Vfet / SIT Amplifiers

@kitr,
Yes, the TA-N7 has the veractor "diodes" / death diodes, but it also suffers of problems with fusible resistors. The thermal runaway of those fusible resistors is horrible and over time they drift substantially from the original value. When they drift past a certain point, they will take out a vfet or two.
Also, the BJTs in the final stage need to be replaced as they are also known to fail.
Net, Net, while the TA-N7 does not have the death rate of the n4650/n5650, it needs to be restored nonetheless. The problem is that there are very few techs that fully understand vfet units and even fewer who would take vfet units for service or restoration. This is valid for all vfet units.
Depending of where you are located (I mean on which continent) I could point you to someone qualified to service your units. 

Yes, my Sony V-fet is full restaured with nichicon and VD1221 have been changed by 1N4148😉

The "bias" must be lowered to 45mv for the ta-4650 and 55mv for the ta-5650.

You meant the ta-E88, not the N88 ?
I speak the preamp E88, i preferred my old 700c, that's a question of preference...
But i prefer again the valve preamp with Sony V-fet, this combo is fantastic!

Enjoying the SIT3 with my Daedalus Athena 95.5DB sensitive speakers. Currently using the Pass XP22 preamp which is kept at the upper range of the volume. I have no issues with low level listening and can crank it up with volume to spare.
Still always listening below unity gain