Power Tube Failure? Thin, Hum, etc...

[mcleanab]

Hey all!

New to this forum, so thanks in advance for any and all information!

My little fire breather Mark V:25 was kicking ass... swapped out some preamp tubes (was loaded with JJ's and the stock 'computer matched' Mesa power tubes), and sounded great. Then I noticed it got a tad fizzy and the low end was fading off... flipped the 25 to 10 watt switch and a very present hum kicked in, on both channels. Made sure the power tubes were nice and snug (preamp tubes as well) and it lasted for a few minutes sounding glorious, then back to the thin, lack of bass, and hum on the 10 watt mode. I can try rolling through the preamp tubes to see if there's a bad one, but I was worried I might damage the amp if it is the power tubes. (I could yank the power tubes and stick them in a different amp to see, I guess)...

Just turned it off and ordered new power tubes (Mesa matched).

Any thoughts or similar experiences? Looking for some guidance and assistance!

Thanks!

 

[tbonesullivan]

How long have you had the amp and how old is it? This could be a single tube, dirty contacts on a tube, or something else entirely.

 

[mcleanab]

How long have you had the amp and how old is it? This could be a single tube, dirty contacts on a tube, or something else entirely.

Got it in a trade about 2 years ago. All Mesa tubes. Replaced the preamp tubes with JJ's about a year ago. It's been great until the other day...

I did swap out the preamp tubes with some other ones, but now that this issue has started, doesn't matter what preamp tubes are in it...

 

[JustPassingThrough]

How old is the amp? The symptoms you're describing sound like those attributed to a filter cap failing, but they typically last 10+ years.

 

[Henz]

Imo, fwiw, when one replaces all of the prebies at the same time , it just makes troubleshooting that much more difficult. Replace one prebie at a time. If problems remain, reinsert and go to the next .

 

[bandit2013]

It sounds more like a power tube issue. Sure, preamp tubes can crap out but usually takes much longer. Since the V does not have any cathode follower preamp circuits like some other Mesa amps (That is a long list but mostly Rectifier, LoneStar, Stiletto, Triple Crown, BAD, and so on).

When playing and the sound changes dramatically, the instinct is to hit the standby switch, before you do that, look at the power tubes first. If the plates are glowing bright orange or red, power tube is at EOL. It is usually hard to tell if it is just one as often times when one takes a hit, the other will look similar due to uncontrolled plate current. When the power tube fails and red-plates, it will loose most of its audible character as it will no longer respond to any AC signal on the control grid. EL84 tubes may behave a bit differently than the big bottle tubes but the failure mode has similar characteristic sounds. The amp chassis may also get hot due to this excess current if the fuse does not blow out. 8 times out of 10 that the power tubes are the issue. Sure, preamp tubes can fail or degrade as well but they usually have a different characteristic when they bite the dust and often times, they do not recover and sound better when things cool down. When a power tube overloads it gets really hot so after letting it cool down and starting over again it will sound normal for a while. It only will get worse from this point moving forward until you replace the power tubes. Do you still have the old one's? If they were in working order, you can use them while you wait for the replacements arrive.