Talking of tubes, the S-400 is equipped with a 12AT7, a 12BH7 and six output tubes a side. That makes it a classical driver/phase-splitter/push-pull output stage design. The output tubes can be 6550 or KT88, and the two do sound rather different. The 6550 offers a fuller, weightier and arguably more powerful presentation. The KT88 is slightly leaner, with a quicker bass, more transparency with greater mid-band delicacy, texture and dynamic discrimination. It’s a little like listening to the Chicago Symphony under Reiner compared to the LPO conducted by Sir Adrian. Both have their own appeal, but being English (at least by birth, if not by inclination) and having spent years attending the LPO, perhaps it’s not surprising that I choose to use KT88 output tubes. Your tastes (or system) might differ, but this is a choice, a musically important choice, that needs to be made if you are going to use the VTL amp.
At the front of the tube deck are a pair of three position switches, one for each channel, that allow users to select low, medium or high feedback/damping factor settings – a worthwhile factor when it comes to room and speaker matching. The rear panel is equipped with balanced and single-ended inputs, one pair of binding posts per channel, an absolute phase switch and all of the amp’s fuse holders, making them readily accessible without dismantling the casework. There’s even a fuse tester built into the rear panel!
Remove the top plate from the chassis (it’s a firm, push fit) and you reveal the tubes. Turn the amp on from the rear panel and a green LED flashes next to each tube until the amp settles into stand-by. Next press the central, red power button on the front-panel. The display will start a 240-second countdown while the amp runs through its soft start procedure, while also automatically checking the operating status and adjusting the bias of each output tube. Once the countdown reaches zero, the display goes blank (reducing any risk of spurious noise from an active display) and the amp is ready to go. But where things get clever is if the amp detects a problem, either on start up or during operation. If that happens, the amp shuts down and the front-panel will display either a green (non-critical) or red (critical) tube fault message. If the fault is non-critical, the amp simply disconnects the affected output pair and carries on regardless. The fault indication simply tells the user that at least one tube needs replacing. If the fault is critical, the amp will shut down and the tube will need replacing before restarting. Either way, the faulty tube is indicated both by its number on the front-panel display and the green bias LED blinking beside it on the top-decking.
But the real secrets lie hidden behind the black face-plate that sits below the front panel display. Remove it and you see two rows of three small push buttons. These allow you to access a host of information regarding your amplifier’s operation (total operating time, tube life, incoming AC voltage, temperature of the rear panel heat-sink and bias current requirement of each individual tube). LEDs associated with each button indicate the nature of the tube fault and used in combination, the buttons also allow you to re-set the amplifier after a tube replacement , or the tube life timer after swapping out the output tubes.
