tubes and running both channels at 50 watts

[toneguy86]

Just wondering how many people here run both channels on their LSC at 50 watts and how they work with tubes in this situation. My assumption is that in this case, it is just the two outside tubes that are working (correct?). Do you only replace those tubes when you retube? Have the inside tubes on reserve and rotate them in at some point? Just curious how others do it.

Mark

 

[djw]

I run at 50 on both, but unfortunately I haven't gigged enough since I got the amp to actually wear out a pair of tubes. (Actually, this is also because I've been experimenting with different tubes so much.) But you're correct, you're using the outer pair only in this case, and if you wore them out you'd only need to replace those. Heck, if you have a matching quad you could just swap the inner and outer pairs when the outers wear out, and you'd be good to go.

Seems like I just had a conversation about this... but the nice thing about going all-50 is you can also experiment with different makes of tubes and you only need to buy them in pairs (instead of quads). In my case, I'm currently running a pair of JJ's in the outer sockets, and I have the stock Mesas in the inners, just to keep the sockets filled. I have a handful of other pairs that I've been fiddling with... so you don't need a matching quad, you just need to match the pairs.

Don't forget too that it's desirable to run a half-mismatched load on the speaker output in this setting: if you have an 8-ohm speaker load, plug it into one of the 4 ohm outputs. It's actually the correct load for this amp at 50 watts, so it sounds much better. (The reason for this is complicated, I barely have a grasp on it myself.)

 

[MrMarkIII]

99.999% of tubes are sold in matched pairs. Matched quads are available from some sellers, but hardly necessary. Otherwise, Mesa and other builders would recommend and/or insist on it, for no other reason than to keep your warranty intact.
The inner pair should match each other, and the outer pair as well.
But there's really no compelling electronic reason to have those two pairs match each other.

 

[toneguy86]

Don't forget too that it's desirable to run a half-mismatched load on the speaker output in this setting: if you have an 8-ohm speaker load, plug it into one of the 4 ohm outputs. It's actually the correct load for this amp at 50 watts, so it sounds much better. (The reason for this is complicated, I barely have a grasp on it myself.)

I must have missed this part about the speaker loads in the manual. Any idea about why this is?

Mark

 

[djw]

Take a look at the back of your amp and you'll see a little message about this under the speaker jacks.

Why does this work? Hmm. I consider myself a pretty smart guy, but this particular item just messes with my head. My amp tech explained it to me, in his typical mad-scientist fashion, and I nodded the whole time, going, "Uh huh, uh huh," with my face all screwed up, and walked out of his shop with a razor-thin grasp on what he told me... which evaporated by the time I'd loaded my amp into the car. So I'll explain as best I can, but I am 100% positive that an electrical engineer, or anyone else here who understands this better than I do, will spot howlers all over it. But it goes something like this.

The output impedance ratings as labeled on the amp are accurate when the amp's running at full power (100 watts). When you cut the output power in half, the output at each speaker jack will also be affected; but it fiendishly means the output wants MORE resistance from the speaker than it does normally at full power. The speaker's higher resistance demands less current from the amp to drive it, which prevents the output transformer from working too hard to push the load; it's an appropriate mismatch that keeps things in equilibrium.

Effectively, it means you'd also be right to match a 16 ohm load with the 8 ohm output in this situation. I thought of it as meaning that LESS resistance in this situation would require the amp to reach farther to meet the resistance properly; the speaker's impedance provides the output with something to push against. If it doesn't get enough resistance, it has to gather more resources to push the speaker properly.

The manual illustrates a bunch of safe mismatches, but I think it avoids this whole question because a) leaving things as-is also constitutes a safe mismatch and b) this gets complicated when you're talking about an amp that can footswitch between channels running at different output settings. I think they just figured it wasn't worth making a big deal out of it because it might look like a design flaw... but I don't hold it against them because I think engineering an amp that would also switch output impedance settings when you switched channels is probably more complicated (and expensive) than it sounds, or is worth. It's also not a sexy thing for the marketing department ("Now with output impedance switching!"). It also is further complicated in amps that have the 10 watt option... which will undoubtedly sound 1000% better if it's hooked up to the proper load, which I don't think it is under stock circumstances.

Dr. Science has spoken. I could be wrong about that last bit -- maybe Mesa did actually figure out a way to noodle with output impedance matching for the 10 watt setting... but I doubt it, mostly because of that little note on the back of the amp.

 

[toneguy86]

Take a look at the back of your amp and you'll see a little message about this under the speaker jacks.

Why does this work? Hmm. I consider myself a pretty smart guy, but this particular item just messes with my head. My amp tech explained it to me, in his typical mad-scientist fashion, and I nodded the whole time, going, "Uh huh, uh huh," with my face all screwed up, and walked out of his shop with a razor-thin grasp on what he told me... which evaporated by the time I'd loaded my amp into the car. So I'll explain as best I can, but I am 100% positive that an electrical engineer, or anyone else here who understands this better than I do, will spot howlers all over it. But it goes something like this.

The output impedance ratings as labeled on the amp are accurate when the amp's running at full power (100 watts). When you cut the output power in half, the output at each speaker jack will also be affected; but it fiendishly means the output wants MORE resistance from the speaker than it does normally at full power. The speaker's higher resistance demands less current from the amp to drive it, which prevents the output transformer from working too hard to push the load; it's an appropriate mismatch that keeps things in equilibrium.

Effectively, it means you'd also be right to match a 16 ohm load with the 8 ohm output in this situation. I thought of it as meaning that LESS resistance in this situation would require the amp to reach farther to meet the resistance properly; the speaker's impedance provides the output with something to push against. If it doesn't get enough resistance, it has to gather more resources to push the speaker properly.

The manual illustrates a bunch of safe mismatches, but I think it avoids this whole question because a) leaving things as-is also constitutes a safe mismatch and b) this gets complicated when you're talking about an amp that can footswitch between channels running at different output settings. I think they just figured it wasn't worth making a big deal out of it because it might look like a design flaw... but I don't hold it against them because I think engineering an amp that would also switch output impedance settings when you switched channels is probably more complicated (and expensive) than it sounds, or is worth. It's also not a sexy thing for the marketing department ("Now with output impedance switching!"). It also is further complicated in amps that have the 10 watt option... which will undoubtedly sound 1000% better if it's hooked up to the proper load, which I don't think it is under stock circumstances.

Dr. Science has spoken. I could be wrong about that last bit -- maybe Mesa did actually figure out a way to noodle with output impedance matching for the 10 watt setting... but I doubt it, mostly because of that little note on the back of the amp.

Ya...way over my head. I did find the little note on the back of the amp. Never saw that before. It does sound really good set up this way. I like the idea of only having to change out 2 tubes also every time. I actually had bought 6 anyway the last time I retubed so I had two spares. It was time anyway and I just put the unused ones in and pulled the two outside ones. I have to say...these are scary amps (in a really good way)! They amaze me every time I plug in.

Mark

 

[marzzz]

Sort of along the lines of this topic- the fan is switched, when should you be running the fan, and when is it OK to have it off?? Nothing is mentioned in the manual.

 

[toneguy86]

Sort of along the lines of this topic- the fan is switched, when should you be running the fan, and when is it OK to have it off?? Nothing is mentioned in the manual.

I just always keep the fan on. I figure it doesn't hurt. I'm not sure there can ever be a down side to keeping the tubes cool.

Mark

 

[djw]

Sort of along the lines of this topic- the fan is switched, when should you be running the fan, and when is it OK to have it off?? Nothing is mentioned in the manual.

I just always keep the fan on. I figure it doesn't hurt. I'm not sure there can ever be a down side to keeping the tubes cool.

Mark

Yep. I can't remember where I read it, but it seems like good advice: if you aren't recording or otherwise in a situation where you need absolute quiet, just keep the fan on.