The servo works very well now. I adjusted the output to around +13V from cold start, turned off, and put the jumper of the opto on. Turned the amp on, and the servo started to work immediately. It's very nice and stable.
Thank you everyone for your help.
Fired up my Carbon last night. Adjusted the current to 20mA but back down to 16mA at the moment as I'm afraid that my transformer can't stand it. Being lazy, I utilize the power supply and chassis from my KGSSHV. So I should consider it as an upgrade. I'll buy a new transformer and adjust the current to 20mA later. The amp is running at 450V. Even at 16mA I can feel that it's running a lot hotter than my previous KGSSHV.
The amp sounds wonderful. Thanks Kevin for sharing this wonderful amp design.
Can you get access to a 3d printer? You can print out the plastic part with a 3d printer and use the pins from XLR male connector. I've done many for my DIY headphones.
Birgir, if you have a chance, could you please make the dimension measurement? I really wonder if the glass can outperform the PCB. I can make a set of drivers for you to try.
I think I use something like these: http://www.ebay.com/itm/10pc-CMC-8P-Tube-Socket-Gold-plated-Pin-Hex-Snap-2x10mm-Hole-/130342224494?hash=item1e5900826e:g:ooUAAOSwZVhWSGjY