Amp buying checklist + how to tell if transformers in amp are good when buying?

[itsGiusto]

I've bought used amps many times, but sometimes it was retroactively a crap shoot because I didn't really check everything I should. This was especially true before I learned how to build and maintenance amps, but it has gotten better now.

I wanted to make a list of important things that one should check for anytime he buys a used amp or combo amp, related to functionality, not cosmetics. I'm also mostly interested in checking things that would be expensive to fix. The things I can think of are:

1. Amp turns on
2. Amp makes sound
3. Speaker cone looks undamaged
4. Speaker frame looks unbent
5. Inside chassis, nothing looks burnt
6. If you know the circuit layout, inside chassis looks like you expect
7. No caps look like they're leaking
8. If you have a voltmeter, check voltages including B+ and tube pins

One thing I've never been sure about is how to tell if the transformers are good. Most components can always be replaced cheaply, except for speakers and transformers. Speakers you can usually tell if they're damaged by looking at them. What about transformers? Is there some way of determining the health of those?

 

[Engine Swap]

If it powers on and sounds OK, they’re good.

 

[bebopbrain]

Not much goes wrong with magnet wire wound around a core. The wires can short or open up when abused. If the amp delivers rated power cleanly, you are good.

 

[Pete Farrington]

Turn the amp volume up full, control the SPL using the instrument volume control.
Can the rig achieve the expected level of ‘loud and clean’, with no unpleasant audio artefacts? eg buzzes, rattles, rasping, crackling.
Turn up louder, to push the power amp into overdrive; again, check for unpleasant artefacts, try to get a visual on the output and rectifier valves to watch for any lightning storms.
Yes, it’ll be loud, but a few minutes loud should eliminate the bad stuff, and save a lot of grief later.

 

[Powdog]

Transformers fail by shorting out. As mentioned, they work or they don’t. No way to predict that. Well made iron should outlast everything else inside an amplifier.

 

[Phrygian77]

Turn the amp volume up full, control the SPL using the instrument volume control.
Can the rig achieve the expected level of ‘loud and clean’, with no unpleasant audio artefacts? eg buzzes, rattles, rasping, crackling.
Turn up louder, to push the power amp into overdrive; again, check for unpleasant artefacts, try to get a visual on the output and rectifier valves to watch for any lightning storms.
Yes, it’ll be loud, but a few minutes loud should eliminate the bad stuff, and save a lot of grief later.

The only thing I'll add to this is that output tubes do give off a little display of what you're doing when they're running full tilt, and that's normal.

 

[CCK1]

I don’t know of any way to determine that a transformer is “sick”, or there’s impending failure. They work or they don’t.

 

[itsGiusto]

Turn the amp volume up full, control the SPL using the instrument volume control.
Can the rig achieve the expected level of ‘loud and clean’, with no unpleasant audio artefacts? eg buzzes, rattles, rasping, crackling.
Turn up louder, to push the power amp into overdrive; again, check for unpleasant artefacts, try to get a visual on the output and rectifier valves to watch for any lightning storms.
Yes, it’ll be loud, but a few minutes loud should eliminate the bad stuff, and save a lot of grief later.

To be honest I don't know if I've ever played a dimed amp that is bigger than 5 watts without it sounding buzzy, rattly, etc. Even if the amp itself doesn't buzz and rattle, it's so loud it'll make other things in the room rattle, so it's hard to tell what the actual problem is.

 

[schmee]

To be honest I don't know if I've ever played a dimed amp that is bigger than 5 watts without it sounding buzzy, rattly, etc. Even if the amp itself doesn't buzz and rattle, it's so loud it'll make other things in the room rattle, so it's hard to tell what the actual problem is.

Yeah. It's not a bad test, and could point out an issue or two, but dont expect it to sound real clean necessarily. The amp could crackle or hum a lot up there though, or the speaker have problems, so it's a valid test.
But the amp should sound pretty clean at maybe half to 2/3 the way up I suppose. Depending on the amp.

 

[Pete Farrington]

I don't know if I've ever played a dimed amp that is bigger than 5 watts without it sounding buzzy, rattly, etc.

I was suggesting to check for unusual artefacts. Sorry for not stating that clearly.
If you hear them, then you’ll know.
But if you don’t push the rig hard, then they may not manifest, so you won’t hear them, and you risk buying a pig in a poke.
An output transformer in the early stages of failure can have intermittent crackling, ie which only manifests on strong signal peaks.
Same for speakers with failing glue joints or cone surrounds, ie it may require a strong signal peak for it to cause a horrid weird noise.

output tubes do give off a little display of what you're doing when they're running full tilt, and that's normal

Yes, the normal condition is the standard blue glow in output valves that tends to get modulated by high power signals.

Lightning storms inside the valve envelope are different, generally they'll be accompanied by super loud crackling.

When you’ve experienced this stuff, you’ll understand what I’m trying to convey, it’ll be obvious when it happens.

My point is that it may not happen unless you invoke the necessary condition of the amp being worked hard.

 

[Lowerleftcoast]

In addition to above.
Check the heat. You should be able to put you hand on the transformers for at least a brief period. Most will only get warm.
Use your nose. Ooooh that smell. Can'tcha smell that smell...
Look for the drippy goo.