Blown tube issue

Russ quoted the bias instructions. "300mV across a 10-ohm resistor".  If he is correct, that would give a plate current of 30mA.  I just checked some EL34 data sheets on-line, and they all confirm what Russ said, that 30mA is very conservative.  Most give a range of plate current from 35mA to 70mA. The point about whether the tube that fails is always in the same position is a very important one. If that is the case, watch the replacement tube carefully.  Is it glowing more "red" than the other tubes?  If so, then you may have a leaky coupling capacitor (the capacitor that feeds the control grid signal from the front end).  If that cap leaks positive DC to the grid, that makes the (negative) grid bias voltage go less negative, which turns that tube ON harder and harder. Moreover, over time, the leak builds in magnitude such that the bias voltage becomes less and less negative and then... blooey.  The tube goes cherry red, sometimes also blue, and dies.  The good thing is that fixing this problem is easy; replace the coupling capacitor.  I especially suspect paper in oil capacitors of leaking (DC voltage, not necessarily oil).

What everyone else said. Also, if you bought the amps used with used tubes installed, it may not be unusual at all to have lost two tubes over a period of 1.5 years. The fact that you do not use the amps so much is neither here nor there. Tubes are most stressed during turn on and warm up.. Just like an electric lightbulb, they are most apt to blow at start up. Especially if you have never checked and adjusted bias during your period of ownership. As tubes age, the bias must be periodically adjusted. However, the tendency during the aging process is to lose transconductance, which means that as the bias stays the same the current through the tube will go down. That per se would not stress the tube; it would cause the sound to grow dull and lifeless.