ohms help!!

Short answer: yes

Long answer:
Your speaker cabinet can safely handle more power than your amp is able to send to it, therefore you won't blow the speaker.
The amp can put out up to 75 watts. In all likelihood you won't have it turned up to full in, so it'll be putting out less than full power anyway.

The Crate speaker will likely be either 4 or 8 ohms (couldn't find it with a quick google), but that isn't critical here. Even though the amp's rating is measured at 4 ohms, I've never seen a guitar amp that wasn't completely safe to use with an 8 ohm speaker.

The wattage measured into 8 ohms will be a lower number, so they use the 4 ohm measurement, 'cause it looks bigger in the advertising.

i have an old polytone amp. it says "135 watts input/ 4 ohms 75 watts output" can i hook this up to a crate G412SL 100watt cab??
OK, here's a couple things. First, is the amp solid-state, or does it have a vacuum-tube power section? If it's a solid state power section, you should be just fine. If it has a tube power output section, then does it have an "Ohms 4-8-16" selector or separate jacks? If it does have a switch or separate jacks for various impedances, again you're good.

However, if the amp has a tube output section, and only has a 4 ohm output jack(s), if you run it into that 8 ohm Crate cab, you're going to be operating the amp into a mismatched load. . This is extremely tough on tubes and output transformers, which isn't a good idea on a brand-new amp, and can be disastrous for an old amp. If I recall correctly, the Crate 4-12's usually have 4 8-ohm speakers, which means that there's no way to wire them to achieve 4 ohms. You would need to have either 16 or 4 ohm speakers to be able to arrive at 4 ohms. A solid state amp generally has no output transformer, and will match any load between the maximum and minimum load ratings.
Short answer: yes

Long answer:
Your speaker cabinet can safely handle more power than your amp is able to send to it, therefore you won't blow the speaker.
The amp can put out up to 75 watts. In all likelihood you won't have it turned up to full in, so it'll be putting out less than full power anyway.

The Crate speaker will likely be either 4 or 8 ohms (couldn't find it with a quick google), but that isn't critical here. Even though the amp's rating is measured at 4 ohms, I've never seen a guitar amp that wasn't completely safe to use with an 8 ohm speaker.

The wattage measured into 8 ohms will be a lower number, so they use the 4 ohm measurement, 'cause it looks bigger in the advertising.

The statement about the power ratings between the amp and that cab are correct. However, the comment about an amp being safe to use with an 8 ohm speaker despite being rated otherwise are not correct in the case of tube amps and can cause undesired sound and behaviors, as well as seriously damage the amp and tubes.

Also, the statement about the power ratings changing with the load impedance only apply to solid state amps, as they have no output transformer. In the case of tube amps, the power..both real and apparent..will remain the same as long as the impedances are matched. Example: a Marshall tube head will put out the same amount of power and sound pretty much the same no matter what impedance you select on the back panel, as long as the speaker(s) it's attached to agree impedance-wise. The differences in sound running that Marshall head into, say, a 4 ohm 4-12 and a 16 ohm 4-12 cab (assuming the impedance selector(s) is/are set correctly) has more to do with differences in the voice coils and other construction differences between the speakers themselves being built for different impedances.

Cheers!

Strat

Even when you change the actual load on a tube amp so that the impedance is not matched to the nominal value, the output power varies relatively little over a wide range of impedances. The reson for this is that pentode or beam power tubes (rather than triodes) are normally used for output, and they're normally hooked up so that the screen grid voltage (which is what mainly controls the attraction of electrons to the plate, not the plate voltage which varies widely with the plate current through the load) remains nearly constant. The actual output impedance is arbitrary, and is usually chosen as the point on the tube's output vs. impedance curve where the distortion is at a minimum (which usually also corresponds fairly closely to actual maximum power.) For various reasons different impedances are often chosen, though. What mostly does change with the impedance is the maximum range of the voltage swing with the signal in the primary winding of the output transformer. If the insulation rating of the winding is marginal, increasing the voltage swing by running higher than rated speaker impedance could lead to arcing and shorting. If the transformer's reasonably rated, it ought to be OK with a 2:1 impedance mismatch, but there are some marginal transformers out there.

I rather suspect that most of the amps brought in with fried transformers allegedly from impedance mismatch actually fried from playing the amp with no speaker plugged in, which makes the output transformer act much like the ignition coil on a car engine and generates voltage spikes in excess of 10KV. One trick many custom amp builders use to guard against this is to put a fixed resistance across the output of about 20X the nominal speaker impedance, with power rated to comfortably absorb the maximum developed across that resistance when the amp's driven at full power through the normal load. That's enough to kill the Q of the transformer and prevent extremely high voltage spikes from "ringing" in an unloaded condition.

On the other end of the spectrum, playing a tube amp with a dead short across the output for a short time won't hurt it, whereas simply the click of plugging in a cable with no speaker attached can instantly fry it. Just the opposite of a solid state amp, which is safe to run with no speaker but will fry if you lower the impedance of the load.

The actual output impedance is arbitrary, and is usually chosen as the point on the tube's output vs. impedance curve where the distortion is at a minimum (which usually also corresponds fairly closely to actual maximum power.) For various reasons different impedances are often chosen, though.

The reason that the impedances are chosen, and that the point is usually at the point where power output is maximum and distortion is minimum, is because the tubes do actually exhibit a certain impedance under a particular operating condition and frequency.

When that impedance is matched, the efficiency is maximized and distortion is minimized, however, variances in voltages, component value tolerances, and other variables in operating conditions including the particular frequency used to measure it means that the maximum efficiency/minimum distortion impedance points aren't always the same, so a compromise is made.
On the other end of the spectrum, playing a tube amp with a dead short across the output for a short time won't hurt it, whereas simply the click of plugging in a cable with no speaker attached can instantly fry it. Just the opposite of a solid state amp, which is safe to run with no speaker but will fry if you lower the impedance of the load.

Shorting the speaker connection causes current through the output tubes and output transformer primary to peak and they will try to pull a lot more current from the power supply and the power transformer than they were designed for. This can cause damage to an older amp, in particular the high voltage windings of the power transformer.

Granted, if only momentary, at most you might blow the high voltage fuse. Still not a good idea though, especially on older amps with old paper insulation between winding layers and where some were built using transformers that were marginally sufficient. You'd probably not hurt an old Marshall with huge Drake transformers, but you'd be treading thin ice on some of the less-well-built makes that were built with components chosen primarily on cost.
If the transformer's reasonably rated, it ought to be OK with a 2:1 impedance mismatch, but there are some marginal transformers out there.

That's the thing..from what I recall of old Polytones, the transformers weren't very 'beefy'. I'd be very careful about stressing one back when it was new, and I'd definitely be wary this many years down the road, especially since one doesn't know what abuse the amp has already suffered.

The other points are spot-on. Thanks for an(other) excellent post Ricochet! :D

Cheers!

Strat

You guys keep using the term "impedance matched." Do you really mean "loaded as specified." In SS amp design, I know true (conjugate) impedance matching is rarely used in power stages, as this results in equal power dissipation in the source (power device and any impedance transformation circuits, e.g. transformers) and the load (speaker cab driver elements). This is condition rarely desirable except in linear RF amplification. For the same reason, I'd be surprised if source-load impedance matching is used in tube power amp outputs: the output transformers would be molten in no time at high output levels. What's the scoop?

Yes, I was using the term loosely, referring to matching the number of ohms on the speaker with the number on the jack.

The plate impedance ratings of tubes are derived from static testing where the signal grid voltage is changed by a tiny increment and the plate voltage is changed to bring the plate current back to where it was. (That's actually the plate resistance, plate impedance changes with frequency but is pretty close to DC values throughout the audio frequency range.) It's so high with pentodes and beam power tubes that it's impractical to match it. Triodes have a much lower plate impedance, and follow the old theorem that power transfer is maximized when the impedances of a generator (meaning the tube) and the load it drives are matched. In practice that leads to maximum power output with unacceptable distortion, so the load impedance applied to a power triode is generally set around 2X the plate impedance. That empirically gives the highest practical power output with low levels of distortion in most cases. (11% less than the maximum power output with impedance matched. Run the load impedance up to 4X the plate impedance, and the output power's down 36% from the value with matched impedances. Not too critical.)