, I have a (big) weak spot for these vintage Yamaha amps (B-2, CA-1000/1010, A-960 etc.), so a challenge like a B-1 I would not let pass by if it ever would come my way
I wouldn't call it a nightmare. The protection circuitry is there for very good reasons...and so are other features such as bias balance, etc.
Well, that's the reason I tackled the Sony. A fairly common amp with the compound topology that I'm a fan of. It is not a very complicated circuit and yet gave me quite some headaches... TA-5650 is similar but the TA-8650 is not because it's a source follower. Just replacing the VFETs with laterals will suffice (and adjusting the bias).
Tjerk, 9ZZ
Tjerk, 9ZZ
slimecity
Super Member
Apparently one of the main differences between the Yammy and the lower-model Sony TA VFET amps is that the protect circuitry actually worked in the Yammy gear.
Most of the TA 4650's or 5650's you find today will have at least 1 or 2 VFETs blown.
The Sony 8650 apparently had a better and improved protect circuit - these are bit rarer, but apparently you are more likely to find more intact VFETs in them as the protect circuitry did its job properly.....
Most of the TA 4650's or 5650's you find today will have at least 1 or 2 VFETs blown.
The Sony 8650 apparently had a better and improved protect circuit - these are bit rarer, but apparently you are more likely to find more intact VFETs in them as the protect circuitry did its job properly.....
jeromach
With sufficient thrust, ducks fly just fine
Well, American VFET / MOSFET exchange aficionado's, here is your chance; https://www.ebay.com/itm/193241503906
PS: It might as well just have VFETs that are ok of course
PS: It might as well just have VFETs that are ok of course
Dunno about Yamaha B-2 (too complicated for a quick glance) but the TA-4650's protection is simple and effective. Only if the VD-1221 goes phut it can't cope with this failure because it wasn't designed for that event. Regular overloading is handled just fine.
Tjerk, 9ZZ
Tjerk, 9ZZ
Just discovered that my nice writeup is not accessable after moving to Jeff Bezos' crap shop AWS. It doesn't honor the automatic redirect to index.html that all other webhosting outfits do. So access it here.
Thanks Jeff. Sheesh...
I need to update my writeup. A 4650 I modded bounced back with catastrophic failure of the laterals due to the Joepiecon 10 µF/400 V caps failing. C403 and C413. Turned out that the reactive current going through them exceeded permissible ripple current and once they failed the negative bias voltage of the laterals disappeared and they went into full conduction. One pair tragically failed but luckily the other survived. With some effort I replaced the electrolytics with 106/100 film caps and that works better. Film capacitors can handle much higher currents than electrolytics but are much bulkier.
More later...
More later...
claudiomas
New Member
Great thread, i was service engineer working on afther sale repair wokshop for more than a decade.
many many Vfet amp blow or runaway becose capacitive and resistive solder flux residual , boards was not washed
by factory, i enen belive the dual diode swap its fake news, just a toot btrush and acetone washing its main thing to do
same problem with many others amps etc,
many many Vfet amp blow or runaway becose capacitive and resistive solder flux residual , boards was not washed
by factory, i enen belive the dual diode swap its fake news, just a toot btrush and acetone washing its main thing to do
same problem with many others amps etc,