How can you tell if a rectifier tube is bad?

[marshall2555]

I have a fender reissue 1959 bassman. The rectifier tube is black on the glass above the base, so I would assume that it needs replacement. The power light on the amp fluctuates as I play and there is slight distortion when the amp is on 2 for volume. Would this indicate a bad rectifier tube? I would assume that this amp would have a little more clean headroom, I just bought it used.

[MWJB]

If the rectifier had failed the fuse would most likely have blown and the pilot wouldn't come on at all. There would also be no sound from the amp at all.

The power light is not related to your distortion, the amp will work perfectly normally without the pilot. WITH THE AMP UNPLUGGED FROM WALL AC, unscrew the lamp jewel and try another type 47 bulb. Quite often there is play in the lamp assembly and the pilot is intermittent.

Distorting at 2 does not sound normal, take the amp to a tech for a check up. It's possible that you might be able to fit a tighter rectifier (5AR4/GZ34/solidstate) to improve headroom, but do not do this without monitoring bias current and don't do it until the abnormal distortion you have is cured.

Try a known good 12AX7 in the 3 preamp tube positions, one by one. You can't damage the amp by subbing preamp tubes (as long as you observe the tube's keyway, don't bend any pins and obviously do any swapping with the amp unplugged). This will eliminate a duff preamp tube as your culprit.

[marshall2555]

thanks for the info I will try the preamp tube swap. Let me ask you this though, the amp has all groove tubes except for the rectifier tube which is a mesa boogie 5U4. Factory says this amp should have a 5AR4. At the risk of sounding stupid, does this make a big difference?

[MWJB]

The 5U4 is generally more reliable than the 5AR4 in this amp, but it drops the voltage and bias current slightly and sags more than a 5AR4 (which is tighter sounding). Some people prefer one over the other, the one you like best is the right one.

[Enzo]

That black coating inside the tube is not a problem. Unless it turns white.