Quick question! buying amp maybe today

[Mister CCJ]

I know this is a stupid question but I'm looking at ordering a Carvin head today and I need to know if I can run it through my Peavey XXX 1x12 combo's speaker just to try it out. Thanks

[microdmitry]

Just don't turn the volume up too much. Also, speakers sound different, so what was designed to sound good with Vintage 30 may sound like ass with another speaker (I'm not sure which one your cab has).

[Mr.WizardNeck]

You could but you just have to match the impedance. Which amp?

[microdmitry]

You don't have to match the impedance if you don't turn it up too much. Impedance only matters when the amp is cranked, and even then it's unlikely that anything bad would happen (you'll lose some volume, but that's about it).

[Mr.WizardNeck]

Oooooooo. Micro. That's dangerous talk for a tube amp. That's a good rule for a Solid State amp but not tube. Tube amps are really finicky when it comes to impedance. I had a V3 and the impedance switch was a bit messed so when I plugged into the cab and had the master volume barely below 1, the speakers were pulsing and looking like they were going to blow. Always match the impedance. With solid state, whole other deal. If you're getting the SX series then don't worry but all their tube heads have impedance switches so you can just match that up.

I myself now just use a Yamaha G100 and I plan to get another one for stereo effects . Best solid state amp I've ever used.

[microdmitry]

That's not "dangerous talk". In fact, it would be more dangerous to do this to a solid state amp. Tubes are very tolerant of impedance mismatches, particularly if set to output impedance lower than the impedance of the cab. This way the impedance seen by the tubes through the output transformer is higher than normal, thus tubes will see less current when they operate, which will lead to a bit of a power drop, but nothing else. But any of this only matters when you really crank it (thereby raising plate dissipation). If, say, your cab is 4 ohms and you set the switch to 8 or 16 ohms, then the tubes will have to dissipate more power, because output impedance they see will be lower. But even then, barring a really severe impedance mismatch, they're unlikely to overheat or blow. Tubes have a pretty generous safety margin.

I could explain in more detail if there's interest.

[Mr.WizardNeck]

Then why do speakers start pulsing in and out as power tubes start flashing?

[microdmitry]

How do I know, maybe your amp was broken? If your tubes can't cope, they'll just red-plate, not flash. Flashing means a short. If you ever see a tube flash, turn off the amp immediately and replace that tube.

[Mr.WizardNeck]

The thing was, all four were flashing but when impedance was switched, they went normal and the pulsing stopped. It was very strange.

[vaitamin7v]

were any medical stimulants involved before flashing lights.

[Mr.WizardNeck]

Originally Posted by vaitamin7v

were any medical stimulants involved before flashing lights.

Nope. It's was the blue flickering and only really visible from the back of the amp.

[elcid]

Here's what I found out

Tube amps are more tolerant of low impedance mismatches, but are intolerant of opens and higher impedance mismatches. Using too high an impedance for a tube amp can turn the tubes or output transformer to smoke.

the load of the speaker is directly linked to the power drain of the output tube section; which means you can nuke the entire power section - fry the limit resistor, cook the transformer and burn up tubes like a christmas tree.

[Mr.WizardNeck]

Damn those intolerant tubes.

[microdmitry]

Originally Posted by elcid

Here's what I found out

Tube amps are more tolerant of low impedance mismatches, but are intolerant of opens and higher impedance mismatches. Using too high an impedance for a tube amp can turn the tubes or output transformer to smoke.

the load of the speaker is directly linked to the power drain of the output tube section; which means you can nuke the entire power section - fry the limit resistor, cook the transformer and burn up tubes like a christmas tree.

Baloney. Yes, it's dangerous to run the amp with no speaker connected. Everything else is perfectly fine. And under no circumstances can you "fry the output transformer" by mismatching impedance. In fact, you mismatch the impedance when you pull two power tubes on a 100W amp to reduce power, and the remaining two tubes work just fine.

[microdmitry]

Just to clarify, I'm not talking about extreme impedance mismatches here (i.e. when you plug a 4 ohm cab into an amp set to 16ohm or vice versa). I'm talking setting amp to 8 ohm and connecting 16 or 4 ohm cab. While not optimal, this will work fine as long as you don't crank the amp to 11. The key thing to understand here is that there's a safety margin in both tubes and OT. This safety margin is required because speaker itself has a highly non-linear impedance when you lay it over the frequency axis. Impedance will gradually rise before speaker hits its resonant frequency, then fall, and then rise again steadily all the way beyond its cut-off frequency.