Life of KT 150 Tubes

Output tubes are usually wired in parallel, not in series.  Thus it is quite possible for one to fail while the others roll along. I have no idea why my previous post was deleted, but it was. Perhaps an overzealous censor.

The OP says that one tube in each amplifier “popped”. That is not the exit mode of tubes that wear out. Normally a tube that is dying due to long use will just begin to sound bad and eventually become impossible to bias properly until the user finally notices a problem and replaces it. Although catastrophic failure does occasionally occur, most commonly at start-up, the amplifier needs to be evaluated.

A schematic is nice to have, but not really necessary for trouble-shooting this particular problem. Basically, if there is nothing amiss with ancillary components (capacitors, resistors, tube sockets, wiring, solder joints), then you have to believe it was tubes. By the way, I forgot to mention tube sockets. Check that all wires soldered to the tube pins on the bottom side are well and firmly making contact. But even that doesn’t always rule out a cold solder joint. Then also, the socket has to have a firm grip on all tube pins. Some of the modern Chinese-made sockets, while they often look really nice, can have a loose grip on tube pins.

You could swap a good tube into a socket where the previous tube blew.  Start the amplifier t up and watch for a red glow.  If it glows red (not orange), shut down the power immediately.  That proves you have a problem in the circuitry.  Sounds like from what others wrote who seem to know these amplifiers, you have a bias current meter on the chassis.  Is that correct? That should be very helpful.

You do not need an "authorized" technician to sort out this problem.  You need someone who is knowledgeable and has the balls to just do it.

The best thing to do is not to wonder why both amplifiers failed at the same time.  The fact is that both amplifiers need repair, and the best approach to that is to fix each amplifier one at a time.  In the process of doing so, it may become obvious why they both failed at the same time, or you may never figure that out.  It makes no difference either way. 

A transformer coupled tube amplifier operates on certain basic rules.  The stress on the output tubes is determined by the plate voltage, the bias voltage, and the current drawn from plate to cathode.  The easiest thing that any good technician can do is to determine those parameters for your amplifiers.  If any of the 3 parameters is way out of specification, then you have a reason why the tube may have blown, and it's easy to fix.  For example, a coupling capacitor between the driver stage and the output tubes may have failed such that there is no bias voltage getting to the grid.  This causes the tube to draw maximum current and burn out rapidly.  Did you notice the tube glowing red instead of typical orange? I admit it is unlikely to lose a coupling capacitor simultaneously in two different amplifiers, but don't assume that both amplifiers failed for the same reason.  For another example, a plate resistor may have failed.  This would also cause excessive current draw, a brief red glow, and tube death.  I don't see why a competent tech won't look at the circuit, even without a schematic, just to test for these easily diagnosed issues and others like it.  If one guy won't look at it, try another.  Also, check on-line for a schematic.  Also, post this on a tube amplifier forum, e.g., TubeDIY Asylum, etc.