First Tube Amp: Advice? Keep spare tubes on hand?

Hate to be the fly in the ointment but there are some amps that require a resistor replacement before attempting to swap out the bad output tube. This depends on how the tube failed. Some manufacturers use these resistors like fuses. If the resistor fails, like a plate, grid, or cathode resistor, it can minimize damage to other parts of the amplifier. If a new tube is installed and the resistor is not replaced, it can cause damage to your brand new tube. I would probably contact the manufacturer to make sure you can easily swap out output tubes. Small signal tubes like the 12AX7 or 6922 can be easily swapped out as long as the amp has been turned off first. G

Xti16 I couldn't agree more. Running an output tube on a machine for 10-15 mins. is a waste of time. Many output tubes require 24 hrs. for the innards to stabilize. I just got done installing new output EL-34's in a Conrad Johnson Amp. And sure enough with brand new tested tubes one of the Wing C EL-34's started to run away.... You have to be vigilante.

First off did that tube arc? Are their any burned resistors around the output tubes? Was it always drawing this low bias voltage, or did this just happen? If no resistors are burned possibly swapping the outputs might help to diagnose the problem. If the problem moves then you know its the tube and the tube will need to be replaced as it has been damaged. If the problem stays at that tube socket then there is a problem with the voltages being applied to that tube socket or the circuit that supplies that socket. That being said, before trying any of these procedures, I would strongly suggest you contact the manufacturer of this amp as they can help you trouble shoot your problem. I don't have your amplifiers' circuit diagram in front of me. Hope that helps....G