How do you know when your valves should be replaced, What are the telltale signs?

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Some information that might be helpful::

How do I know if I need to change my tubes?

If your amplifier makes strange noises (hisses or squeals), or if it experiences a lack of power or a lack of bottom end.

If I change my tubes, do I need to re-bias my amplifier?

For preamp tubes, which are self-biasing, the answer is no. Install the new tubes and you’re done. For output tubes, re-biasing is recommended as the bias will drift. If you are changing tube brand or type, a re-bias is required.

How do I determine if my preamp tubes or my output tubes need to be changed?

If you're experiencing a lack of bottom end and power, it's probably your output tubes. If you're hearing strange sounds coming from your amplifier, turn your gain section up in volume, and slowly turn the master volume down. If the noises persist after the master volume has been turned down, it's an output tube. If the noises go away after turning the master volume down, it's an input tube.

Should tubes glow?

Yes! If they are not glowing they are not working! The plate structure should never glow orange or have any spots on it that are orange. The heater or filament, which runs up the center of the tube, is the only part of the tube that should glow orange.

A blue or purple glow that pulses under load is normal. A strong purple glow that shows up when a tube is not under load at idle means the tube is "gassy" and is on it’s way out. This is usually caused by improper vacuum and when under lead shows a very intense purple glow.

Should tubes make noise?

All 12AX7 type tubes are microphonic. Some are just more so than others. You can check your pre-amp tubes by turning the amp on, gaining it up and tapping lightly on each tube with the end of a pencil or a chop stick (my favorite). You will be able to hear the tapping through your speakers, which is normal. It is not normal for a tube to ring like a bell after it’s tapped. If it does ring then it’s what is called microphonic and should be replaced.

In the case of very high gain amps like the Peavey 5150 and Ultra or the Mesa’s and Marshalls you will find that by simply swapping the pre-amp tubes around you can find a combination which will yield the least amount of noise.

Power tubes on the other hand are rarely microphonic. EL84’s seem to be the most susceptible to being microphonic and having filament rattle. The worst combination of course is a combo amp using EL84’s especially some of the high powered one’s. The speaker vibration can rattle those little tubes to death in short order. All power tubes can be checked for being microphonic just like pre-amp tubes.