Which came first? Blown screen grid resistor or 6L6GC power tube?

['66Cuda]

Is this cause and effect or sheer coincidence? Amplifier is a 1979 Silverface Super Reverb and is not mine. The other day after I replaced the reverb tank and cord on it, I took it off standby to test it and heard a terrible buzzing sound, then snap, crackle and pop. The V7(right) power tube screen grid resistor had catastrophic failure, to include blowing it in half. I got it shut down right away as I was standing by the switch. I figured the fuse had blown, so I first checked it. Nope! 30 amp auto fuse! I forgot to check it BEFORE I began messing with it. I was worried that there may be other problems now. I tested the JJ 6L6GC tube on my EICO tester, and it tested for no shorts but was now "replace" on the conductance. I had previously tested this tube a few weeks ago and it tested fine. This tube was missing the indexing tab (broken off at some point) but the owner was still using the tube, and it was indexed properly when I lit it. After I replaced the 470 ohm screen grid resistors, 1.5k stopper resistors and put a new tube in, I tested it and two of the speakers had scratchy voice coils. This amp played fine before the catastrophe. Can these two events be at all related, or is this strictly coincidence? So did the tube take out the 470 ohm resistor, or did the resistor take out the power tube?

[Enzo]

tested it a few weeks ago? Everything works until it doesn't.

Resistors are passive, they fail when too much current runs through them. A bad tube takes out the resistor. If the resistor opens, the tube stops conducting. WHich is the same as the tube not even being there.

Tube testers can tell you a tube is bad, but cannot really tell you a tube is "good".

Just because the tube was in the socket correctly when you got it, doesn;t mean it was always in teh socket correctly. A broken off index pin allows a tube to be inserted eight different ways, Some of which can damage the circuits.

There is no way for a tube amp like this to put DC on the speakers. So it is not likely scratchy voice coils are the result of the tube failure. The speakers may have already been damaged from use or abuse.

['66Cuda]

Good points and thanks for the reply. I expected to find a short in the tube while testing, but only found low conductance. Shouldn't I expect to find a short or at least something that caused the rush of current?

[Chuck H]

Not necessarily under low voltage conditions such as a tube tester would offer. It is just like Enzo said. A tube tester can only tell you if a tube is bad, but it can't tell you if a tube is good. A short due to an arc at 450V may not show up at only 250V. Be sure and check out the sockets for conductance between pins or visible charring.