(Apparently) failed power tubes in Studio 22+

[Danavenell]

Hello, first time poster here. A Studio 22+ came my way for a bargain price 18 months ago - the 5 pre amp tubes were replaced just before I collected it, and I replaced the power tubes (with a pair of Mesa EL84's) just over 6 months ago.

Yesterday, I changed channels (from rythm to lead I believe) and suddenly started getting a very loud popping noise, which then continued across both channels, and even with the amp on standby. This morning I put the old tubes back in....absolutely fine (albeit with a little more general noise on the clean channel than the newer tubes). Tried the new tubes again and got exactly the same loud continuous popping.

Band practice is this Saturday so relieved that putting the old tubes back in has sorted it for now - but is it fairly common for new tubes to fail after 6 months? (literally about 4 days after the Mesa guarantee has expired!).

As the old tubes have rectified the problem I'm assuming it is the new tubes and nothing in the amp that's causing it. Anyone else had similar issues using this (brilliant) amp?

 

[dodger916]

It's not unusual for a tube to go bad in 6 months. Some are bad out of the box. When you say "old tubes" do you have the old power tubes and preamp tubes? If so, I'd start by installing the new power tubes with the old preamp tubes to see if it's the power tubes. If no noise then it's a preamp tube, and you'll want to isolate the bad tubes(s). With the old (good) preamp tubes installed, I would start "rolling" the new tubes, meaning I'd replace the V1 old tube with one of the new tubes and see how it sounds. Then place a different new tube in V1 until you find the bad tube(s).

 

[Danavenell]

Sorry, when I say "old tubes" I mean the old power tubes - the 5 pre amp ones were replaced just before I got it so I don't have the old 12AX7's. I think the pre amp tubes are fine as the problem is rectified by putting the old pair of power tubes back in.

Thanks though, I'll rotate them to see if it's just one that's bad or both....you've answered my main question though, in that I wasn't sure if it was normal for a tube to go bad after 6 months...thanks

 

[bigbean]

Mesa Amps run Hot / Heat is an enemy of component life.

According to the Philips tube chart for EL84s, ( http://tubedata.milbert.com/sheets/010/e/EL84.pdf ) they should have voltages of 250v to 300v with -6 on the other side.

According to the Studio 22+ schematic ( http://www.schematicx.com/view-schematic/mesa-boogie-studio-22-caliber-plus-schematic ) the design drives the tubes @ 390v with -12 on the other side. That extra stress on the output tubes is where part of the magic is. An amp run at the Philips spec would have tubes that last forever but the amp would sound blah. To quote Joe Walsh, "Burning the candle at both ends, twice the light but half the time".

When I'm done with a gig my 22+ is hotter than firecracker. I personally don't subscribe to the "holy grail / NOS / bugle boy made in the middle of the week" line of heuey. Get some Sovteks (probably what the Mesa tubes are anyway) buy an extra set and tape them inside the bottom of the amp. Anything that runs hot is going to eat tubes.

One of the Mesa amps designed during this same period (higher wattage) runs so hot it will desolder the tube sockets. Lets not even talk about AC-30s.

Nothing like a good print rant to vent the cares of the day.

 

[Danavenell]

Thanks for the info bigbean; I had noticed how hot the little fella gets actually; when I put the new tubes in I could smell 'em burning for the first week - I suppose that's a normal part of the bedding in process, but this being my first tube amp I wasn't sure at the time!

I suppose I will have to get some spares...small price to pay for such a brilliant amp though.

 

[bigbean]

Some interesting reading on tube quallity issues. If it wasn't for us pickers there wouldn't be any tubes made at all, anywhere.

http://www.vacuumtube.com/faq1.htm

Pick on.