EL 34's in Mark III

Yesterday, I bought a pair of el34 Mesa tubes at Guitar Center. I pulled my two outside power tubes, replaced them with these, and when I powered the amp up, my fuse blew and I smelled electrical burn. I replaced these tubes with the 6l6's that were in it originally and also replaced the fuse. The EL34's were blown, just fried. Also, the amp has a hum at "idle" now that it didn't have before, even with humbuckers. What on earth is going on here? Everything in the manual recommended these tubes and said you could just plug and play. My 6L6's have little paint dabs on them; the EL34's did not. When I re-read the manual, it said that 300 series amps have the EL34's on the inner two sockets. Could this have been it? I think I screwed my amp up by changing these tubes, and I'm pissed.

Is it a simul-class amp? If not, you're not supposed to put EL-34's in it. Does your amp use 6 power tubes? If so it's a coliseum (300 series) amp. If it uses 4 power tubes it's not.

Paw said:

Is it a simul-class amp? If not, you're not supposed to put EL-34's in it. Does your amp use 6 power tubes? If so it's a coliseum (300 series) amp. If it uses 4 power tubes it's not.

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Ouch. Man, sorry to hear about the fizzle event, but all is not lost. Our MkIIIs are built like tanks and can take it...even if a little repair work is called for. More than likely, your amp is NOT Simul-Class and you found out the hard way or you swapped the wrong tubes in a coliseum model. Also likely is that the grid resistors on the that pair have fried. You'll have to pull the chassis out to visually confirm. Search these boards for images (if I get a chance I'll update this post with a link). This will require replacement of those resistors ASAP. Don't attempt to turn on the amp until you either make these repairs or seek professional help locally.

Feel free to post photos for review.

Thanks for the reply. This amp is a blue stripe w/ a switch on the back that says "simulclass" or "class A". Four power tubes in all. I believe the damage done was limited to the new tubes. Otherwise, I get a hum that I don't think was there before, but full power and good sound as before with the 6L6's back in. I'm concerned that you think I should even be using it. Based on the above, am I OK? Thx...LPD

I guess I misunderstood. If replacing the previous tubes returns the amp to normal operation, then you should be good. But a general visual inspection would still be a good idea. Anytime something goes nuclear, there can be collateral damage.See this thread for photos of damaged resistors. Though you have a blue stripe, the appearance of damaged resistors is the same (and can cause the hum you described).

Regardless, there still remains the question of why those tubes did their best impression of Chernobyl. That amp, if Simul-Class, is designed to take EL34s out of the box. Something is missing from this equation.

In your opinion, is the EL34 thing worth doing anyway? To me, the 6L6's sound great. I am going to have the amp looked at and also call Mesa about the issue.

Personally, I prefer the EL34s. I went to all 6L6s for about a month and switched back many years ago. The 6L6s were too bright and brittle for me. Plus, the factory heavily recommends using EL34s for improved reliability and tube life (check the latest version of the manual). In all, this is a totally subjective decision and only your ears can decide. If I were trying to emulate a Fender Twin, I'd go all 6L6s...but I already have one of those.

I would recommend calling MB and bouncing your issue off of the support department. Could be nothing, but something doesn't add up. I've never had a pair of tubes, new out of the box, blow up and take a fuse with them.

Sounds like something failed in the amp. Id take it to a tech for a once over. Shouldnt be a difficult fix. Anytme something like that happens and you smell burning, take your amp to a tech. Something is obviously damaged if you now have a new hum.

As far as tube types...

6L6s are more in your face and tighter and punchier in the mark III. Adding 34s to the outers makes the sound spongier and more organic. Less dynamic due to the earlier breakup of the 34s i guess.

If you bought a set that where rated very hot this could have caused the issue. You generally are looking for a pair that fall into the 35-40ma range.
Not all tubes are created equal. It is possible you just bought a pairs on the wrong side of the desired range.

Thanks, guys. Very helpful. LPD