Help with Nomad 45 troubleshooting Tubes

Hi, I'm new here and I'm just looking for a little help with getting my Nomad 45 2x12 up and running properly. Last week it road in the back of truck to a friends house for some practice. For some reason the Input knob or the tube that controls the input levels is having some issues. When adjusting it it seems to only want to go to a very loud volume or a very quiet volume. Lots of static when adjusting the knob, but no consistent change in volume level - it will just jump from loud to quiet when turning it up. I'm not sure if moisture or cold air form riding in the back of the truck would have done me in of if it is just a bad tube. Any help would be very appreciated. I tried searching the forums and didn't find much help. I should note too that I've tapped each tube when it is on and i don't hear any crackling or noise. The problem occurs with every channel and seems to be only effecting the output. Could it be the knob itself as the problem? Perhaps it got bumped when it was being carried in?

Thank you

Let's be clear...

Is it the Gain (Ch 1/2/3) Knob?
Is it the Master (Ch 1/2/3) Knob?
Is it the Output (All) Knob?

It sounds like your input stage preamp tube is going bad.

I would replace all of them to be sure.

You may want to get at least 1 known working tube and swap it in your amp until you find the problem tube.

Thanks for replying guys. It is the output (ALL channels) knob. While I don't have a brand new tube, I've take a few of them out and swapped them around, but it doesn't seem to make and changes. I will order two tubes - one of the bigger and one of the smaller ones though. Until then, any other suggestions? I've tapped each tube and no noises and all bulbs are lit pretty evenly.

I would try cleaning the pot before any tube changes. It sounds like a dirty, or faulty pot. :idea:

FWIW, it's best to change output tubes (bigger tubes) in matched pairs (two inner OR two outer), or matched quads. Not a good idea to change only one output tube. You can change only one or several preamp tubes (smaller ones).

shimmilou said:

I would try cleaning the pot before any tube changes. It sounds like a dirty, or faulty pot. :idea:

FWIW, it's best to change output tubes (bigger tubes) in matched pairs (two inner OR two outer), or matched quads. Not a good idea to change only one output tube. You can change only one or several preamp tubes (smaller ones).

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It seems like if the issue was with a preamp tube that the manipulation of the individual gain and volume knobs would have resulted in the noise also.

Since it seems to happen only with the common output knob, I would tend to agree with shimmilou. It sounds like a bad or dirty pot. Spray some cleaner into the pot and (with the amp off) turn it up and down multiple times. It could be that some oxidation has built up inside the pot. If so, cleaning it should help.

Also, shimmilou is absolutely correct in their advice concerning tubes. Replace them in pairs or in quads. The only time I ever replace a single power amp tube is when I am trying to quickly swap one in and out to see if it is bad. When I replace them 'for good', I either replace a pair, or all 4.

Plus, when you order your EL84 power tubes, tell them that it is for a Mesa Nomad 45. They should be able to give you tubes that fall within the ratings needed for your amp.

Hopefully, the issue is simply a dirty pot. If so, should be a quick and painless fix.

Hey guys thanks again for the help. I've tried to remove the chassis by removing the 4 bolts from the top like the manual says to. However, it will not come out! The manual says that it should slide out like a shelf, but something is still holding it at the front. I tried removing the all the knobs thinking that maybe the front panel is mounted to the front of the amp but no go. has anybody done this before?

I've never cleaned the pots before so I am assuming that I have remove the chassis to get to them, is this correct?

ok so i finally got the chassis off and took a look at the pot. Everything seems perfectly clean. I am quite sure it is one of the preamp tubes. So now my new question is, is there a specific type of preamp tube I have to use for the nomad 45? The tubes that were on there are Russian made Mesa 12AX7-A models. Boy are they expensive and I can only find them on ebay - Mesa doesn't even have the same model on there site. I see some cheap ones on ebay, are those okay to use?

any suggestions for tubes? I play jazzy and heavy stuff.

It's not the outside of the pot that you clean, it's the inside. There should be a small opening, or hole, in the back of the pot, or where the three terminals come out, to allow some contact cleaner to be sprayed inside the pot while turning the pot back and forth many times. :idea:

First, go to the Mesa website and download the Nomad manual.
There are lots of bells and whistles on this amp, lots of knobs and switches that can seem to cause malfunctions.

Second, if all of the small 12AX7 pre-amp tubes are Russian, don't use that particular (Russian) tube in V3. When I refer to "V3", I'm talking about the third pre-amp tube, which on the Nomad is the middle one in a row of five. It's a long story, but suffice to say that Russian 12AX7s don't do well in V3. This is one reason why Mesa no longer sells them.

How do I know what and where "V3" is on an amp I've never even seen in person? Because I have read the manual.

You can use any brand of 12AX7 as long as you stay away from the Mesa re-branded 12AX7s that say "Russian". I say "re-branded" because Mesa does not make tubes. They buy them from Asian and European factories like everyone else.

Now, everybody chime in on which tubes are the best....

MrMarkIII said:

...Now, everybody chime in on which tubes are the best....

Click to expand...

The glass ones are way better than the metal cans. :lol:

I actually took the pot completely apart. The solder connections were clean, and aside from a little grease for turning the knob it was clean on the inside too.

All 5 of the preamp tubes are 12AX7-A Russian made. Perhaps the problem is that the V3 is using that tube.

:| You can't always see the little bit of carbon/plastic/grease/wax that sometimes builds up on the underside of the wiper on the pot. It's good practice to spray some contact cleaner inside a scratchy pot, but you're determined that it's a tube, so have you tried swapping tubes around to try it? And $10 to $20 is too expensive for you to buy a tube? You can use one known good tube to try in each position, one at a time to see if that cures your noise.

If the tube that's in V3 has been working for you, and Mesa put that tube there in the first place......., V3 has nothing to do with channel 1, so if you have the noise on all channels, it's not V3. The tubes V1, V4 and V5 are common to all channels. :wink:

As suggested, you should download the manual if you haven't already. The tubes functions: V1, both halves are first stage for all channels. V2, half is second stage, and half is third stage channel 1 only. V3, half is third stage and half is fourth stage channels 2 and 3 only. V4, half is second stage channels 2 and 3 only, and half is effects return. V5, both halves are PI for all channels.