Try changing the fuse anyway. Sometimes there is a very small fracture that can't even be seen in the fuse. If that doesn't work, call Cary.
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most-likely that is the fault of a power supply and less-likely you'll be able to perform that troubleshooting by yourself if you've never worked with high-voltage circuits (nearly 500V).
there are definitely fuses on the amplifier and i guess one main for power supply and the other is an output fuse. changing these fuses will unfortunately less-likely help.
were the previous tubes blown? did you try them on another side of the amp or placing them onto the working monoblock?
You should call Kirk at Cary Audio and explain which tube positions are not getting heater voltage. Chances are good that there is a fuse associated with that group of tubes.
In general tubes rarely all fail at the same time. So the catastrophic failure of three tubes is very unlikely. Normally there is one portion of the power supply that provides the heater voltage and one that provides B+ or High Voltage. I find it unlikely that there is a power supply failure if 9 of the 12 tubes are getting heater voltages.
With all that said you are most likely looking at a fuse or at very worst a burned resistor or resistors which in some designs like Audio Research act as fuses. Kirk should be able to help you further. Do be careful if you open the amp up. The DC caps in a tube amp can hold a charge for many days in some designs.
Give Cary a call, ask for Kirk or Dennice Had (the owner), both will gladly take your call. I to have a V12 and a couple of issues came up with it and they talked me through it right over the phone. I would doubt any of your fuses are bad, the fuses controle each rail or side, not three tubes of the six per side, good luck would be interested on what you find out........Bob
Thanks to all those who responded! Fellow and knowledgeable Audiogoners, such as yourselves, make this such a helpful and informative site. Just found out that the tubes I thought were defective were still good. Switched sides and the supposed-to-be bad tubes lit up plugged into the other channel's sockets. Turned the amp upside down and opened the chassis, could not find any fuses inside. The fuse located outside the amp for the tube rail is intact. Looked and sniffed around for any burnt component; did not find any. I think CATMAN's prognosis is correct. Will try to call CARY this morning. Again, thanks to all of you. Will keep you posted.
sorry i made a-bit unprecise statement about power supply which truly meant to be the individual DC supply for the tubes that might be for a few at the same time depending on the design topology.
the mentioned resistor-fuse circuit protection as mentioned by Catman is integrated onto many amplifier brands and I guess with the simple 10Ohm/5...10W bias-measurement wire-wound resistor. the grid supply resistors 1k...5k can also act like fuses if something happens with tube or outside(grid-cathode shorts).
these problems can be easily traced with multimeter when the amplifier is off BUT discharging powerful filter caps is a-must before even touching anything on the circuit. use 1k wire-wound resistor(RadioSnack is OK) with isolated clip leads on each filter cap and any high capacitance caps.
when the problem is identified in order to properly fix it you need to carefully replace the faulty element without damaging next-standing elements(use heat sinks for soldering) and without boiling the solder(use 15...20W setting on your iron). use tin/silver solder that is more strong and heat-proof.