I have recently bought a few cabs and amps so I have been searching and reading about this a bit.

There is lots of good info in this thread. If you really want anything that is close to a hard and fast rule, here is what I have ended on. (There are always exceptions and it's up to you to decide what works/sounds best for your equipment. Keep in mind that I'm also some random internet chump, so take this with a grain of salt.)

Most important is to know if the amp is tube or transistor.

Transistor amps can go higher (ie if the head is 8ohm, it can deal with 16ohm), but not lower (8ohm into a 4ohm cab will make the transistors try to pull twice as much current and they will get nice and smokey.)

I found a good piece of advice from Scott in the archives about tube amps:

The way I understand it, you can tell if you're causing a problem by running too low impedance speakers... all you need to do is play the amp like you normally would, and after a couple minutes, touch the output transformer with your hand and feel how how it is... if it's warm or even a little hot, that's alright. If it's so hot that it burns your hand to touch it, you shouldn't run that low of an impedance. I usually check the output and power transformers, both of them, after about a minute, and again after another minute or two, and again after 10 minutes or so... I've never had either transformer be too hot to touch.

My amp tech guy has told me:

Run the head into a higher ohm cabinet. Ok, but not ideal.

Run the head into a cabinet that is rated lower. Buy a new transformer. (and probably power tubes, maybe some caps, a new fuse, and you could use some new tolex.)

J. Burns wrote:

Thomas the Hungarian wrote:as far as a finally word on the subject, not sure if that will ever happen there will always be a debate.

Fuck that. We're settling this right here, right now.

juice wrote:sorry, a third post...

No apologies necessary.

Running the bassman at 16 ohms would be bad because the impedance the plates would see is around 17K. For 6l6's, this is very bad.

Running it at 2 ohms would theoretically be around 2200 ohms which is also pretty bad on 6l6s unless you had four of them.

What would be normal like if say, I was running a 4 ohm load? Would 8 be bad as well?

For that transformer, (primary of 4.3K) running it at 8 ohms would be pretty much fine, because you end up getting an impedance around 8K which is still pretty close to the design range of a 6L6, and the schumacher transformers in the fenders (especially the bassman) is generally well built. This is not so for many amps/transformers though. However, running an amp at a higher impedance is generally fine on a transformer but it will take it's toll on the tubes which are operating at an improper impedance.

The reason there is no *definitive* answer is because not all amps have the same transformers, tubes, or operating voltages.

Howver, if you have a 16 ohm cabinet with an even number of speakers, it can be rewired to 4 ohms. You either have:

A. 2 - 8 Ohm speakers in series, so wire them in parallel.
B. 4 - 16 Ohm speakers in series-parallel, which can all be wired in parallel
C. 4 - 4 ohm speakers (not likely) wired in series which can be wired in series-parallel.

If you're really worried about rewiring the cab because you use it with another amp, you could wire in a switch next to the jack that would let you switch between 4 or 16 ohms depending on which amp you are using.

Hammond transformers are beasts. These are what was used in traynors. I'm putting a hammond 1650N in my home built amp, and it's bigger than my power transformer (which is generally designed for a 50W amp) Hammond rates the 1650N as a 60W transformer, however this figure is basically with very little THD and a broad response. For guitar, it's probably more like a 85W transformer.

So, if I can condense this, I could tell myself; 'high ok, low go pay'?

J. Burns wrote:On impedance mismatching.

O.K I'll try.

1. Transistor amps are voltage sources thus the only variable is the load Z. Reduce the load Z and ohms law accurately indicates the the output power increases proportionally. The speaker Z doesn't matter unless you exceed the specs.

2. Tubes. Some amps are more tolerant of stress than others. Some tubes are more tolerant than others. Things that stress and ruin amps and/or tubes.
a. Too loud for too long
b. Too hot - poor ventilation
c. Age
d. Speaker Z too low and you risk overcurrent in the tunes and transformer. You will shorten tube life and in rare cases the transformer as well. IT DEPENDS on the quality of the amp (see previously mentioned Traynor)
You may saturate the transformer with too much magnetism to the point that it reaches diminishing returns and create distortion (may or may not be bad as far as tone goes).
You may also saturate the tubes to the point that the tubes overheat, the plates turn red and reach diminishing returns and create distortion (may or may not be bad as far as tone goes).
e. Speaker Z too high, and you risk damage from back EMF. This is more likely in high power (especially British - Orange, HiWatt, Marshall type) amps 100 watts and over. Back EMF can arc and destroy power tubes instantaneously, arc and burn tube sockets and very rarely arc inside a transformer. Low power amps this is unlikely.

3. Don't be fooled. There is NO "best practice" advice for mis-matching impedance because every model tube amp and tube combination and amp architecture (Class A, Class AB, ultralinear, feedback circuits, etc.) reacts differently. Not even all amps of the exact same model will tolerate stress the same way due to manufacturing tolerances, tube quality, abuse and age.

4. There is a BIG REASON they put the Z on the back of the amp and often a method of selecting it.

ubercat wrote:So, if I can condense this, I could tell myself; 'high ok, low go pay'?

It appears you can go high to a point. I'd be wary of going more than double the rated impedance of the amp. I'd say it's more like "Four is swell, eight is great! Go to sixteen and you're gonna get raped!"

I threw in a new fuse and some old power tubes, and the Bassman appears to be okay for now. I'm having the cab rewired for 4 ohms as I speak. A layman's guess is that 16 ohms arc'd the power tubes. New ones are on the way. All's well that ends well, I guess. I just bought this amp a couple weeks ago, and am totally in love with it, so I was despondent when it died.

A mod that I'm considering doing is replacing the OT with a Mercury Magnetics one that has 4, 8, and 16 taps, and then installing a three-way toggle where the second speaker jack used to be.

Thanks for your help. everyone!

J. Burns wrote:
I threw in a new fuse and some old power tubes, and the Bassman appears to be okay for now.

Whew, you had me worried James!

could be that the new tubes (one of them, most likely) was bad. I bought a fricken sextet of matched 6550's for my SVT and spent forever trying to figure out what was wrong with the amp, when it turns out what was wrong was that three of the tubes were either dead or not working so well.

they should have a warranty, if you bought them from a dealer, even an eBay dealer, who's legitimately getting new tubes from a manufacturer.

for what it's worth, if you're not checking the bias when you're putting in new output tubes (and even if you are, really) I'd recommend taking standing there and looking at the tubes for at least the first 5 minutes of operation, even 10 or 15, to make sure that the new tubes don't start glowing red/orange. a little blue glow is normal, and tiny glowing spots (the heaters) are normal, but if the metal plate inside the tube starts to glow, shut the amp off right away. it's either a fucked up tube, or the amp's biased way too hot and it's going to kill the tube very quickly.

Okay, in the solid state world, amps have a lowest impedance setting. Anything higher than that is okay.

On most Fender TUBE amps, you can go up to double or half.

On any tube amp, just make is match.. get it as close as possible,and you're never totally safe unless it matches, especially at very high volume levels.

Ben

Worst mismatch I ever tried (since I learned about impedance anyway) was a YBA-1 with three cabs hooked up to it (two 8x10" and one 6x10") for a total load of less than 2ohms. Expected load for that amp is 8ohms. It sounded pretty great with (formerly) my '74 SG Special plugged straight in. Amp still works fine. I only really beat on it for maybe a half hour though.