I recently found a Marshall 1960ax 4x12 cab at a pawn shop for a decent price ,I believe.so I put it in layaway will be picking it up in a few months. It says 16 ohm ,100 Watt rms. On the plate.I know my Randall rh200 and my fender 100h are not compatible. So until some day when I can get a matching Marshall head I will be using my peavey valvking 100 Watt head. I was talking to a guy that told me the1960 ax is more for classic rock tone so I don't mind turning the gain down some but my question is so simple buy I just want to make sure. If I turn my valveking switch down to 16 ohms to match the cab is it safe to run a 100 Watt head through a 100 Watt cab or is that walking a dangerous line?
When running tube amps, it is considered safe to either match ohms between the amp and the cab, or run a cab that has a higher impedance rating. In other words, it's safe to plug a 16 ohm cab into an 8 ohm amp/head. It's considered a safe mismatch.

What is not safe is running an 8 ohm cab into a 16 ohm amp/head. That is considered an unsafe mismatch and can get you into trouble.
That's good info,I didn't know I could run my head at 8. Thanks. How about the wattage though is it safe to run 100 into 100 or can that be potentially dangerous?
With the mismatch, you're actually getting less power, or wattage, to the cab. Is it a good idea to run the amp at "11" for a long time? I wouldn't do it. If it's a vintage amp, there are some other things to consider, too.
ok, im thick ,bare with me .so if there is actually an advantage to seting the amp to 8 ohms? im pretty sure the cab is not vintage but i dont think the greenbacks in it are desighn to handle extreme power .so i plan to respect that. i apreciate your input. i will play easy till i can find a nice 50 watt marshall with in my price range
Intentionally creating a mismatch will have a slight impact on the tone produced by the amp. As I already mentioned, it also reduces the amount of power being sent to the speaker or cab. Mesa Boogie discusses this in their amp manuals.

If the amp is vintage, there would be a concern over some of the aging components. Electrolytic capacitors do not last forever. Most technicians recommend replacing them every 20 years, or so. Carbon composition resistors, which were quite common back then, are known to change value over time - they aren't very stable. Lastly, most of the old vintage amps still used only 2 prongs for the power cord. That one is a safety concern. Today, it's not uncommon to find a vintage amp with a modern 3-prong power plug. This means that it's been retrofitted to make it safer. So, playing one of these older amps at full volume for extended periods can be bad for the amp's health - not to mention the fact that tubes don't last nearly as long when constantly run full out. Personally, I wouldn't even run a modern amp at full throttle. It's just too hard on them.