Posted Feb 2, 2021, 8:26 AM UTC
Just remember the two things you must never do...don't run mixed loads (8 and 16 ohm) off same amp head at same time and most importantly don't EVER run your head at an impedance higher than the load. For example don't hook a 4 ohm load into a 16 ohm head. It will burn up the amp. You can however run a higher load into a lower load amp safely like 16 ohm cab into 8 ohm head but it will be quieter than if you impedance matched.
Most of these rules only apply to solid-state amps.
I’m pretty sure the ohms thing applies to any type of amp
https://www.guitartricks.com/blog/Understanding-Guitar-Amp-Watts-Ohms-and-Speaker-Wiring
This has a little section about it
No.
With tube amps, the speaker impedance is reflected back through the output transformer to create the plate impedance for the power tubes. Tube are reactive and those power tubes need optimal plate voltage and optimal plate impedance for full output. If that plate impedance goes up or down, by using a mismatched speaker load, the tube output drops, as shown in this data sheet here.
A tube amp can't burn out from driving a lower than expected impedance. The tubes are already putting out the most power when properly matched, they never put out more than that. A power tube is never going to put out more power than the output transformer (OT) can handle. At worst, the higher bias will wear out the power tubes a little faster.
The big danger with tube amps and mismatched loads is flyback voltage from a higher than expected load. This is voltage that works its way backwards through the OT, being increase exponentially by the transformer ratio. This can burn out the OT or do the damage. This is all dependent on conditions...the higher the wattage the amp and the higher then mismatch the more flyback voltage is generated, and the threat depends on how overbuilt the OT is or whether there are diodes to block flyback voltage (not common, but more common on modern amps). A good case study is the '70-s Marshall Superlead, which is infamous for burning out the OT when driving a higher impendace cab.
There is a reason tube amp designers wire up output jacks in parallel, if they have to mismatch, they want to drive the load impedance down, not up. All vitange Fenders ext speaker jacks are intentionally wired in parallel to drive the impedance down when used on combos.
Solid-state amps work very different, their output transistors/mosfets react linearly to load. Double the load, have the output; halve the load, double the output. If the load goes low enough, the transistors will burn themselves trying to put out more power than they are rated for. This is why you NEVER ever got below the posted minimum load on SS amps. BNut, in the hi-fi/radio world, solid-state has been the industry standard for 60 years now, and EEs and amp designers began confusing the rules and concepts as early as the '70s.
what i was saying, for example you shouldn't put a 16 ohm head through 8 or 4 ohm speakers. You can always go lower, but not higher. Thats what the link i included said. I think the damage is to the speaker cones, not the head. this is something i was taught a long time ago. Is that not accurate?
You wrote a lot of text there, and seem to know more about this than i do but im not sure i see this specifically addressed. I really wasn't talking about mixed loads.
again, for example this is what i always thought:
4ohm output head - can be used in 4, 8 or 16 ohms
8ohm output head - can be used in 8 or 16
16 ohm output head - can be used in 16.
i always thought that speaker should match or be higher. is this not the case?
With tube amps, it's actually safer to run a lower speaker load than the amp is rated for (like an 8 ohm amp into a 4 ohm cab), although most tube amps can handle a small mismatch in either direction. Here is an article that goes into more detail.
thats interesting, but ive read so many places that the output should not exceed the ohm rating of the speaker as it is bad for the speaker cones. here is just a few from a quick google search
https://www.guitartricks.com/blog/Understanding-Guitar-Amp-Watts-Ohms-and-Speaker-Wiring
https://blog.live4guitar.com/article/impedance-ohm
https://guitar.com/features/opinion-analysis/what-is-impedance-guitar-amplifier/
they all say they can cause damage. Mesa has this on their website showing "safe mismatches" and none of them show a higher output load than the ohm rating of the speakers. https://mesa-boogie.imgix.net/media/Amplitudes/2013/June/Speaker%20Impedance%20Matching%20and%20Hookup.pdf Unless i am missing something...
Solid state amps are actually more forgiving when mismatching impedances than valve amps and absolutely every rule applies to all kinds of amps. You can easily melt a transformer on a valve amp by mismatching impedances incorrectly.
You can easily melt a transformer on a valve amp by mismatching impedances incorrectly.
Only by using a much higher than expected speaker load.
A power tube will NEVER put out more power than a output transformer can handle, only flyback voltaged from too large a speaker impedance can damage OTs. See my much larger reply below (or above, I never know with Reddit)
don't run mixed loads (8 and 16 ohm) off same amp head at same time
BTW, in theory, this could be done on a tube amp, ...
If we connected the 16-ohm cab to a 8-ohm tap of the output transformer and connected the 8-ohm cab off the 4-ohm tap...then the math of the winding ratios should add up and everything should work out — the tubes would see the intended plate impedance, and output wattage will be balanced between the two cabs.
But that's a "the math works on paper" idea and would really depend on the amp, the transformer, and how the jacks are wired (and how those jacks are labeled so we don't make assumptions). For anyone reading, I don't really recommend trying it, it's just for academic discussion.
We could just mix the loads (5.33 ohms in parallel), and many people do, and then drive it with either the 4-ohm or 8-ohm tap (close enough for rock and roll)...but the tube output will drop some and the 8-ohm cab will get 2/3 of the wattage and the 16-ohm will get the remaining 1/3. Path of resistance and all.
Yes, assuming the speakers you're putting in the cab are 16 Ohm speakers. You can make an 8 Ohm cabinet by wiring the same way (series/parallel) with 8 Ohm speakers.