Mesa LSS, problem apparently not solved....

[Jared Purdy]

Loud, audible "pop" when switching from 5 to 15 watt mode.

I have read this elsewhere (not sure if it was here), and my Mesa LSS is now doing it in both the 1st channel and the 2nd channel. It doesn't do it when switching from 15 to 30, or back to 15 from 30, but it does it back and forth from 5 to 15. It started doing it today (I noticed), and coincidentally, the amp blew another fuse and a new (2 week old) Mesa 5Y3 last night as I was playing quietly.

Any idea as to the culprit??

I'm taking it in on Moday or Tuesday next week as they can't see it until then anyways (3 Mesa 5Y3's and five fuses in about 7 weeks!). Mesa insisted when I called them about two weeks ago that it was a bad batch of tubes, so I replaced the power tubes with new Mesa's, and put a 3rd Mesa 5Y3 in, which blew last night. Funny thing is, when I put a new fuse in and tunred the amp on (in standby) in the 5 watt mode, it turned on, but there was no sound, and the Mesa 5Y3 didn't light up at all. It remained cold. I put in the Sovtek, and it lights up, and there is sound. For now, I'm running the Sovtek 5Y3 in it, and it's holding up.

Any thoughts are appreciated.

[Steve Conner]

Did you see this thread? Another guy with an exploding LSS.
http://music-electronics-forum.com/t23395/#post197837

Maybe they dumped the bad 5Y3s on Canada?

[MWJB]

"Loud, audible "pop" when switching from 5 to 15 watt mode. I have read this elsewhere (not sure if it was here), and my Mesa LSS is now doing it in both the 1st channel and the 2nd channel. It doesn't do it when switching from 15 to 30, or back to 15 from 30, but it does it back and forth from 5 to 15."

According to reliable sources (Alex R for 1) when switching from 15 to 5 & back, half the remaining pair of tubes is muted/unmuted. In 30W mode it uses all 4 EL84 tubes, 2 on each "end" of the output transformer, 2 "push" & 2 "pull" (you might hear of push-pull amps?). In 15W mode it switches to 2 tubes, one on each end of the output transformer, one pushes the other pulls. In 5W mode it only uses one tube on one end (hence the term "single ended" which you may have heard when discussing amps?) of the output transformer, the other is muted/unmuted. This action doesn't take place when switching from 15 to 30 & back. Can you mute the pop with the amp's volume control? The standby switch can also be thrown when switching from 5 to 15 & back, to mute the pop. Honestly, I have no idea how "normal" this is with regard to the LSS.

When the rectifier fails, it takes the fuse with it.

The other thread mentions a 68ohm 1W resistor, used in 30W mode...if this is the EL84 cathode resistor (I don't know for sure, check with the tech in the store) for the power tubes, it is too small a wattage, I'd expect to see more like 4-5W

[Jared Purdy]

Thanks Steve & MWJB, Steve, no I have not previousky seen that, though I did just read it. MWJB, I am familar with terms that you use, but I'm afraid that does not mean too much!! I mean, yes I am familiar with the terms "push pull", and yes I am also aware that the amp only uses 1 tube in 5 watt, 2 in 15, and all four in 30. Also, in the 30 watt mode it doesn't use the 5Y3, but the solid state rectifier, hence, you can play it in 30 watt mode without the 5Y3 even being installed. That said, according to the Mesa LSS manual, in 30 watt mode, it does use the 5Y3 (for something??), which helps improve the tone.

But I digress....So, is Alex R saying that the pop comes from flipping between 5 & 15 watts?? I've noticewd that it gets more noticeable the warmer the tubes get i.e, the longer I play. Aside from the power tubes, what else would cause the rectifier to fail? Power tube circuitry and related components?

[GregS]

Aside from the power tubes, what else would cause the rectifier to fail? Power tube circuitry and related components?

Possibly the circuitry in the power amp. For example if the power tubes are biased to run very hot.

I can't remember if this was already discussed or not but you might want to observe the power tubes while the amp is on to see if they are redplating (i.e. the outer grey structure in the tube) glowing orange. This indicates conducting excessive current. You can also get an idea if you have one of those cheap infrared temperature measuring devices. Point it at a tube and take a reading. If it is well over 300F this also indicates high current.

[Jared Purdy]

Thanks Greg, I don't have one of those, but I have no aversions to buying tools! However, I doubt that is the problem as Mesa biases their amps on the very cool side of things (30%). Still, their amps are fan cooled which certainly alludes to something hot.

[MWJB]

Hi Jared,

Your Mesa LSS is cathode biased, therefore it won't be biased to 30%, cathode biased amps are always biased hotter than similar designs in "fixed bias" (these are the kinds of amps that are sometimes biased cool). A fan signifies that they are aware heat is an issue, the components that generate the most heat (other than the tube rectifier) are the power tubes and the thing that generates the heat in the power tubes is the bias current.

[stokes]

This has been discussed to death,here and on other forums,but here goes,one more time.The problem with the rectifier failing in these amps is that the 5Y3 is run at the edge of its rated current capacity.It may be fine with NOS 5Y3's,but although Mesa claims to have gotten some heartier 5Y3's made by Sovtek or some such,they still dont have the oomph to run very long.Get yourself a good NOS 5y3,but that isnt foolproof either.Best to put a 5V4 in there and forget it.