I have a small 4ohm tube amp. I generally bypass the tiny 4 or 6 “ speaker and plug into an external cab with matching ohms.

What is the most watts of the speaker that can be run without ruining the 4 ohm amplifier ? Thank you

If the external speaker has the same Ohm rating, and has a power (Watts) rating equal to or larger than the original speaker it will not harm either amp or speaker.

Well the ohms of the small fender champion 600 is 4ohms and so is the external speaker.

the watts of the amp specs are 5 watts ...the external speaker watts are 80 Bad ?? Something doesn’t make sense to me. The ohms match but the watts of the amp as opposed to the speaker watts 80 vs 5 ?
I can’t find the specs of the fender champions internal speaker but I think it’s 15...

The power rating for speakers is the max power they can dissipate without cooking, so the general rule would be to pair an amp with a speaker that can handle at least as much power as the amp puts out - so a 100w amp with a 100w-or-more speaker.
With tube amps you ideally have to match the output impedance of the amp with the input impedance of the speaker(s), but in reality if the output section of your amp is well designed and has enough tolerance you can plug an amp into something with a higher input impedance; I've been plugging my 5w tube amp's 4ohm out into a single 16ohm speaker without any problems or transformers heating up for a couple years without a hitch now.

I recommend you post this kind of questions in the newbie audio engineering section next time, you're likely to get faster and more detailed responses there.

Thank you . I will do that next time.

in electrical theory V=I x R (ohms law)

voltage equals current times resistance.

W = V x I

wattage equals voltage times amps (I).

resistance is measured in ohms, so decreasing the ohms values in a static circuit increases current flow in amps.

if you put a cab with a lower resistance in ohms, than the one specified for the amp, then the current increases and the amp can blow.

a higher resistance cab will cause less current to flow, you wont blow the amp, but less current means less wattage output.

hope this helps.

Buddha

I think you have a misunderstanding of the relationship between ohms and wattage.
If your output is 4 ohms, you’ll want a speaker with an input of 4 ohms regardless of the wattage.
If your amp has a max power rating of 18 watts, you’ll want a speaker with a power handling of at least 18 Watts.
Now, I’m going to touch on something that I don’t know much about, and hope that someone more knowledgeable than me chimes in.
The output stages of tube amps is different than solid state, and they are not as capable of driving speakers that have an impedance rating that does not match the amps output. So for a tube amp, it’s especially important to match the impedance output with the speaker load.

There can be some side effects of running a speaker that has extremely high wattages but they only affect the sound quality, not the amps functionality.
So long as the speakers wattage matches or exceeds the amp its "Safe" to operate. The impedance also needs to match to get the maximum power transferred.

There are other specs involving speakers you should be aware of.

A speakers SPL rating tells you how efficiently the speaker works. Its kind of like miles per gallon with a car. If the SPL rating is high it converts more of the wattage to sound. For every 10dB increase the loudness doubles. A 100dB SPL speaker will be twice as loud as a 90dB. An 80dB will be half as loud as a 90dB.

Next you have frequency response and resonant frequencies. If you've ever heard a speaker removed from a cab it sounds thin as hell. Speakers need to have a certain cabinet size to produce its bass responses. Guitar speakers are NOT high fidelity so many of the cabinets used to house the guitar speakers aren't even tuned. Marshall for example built 8X12 cabs for the Who and when they found out they were too tough to move they got out a saw and ripped them in half. There was no calculations done to get the air volume right. Didn't need it, guitars are midrange instruments so there isn't a need to produce accurate bass frequencies.

PA, Hi Fi and Bass guitar is a whole different ball game. The speakers have to match the boxes and vice versa or bad things happen.
You can have frequencies cancel out, boom when they aren't supposed too, damage the speakers or simply sound like garbage. The air in the cabinet acts like a shock absorber to the speaker and if you've ever driven in a car that has shocks which are worn out or too stiff you wont have a smooth ride.

The materials the speaker are made of and even the designs play a role too. As I said a High wattage speaker can have negative side effects. They typically have much stiffer cones and surrounds. It takes a minimum wattage to just getting the cone to move. Typically you want to push a speaker using between 50~75% of its maximum wattage to nail its sweet spot. If you're only running 6% its sensitivity to guitar strings is likely going to suck and its dynamic response will likely suck equally bad. Its not like you'll hurt anything but tone wise its probably not much of an improvement over a stock combo speaker.

The other thing is most combo's EQ's are "Voiced" to make its stock speaker sound its best. Especially Fenders which I'm very familiar with. You connect them up to an oversized speaker they typically sound flubby and muddy. I've used other combo's like Marshalls and fared better. Most Fenders aren't too hard to re-voice to sound better with a larger speaker but you still want to keep the wattage in the same general range so the cone is lightweight and highly responsive to the strings.