Mesa Nomad 45 preamp tube acting up?

[MickeyB]

After about 10 to 15 minutes of playing on 3rd channel the amp would start getting louder and bad distortion. Acts up on channel 2 and 3 (gain) but not on 1. I checked all the preamp tubes on my BK and they all test good. Took the chassis out and checked the plate load resistors, they check ok at 100k. Turned it back on and waited until it started acting up, checked the resistors again and when I touched the plate load resstor for V4 it stopped. Thought "bad solder joint". Resoldered it, turned it back on, waited again, it started acting up again. Check the voltages, looked good, tapped on them with chop stick, nothing, tapped on V4 and it started humming very loud. Put it on standby, put in a spare 12ax7 I had, turned it back on and have not heard it so far. I checked that Mesa tube again and it still shows its good. Has anyone ever had this happen with a preamp tube? Or is it maybe the socket?

[52 Bill]

I think that what you've discovered is that a tube tester is fine for some things, but it will not test for all problems.

A tube tester will usually test the tube at a much lower voltage than what is normal in a guitar amp. So it may be that the tube would work fine in a radio but not in your amp.

The tube is bad, throw it away and the next time you test tubes, know that the tube may still be suspect even though it tests fine on your tester.

[Mars Amp Repair]

Yep,
You really need to do the 'tap' & see test particularly with the preamp tubes as they are just finicky...esp these days.

You can also have issues at times with preamp tubes that aren't particulary 'bad' per se, but will cause hum in the higher gain preamp positions.

In that case you can usually move the offending tube to the lesser gain stages ie: driver or reverb driver.

glen

[MickeyB]

This is my 6th tube amp that I've owned and never had that with a preamp tube. That does make sense, the tester is not the same as higher voltages and vibrations from the speaker. I was surprised it happened in the Mesa. I guess their tubes can fail as well as anybodys. Thats why their in sockets. Thanks.

[Mars Amp Repair]

Yeah,
I would say preamp tubes are particularly finicky in Mesa's where the gain is so high to begin with.

According to Mesa, you should only use their variety of the Ruby (Chinese) tubes in the cathode follower ckts given the difference in voltage potential between the filament & the cathode.

Mesa tubes are nothing more than re-branded albeit extensively tested Ruby tubes. So, really any brand can have issues. Mesa has talked of changing to JJ tubes, but I haven't seen that happen as of yet.

Live & loin...glen

[MickeyB]

All I had to test with was a JJ out of my Fender HRD. There is a difference in sound in the gain. I thought about changing over to Mullards. I think I better just order a couple Mesa's. I like the gain where it is. Thanks for the heads up.