Does a safe mismatch reduce the effective wattage?

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GJgo

GJgo

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Guys, question for you. Say for example I have a 100w head. Say I plug an 8 ohm load into the 8 ohm jack of the cab for a correct match, we can assume that at full tilt the head is putting out 100w to the speaker.

Now let's say we move the speaker to the 4 ohm jack on the head for a safe mismatch. Does this effectively reduce the wattage that's goig to the speaker, or is it still the same?

In some situations I have noticed a change in volume when doing this.
 
It does noticeably lower the perceived output, and is darker to me. But, I'm not sure if it actually reduces the wattage itself, or if it seems that way without actually doing it.
But, I always notice this effect with every amp I've owned...same result.
 
Yes it does. To maximize power transfer the impedances on the primary of the OT must match the total impedance seen with all tubes in parallel. Going in the opposite direction, if you mismatch the secondary OT to the cab, then there’s a reflected impedance mismatch seen on the primary of the OT (through what’s called flux linkage) of what’s expected by the tubes. The mismatch affects leakage inductance and winding capacitance of the OT as well which will change the distortion factor/THD rating.

Many things happen to the tubes as well - their load lines can become nonlinear which change their operating points unexpectedly which is also a part of the perceived power loss.

So there’s distortion, heat losses, nonlinear frequency losses, and then max power losses.
 
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Random but related question, is a “safe mismatch” like the above example ALWAYS safe? With every amp?
 
glpg80 said:
No. It’s hard on tubes and really stresses them. Don’t do it on tubes you don’t trust and definitely don’t do it on POS iron not built well. I’d never mismatch an Engl for example.
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Interesting. I only run Boogies and they all seem good to go with the safe mismatch, never thought about it with other amps in terms of quality.
 
glpg80 said:
Yes it does. To maximize power transfer the impedances on the primary of the OT must match the total impedance seen with all tubes in parallel. Going in the opposite direction, if you mismatch the secondary OT to the cab, then there’s a reflected impedance mismatch seen on the primary of the OT (through what’s called flux linkage) of what’s expected by the tubes. The mismatch affects leakage inductance and winding capacitance of the OT as well which will change the distortion factor/THD rating.

Many things happen to the tubes as well - their load lines can become nonlinear which change their operating points unexpectedly which is also a part of the perceived power loss.

So there’s distortion, heat losses, nonlinear frequency losses, and then max power losses.
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This is great, thanks for the info. The real question here is, say I have a 100W GB cab and I want to throttle it with a Simul 75W Mark head- I think this could get pretty iffy for the GBs. I know I could just run it in 25w Class A mode but that loses a little body, so I was curious if safe mismatching would effectively reduce the power output to make it less likely that I could smoke the speakers.

Or say for example you have your 180W Coli, that on half power mode drops it down to around 60 (still very loud and thick) watts. If you safe mismatch it into this GB cab would it hedge your bets against smoking the speakers?

...Or should I just have two GB cabs and runs a correct match? :rock:
 
^ I think you'll be safe with just the one cab with a correct match, just my opinion. If it was a 100 watt head i'd worry a little... just don't push the master past a certain point and you should be good I think?? But yeah, putting it into a 4 ohm load will reduce the wattage but make it darker sounding from my experience, or you could run it Class A just to be safe.

Ya, best bet is two GB cabs.
 
A "mismatch" creats all kinds of problems.
Can you do it ? yeah. Should you ? No not really.
For best results always correctly match impedance.
 
shar-vell Dan said:
A "mismatch" creats all kinds of problems.
Can you do it ? yeah. Should you ? No not really.
For best results always correctly match impedance.
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In Mesa's manual's they say a impedance mismatch is safe for the amp. This is with Mesa's though, I know that with some other manufacturers out there it's not a safe thing to do.
 
Ran Fender Bassman heads into 16 ohm Marshall cabs for a few years. Was always loud, sounded great and never caused a problem. Knowing a lot less back then, the ignorance is bliss labelling Fender put on the speaker outs (ie. None) was a great way to reduce stress 😉
For a long time now I’ve always tried to have things match.
 
I've run mismatches without issues for years...but, not set permanently. Just for a tonal change. It's quite a different exp going from 16-16 to 8-16, or even 4-16. Tone becomes darker, more perceived low end. Ultimately I match 90% of the time but it's fun to run it differently for the change in tone.
Only tube issues I've ever had were with new amps, with the crap new tubes that came with the amp.

Now, if you run high-low, like 16 ohm out to a 4 ohm cab, that is more risky to the OT and not recommended...
 
Mesa OT's are spec'd for certain mismatches.
That doesn't jive with Marshalls especially old ones.
Ive done Mesa mismatches w/a loadbox.
The Boogie 4 & 8 ohm tap only has annoyed me for decades.
 
GJgo said:
This is great, thanks for the info. The real question here is, say I have a 100W GB cab and I want to throttle it with a Simul 75W Mark head- I think this could get pretty iffy for the GBs. I know I could just run it in 25w Class A mode but that loses a little body, so I was curious if safe mismatching would effectively reduce the power output to make it less likely that I could smoke the speakers.

Or say for example you have your 180W Coli, that on half power mode drops it down to around 60 (still very loud and thick) watts. If you safe mismatch it into this GB cab would it hedge your bets against smoking the speakers?

...Or should I just have two GB cabs and runs a correct match? :rock:
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For my Coli my cabs are 16. I run one 16 ohm cab mismatched to the 8 ohm input with no worries. One greenback cab is not going to hurt this thing lol. Now if it was an EV loaded cab that’s a different story.
 
glpg80 said:
One greenback cab is not going to hurt this thing lol. Now if it was an EV loaded cab that’s a different story.
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I don't follow. I'm not worried about the cab hurting the amp, thinking the other way around. Please explain? Do you mean to say that a higher wattage speaker puts more load on the OT, all else equal?
 
GJgo said:
I don't follow. I'm not worried about the cab hurting the amp, thinking the other way around. Please explain? Do you mean to say that a higher wattage speaker puts more load on the OT, all else equal?
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Higher efficiency speakers can produce a larger slap back flux that can cause more damage to your amp when mismatched. Pull a skinny rubber band against your leg and pop it, not really a problem. Pull back a thick mother fucker and let it go against your skin. Shits gonna welt. The same happens with reactive mismatching.
 
When I do a safe mismatch my amplifiers sounds like a Bogner amplifier, with a blanket in front the speakers.
 
I had a nasty tube failure in a Champ with a 4 ohm OT into a 16 ohm 15" speaker. I was playing bass at bedroom volume and hit a hard low E and it arced big time. Had to change the tube socket.

I can notice my 4 ohm Bassman start to suffer at 16 ohm. Screams at 8 ohms
 
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