wouldnt it be nice to know the signs of a bad tube? heres some info i found. feel free to add to this.
Power tube issues. #1. If your amp blows a fuse and you have not changed your power tubes in over a year then stop. Do not pass go. Do not replace the fuse and play the amp again unless you had a power surge caused by a power outage, a blown breaker, or you just decided to play your guitar while bathing and survived, or you own a 5150. If you did have a power surge or a blown breaker and popped a fuse then replace it and try the amp again. If you blow a second fuse STOP! You probably have a power tube going short and if you keep replacing fuses or "God forbid" use a 10 amp fuse to cure the problem you risk causing serious damage to the amp. If your 5150 blows one of the conveniently located internal fuses on the PC board then pull the chassis out of the box and replace the fuse, usually all will be well. These amps like to blow these fuses just to screw with you... #2. If you have just installed new tubes and you have blown a fuse then carefully remove the power tubes to make sure you have not miss-indexed one. There is a guide pin on the bottom of all octal tubes including 6L6's, 6V6's, EL34's, E34L's, KT77's and KT88's, these power tubes have a timing key on them and this has to lineup with the notch in the socket. If you turn an amp on with a tube miss-indexed you can damage the tube and the amp! You may think this is impossible and get a good laugh about it but believe me, we get lots of calls from players and customers who have done this! If the tubes were indexed properly and you did not clean the sockets or the sockets are loose and do not grip the pins on the tubes tightly then clean the sockets and retension the sockets if necessary. I cannot emphasize enough that having good connections between your tubes and sockets is of paramount importance!
#3. So lets now assume that you have the tubes installed properly and there are no connection issues but the amp still blows a fuse. Remove the power tubes, replace the fuse and power the amp up. If the amp blows a fuse with no power tubes installed then it's not a tube issue and the amp needs to see a good tech. If you can power the amp up with no tubes installed and the fuse does not blow then power down and install one power tube and power up again. Repeat this process adding the second tube to the equation. If the fuse blows then remove the tube you just added and you will be holding the trouble maker in your hand. You can use this trouble shooting procedure for amps with any number of power tubes.
#4. If your amp uses a rectifier tube and you simply lose power or blow a fuse then this tube can cause both of these things.
#5. If your tubes are more than 6 to 8 months old and have seen regular use do not simply replace one tube, replace them as a set. If your tubes are new then contact your supplier about the failure. If you bought them on ebay do not pass go... Preamp tube issues. #1. If you have a single channel amp and you suddenly lose power or your signal drops down substantially and your power tubes look fine then it's probably a preamp tube causing the problem. The first thing to do is to clean the sockets and reseat the tubes. If a tube was making a marginal connection to begin with then as the components heat up and parts move a bit due to expansion a marginal connection can become no connection. I can't even count how many times we have instructed players who have called with problems to clean their sockets and reseat their tubes only to be amazed that their amp came back to life. Connections people! You know what they say, if you don't have connections you don't go anywhere! #2. If you have cleaned the sockets, reseated the tubes and still have nothing, then replace the preamp tubes one by one using an old tube that was known to be functioning. This will allow you to find a malfunctioning tube. If you simply replace all the preamp tubes then you will never find out which one is causing the problem. #3. If you have a multiple channel amp and all channels go down then it's probably either the V1 preamp tube which in most amps is the preamp tube closest to the input jack or the phase inverter tube which in most amps is either the preamp tube that is farthest away from the input jack or closest to the power tubes. So trouble shoot these tubes first and you will likely find the source of the problem.#4. If you have one or two channels go down on a multiple channel amp but your clean channel still functions then look to the tubes located between your V1 tube and the phase inverter tube. In most modern multi channel amps the signal is cascaded thru several tubes compounding the gain.
yeah i just saw it on another site and noticed theres a few threads about whats wrong with my head so i figured if everyone puts their knowledge here then it would save someone the time of searchin thru 50 threads.