Dead Roadster - HELP PLEASE w/ Tube Diagnosis

[fjk1138]

Hello to All,

My Roadster is about 2-3 mos old, fresh out of the box (I watched the salesperson open it), and today it died on me (I miss it already....). I suspect a tube problem, but would very much appreciate an experienced opinion please.


Here's what happened:

1. I turned it on in Spongy mode. The power came on, I switched through the 4 channels in standy to reduce the classic pop on Ch 3, and selected channel 3. After about 1 min, I took it off Standby.

2. There was no guitar sound, but it made this really weird "farting" sound (or, like a deflating balloon -- sorry, not sure how else to describe it) which eventually slowed to a stop, but no sound would come out. Changing to any of the other 3 channels made no difference. Switching reverb on or off made no difference.

3. The amp seemed like it was turned up in volume slightly (there was a noticeable hiss). I switched to standby, let it sit, put it back on -- no change.

4. I shut the amp down, let it sit a few seconds, turned it on to Bold. No difference, other than the hiss was slightly louder and Ch 4 seemed to have this high piched squeal that varied in pitch but generally went up (I am assuming this is the "microphonic squeal" described in the manual).

5. I shut it off, unplugged it, switched it on to drain it, etc. I checked the fuse -- appears to be OK. (Note to Mesa -- you really need to specify in the manual to push the fuse knob in and then turn it. The knob was too tight, and I could have almost broke it trying to remove it).

6. I took the spring caps off the 12AX7s . With the covers off, and the power back on in Spongy and in standby, the tubes appear as the following (from left to right when facing the back of the amp):

4 of the 6L6GCs:

1 -- Glows dimly
2 -- Glows brighter
3 -- Glows dimmer than 1
4 -- Glows slightly dimmer than 3


2 of the 5U4GB -- glow very dimly, but equally


6 of the 12AX7-A (left to right):

1 -- Can't see it (prior physical inspection it seemed OK)
2, 3 -- Glow nicely
4 -- Appears to glow dimly or not at all (hard to tell with the reflections)
5 -- Glows nicely
6 -- Glows very dimly or not at all


So....

A -- Are the 12AX7s in 4 and 6 glowing normally?
B -- Are the 1, 3 and 4 6L6s faulty? Or, is 2 (brightest one) the faulty one?


And -- as a side note -- I found the FX loop to be somewhat noisy fresh out of the box, so I run the amp with the FX loop bypassed. I believe I saw somewhere else on the forum that the 5th tube should be this one. It lights up, but the black/silver finish at the top of the tube is pretty much gone and one of the metal parts towards the top has what looks like it has some blackened spots. Should this one be replaced, even though it appears to be working (though not well)?

Thanks to anyone who can help me.

 

[MrMarkIII]

All tubes glow differently, so don't rely on that. There's really no objective criteria.
Do you have any spare tubes, like one 12AX7 and a pair of 6L6s?
Start with V1, the first 12AX7.
With the amp on standby, pull it and replace it with a good spare.
Sound check.
If this doesn't fix it, back to standby, and move on to the second 12AX7, V2.
Rinse and repeat.
It's important to do one tube at a time, you will not "save time" by just pulling and replacing all the tubes at once.
Hope this helps

 

[fluff191]

Dude your power tubes should be more or less glowing the same. Sounds like its time for a full re-tube.

 

[mikey383]

Dude your power tubes should be more or less glowing the same. Sounds like its time for a full re-tube.

Not necessarily. Some glow brighter than others. What you're seeing glow is the heaters. I have one tube that doesn't glow as bright as the other 3, and it still works the same. They should all glow, however.

Try swapping out the tubes with a known good one, one at a time.

BTW, that black/silver coating at the top of the tube is barium. It turns white when exposed to air, which will let you know the tube has lost it's vacuum.

 

[fjk1138]

BTW, that black/silver coating at the top of the tube is barium. It turns white when exposed to air, which will let you know the tube has lost it's vacuum.

Well, the one I was referring to was not really white, it was almost totally clear at the top, but I would assume this would mean the same thing...?

Assuming this one tube is the culprit, do you think it would be enough to put the whole amp out of commission?

And yes one more question (sorry) -- I know these are under warranty -- do you have to take the whole head to the dealer to let them diagnose it, or should they be able to give you a few for you to try? Unfortunately, I have no other tubes that I can swap out.

 

[mikey383]

I've never seen one that went totally clear. I've seen them turn white, but not clear.

One preamp tube can cause the entire amp to go out...especially if it's the input section or the phase inverter. I recommend buying one or two (they're only about $10) and swapping them before taking it into the shop.

 

[fjk1138]

One preamp tube can cause the entire amp to go out...especially if it's the input section or the phase inverter.

Well, since I am complete novice with the tubes, I took the whole head to the only non-Guitar Center Mesa dealer near me today (and that is still 1 hour away). They thought at first it was the Rectifier tubes (just by how they looked), but, after some swapping, it was the V1 12AX7 tube, and you were not only correct about that, but it also fixed the noise issue in the FX loop.

It didn't cost me anything (thanks to the warranty), but I picked up 3 more for the future.

Needless to say, I got it home, and it sounds even better than it did when I first got it.

Thanks again to everyone who responded.

 

[mikey383]

Good to see you're all fixed up!

The vast majority of all problems with a tube amp can be traced back to bad tubes. Always check the tubes before suspecting anything else.

 

[fjk1138]

Thanks Mikey.

I was starting to get separation anxiety without my amp

 

[jdurso]

I've never seen one that went totally clear. I've seen them turn white, but not clear.

That happens when they blow open... i had one that blew half its bottom and the tube was dead clear.

 

[mikey383]

That happens when they blow open... i had one that blew half its bottom and the tube was dead clear.

Wasn't saying that it couldn't happen, just that I've never seen it happen. I can't explain why the flashing would disappear when the tube goes out. Makes no sense to me really.

Just for some added knowledge, barium, when exposed to air, turns into barium oxide...which is white. So, that's why barium is used for the flashing on tubes.