Help Adjusting bias on Sony TAN 5550 VFET

i have a sony vfet 5550 an i don t want to make something wrong to damage the vfets..... they are still good !

1. Does any one knows the Voltages which drops on the Resistor r511/ r611 ( on the Gate from Vfets ) whithout the Vfets .
2. - and the Voltage on the Source will be 0.1 V . I m right?
3. After all measurements if all it is ok it
to protect the vfets... on the +/ - rail better with resistors or Lamp ? , if lamp which power or Ohm must have .. if resistor better Fusible i think? ...which value and Wattage
4.- the measurements without the Vfets must to be from the output ( - ) or from the Big caps Ground?

Thanks a lot!
I've attached manual from the web (via and pasted a warning note from page 5. This would make me hesitant about resistors and lamps until better understanding.

Good luck!



  • sony_tan-5550.pdf
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The V-fets are like vacuum tubes: apply the wrong bias and they're dead: no resistors / dim bulb testers here!

What voltage across R511/611? Depends on the grade stamped upon the V-fets.
There are several topics here on DIYaudio about the 4650/5650 and other V-fet amplifiers by Sony.
In the Service Bulletins, more is there to be informed, and also covers the 5550.

I've attached my version of the ServMan of the 5550, just somewhat larger then BSST's, so maybe some bonus info is included.
Also the ServBull's & translations to EN (by me, recently), to have these readily available.

Never worked on the 5550, but I guess the R511/611 voltage is twice the rank voltage, to have the V-fets nearly closed.
And, as the 5550 is a more classic design with the sources to the output, no fear for suicide diodes!


  • Sony_TAN-5550_ServMan.pdf
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  • Sony_TA-4650_5650_8650_TAN-5550_8550_ServBull_62.pdf
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  • Sony_TA-4650_5650_TAN-5550_ServBull_EN.txt
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  • Sony_TA-4650_5650_TAN-5550_ServBull_nl.pdf
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  • Sony_TA-4650_5650_TAN-5550_ServiceBulletin_NL.txt
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  • SSB49_Vranks.jpg
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Joined 2010
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I have a TAN-5550 which I rebuilt several years ago. It does have the suicide diodes, which I replaced along with all the electrolytic capacitors and carbon composition resistors.

When setting up the bias with the VFETs out of circuit, service bulletin 62 advises setting the bias voltages (RT-501, RT601) to the VFET gates to maximum before reinstalling the VFETs. This will ensure that the VFETs will not be conducting or at least conducting minimally on power on.