I have been shopping around for a new amp, and I am finding it somewhat difficult to narrow down. I am looking for an all tube combo, but am finding a lot of variation within the same model amp. What could account for this?

For example, I have plugged into 3 peavey classic 30's, two of them sounded really good, for what I am looking for. The third sounded horrible, thin and lifeless. I always use a new MIM strat when testing amps, because that is what I use at home.

I have found this with almost all of the amp models I have tested, so sound great, some sound bad. The lesson I have taken from it is never buy an amp sight unseen.

What could be causing this? old tubes or just variation from the factory?
If it is the tubes, why would a shop not throw a new set of tubes in an amp they are asking $500+ for (not the classic 30s)? it could make the amp sell a lot faster.
Who knows? Laziness or the hope that someone will just buy it online and they can ship the shitty one? I don't have a valid answer really. Interior components could still be an issue, but it's less likely
It's also possible that the speakers aren't broken in on the "bad" one.
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^ yep

It could be a combination of lots of things really. It is not uncommon at all for amps within the same model to sound drastically different from each other. This was particularly noteworthy back in the 60's and 70's with Marshall amps for example. There were so many differences in the components and tubes sourced you could get a dud just as easily as you could get one 'those' amazing ones.

Now components today are manufactured with fairly strict requirements but even still. Tubes will vary, speakers may vary, amps made on Wednesday will be better than Monday, you get the idea.

Now the Classic 30 is a damn fine amp imo. I wouldn't buy one and some may disagree but for the tone/$$ ratio it is pretty good. Therefore, if you played 2 and liked it then chances are your average C30 will be fine too.

Plus, the amps you play at the store are played by lots of people. How many times have you walked into Guitar Center and an amp head is on and not connected to a cab, or it has tubes missing? Or like Matt said, it has a speaker that has not broken in. That could also explain a thin, lifeless tone.

Most of your lower budget (even higher end) amps use crappy tubes anyway.
Quote by gersh_uwec
If it is the tubes, why would a shop not throw a new set of tubes in an amp they are asking $500+ for (not the classic 30s)? it could make the amp sell a lot faster.

The same reason so many amp manufacturers stick crap tubes in amps from the factory. Personally I think if they released them with good tubes they'd sell more and make more money overall but their bean counters obviously disagree.
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May even come down to the room you played the amps in. Every room sounds different and also dependant on where you sit in relation to the amp.
Everything people have been saying. On top of it all, part of the equation could be the variation in the guitars you're using to test the amps. They may be the same make and model but there will be small variations so it's probably best to bring your own guitar with you when you go amp shopping.
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I always suggest bringing your own guitar even though it is inconvenient.
I also suggest bring a pen and paper to take notes.
I also suggest asking to take an amp into the loud room.
I also suggest going to stores at off peak hours.
I'm going to start bringing my own JJ preamp tubes too for the amps I'm serious about
Tubes. One bad preamp tube can completely **** the sound of any amp.
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Quote by gersh_uwec
If it is the tubes, why would a shop not throw a new set of tubes in an amp they are asking $500+ for (not the classic 30s)? it could make the amp sell a lot faster.

A new set of tubes is more money. No one is going to cover the cost of new tubes. The store isn't going to give you a free tube upgrade without charging you extra. And the consumer isn't going to want to pay more than the retail price just because there's a new set of tubes. Most non-boutique amp makers aren't trying give you the best sound. They're trying to give you the best sound they can for a certain price. That means they won't use the best speakers, tubes, and other components that would make an amp sound a lot better.
I think you've probably got this by now. It could be a faulty tube and rooms make a huge difference, but what you are probably finding is manufacturing tolerance. All the components inside the amp and guitar are subject to variation. Resistors by only 5% but capacitors can vary by 30%, valves (tubes), transformers all vary. Add up all these variations and every now and again manufacturers throw up a dog, even Peavey who have a better track record than most. MIM Strats vary MIA Strats even more so.

You know what to do, buy one of the two good ones. That's why shops are so much better than the internet or mail order.