Mismatching Cab Impedance

[James Freeman]

How it changes the frequency response of the amp.
The graphs are normalized at nominal impedance, around 400Hz.

Mismatching High flattens the curve.
Mismatching Low enhances the curve.

Real amp + Cab:

AxeFx "Speaker Impedance" parameter in the "Power Amp" section, pretty accurate.

I will post how the impedance selector affects volume in the next post.

 

[James Freeman]

4ohm Tap -> 4ohm Cab produces 6dB (half) the VOLTAGE of the equivalent 16->16 setting, with the same curve.

While the ACOUSTIC output is the same.
SM57 on the Cab:

---

That means the CURRENT has to be higher (double the current) in the 4->4 setting to achieve the same power or acoustic output as the 16->16 setting.
So what is healthier for a tube amp in my opinion? The 16->16 setting because current is what causes power loss, vibration and heat at the output transformer.

 

[James Freeman]

Mismatching DOWN

Voltage

Acoustic

-------

Mismatching UP

Voltage

Acoustic

----


Biggest Mismatch In Each Direction

Voltage

Acoustic
+6dB at the resonance with the 16->4 setting.

 

[Humbug]

This is true. Impedance matching is always preferable from a tube and transformer life perspective, but if you're going for an intentional mismatch to tune your amp - one step up (i.e 8ohm tap, 16ohm cab) is usually fine as long as screen current is kept in a reasonable range, and one down can be fine as long as your output transformer is up to the task. I wouldn't go more than one for any length of time, though, especially not if you intend on pushing any volume.

Cool to see actual graphs of the effect! I like graphs!

 

[James Freeman]

CONCLUSION

16->16 healthiest with balanced sound.
4->4 same sound as 16->16 but with more current and heat.
Mismatching Up will make the sound a little flatter.
Mismatching Down will enhance the V curve a little.
Mismatching has very little effect on average acoustic output (volume).

Done and done.

 

[Humbug]

... also, I believe less current through the power tubes will help with their longevity.

Acktchually: A 4 ohm cab on the 4 ohm tap will present the same load to the tubes as a 16 ohm cab on the 16 tap.

 

[James Freeman]

Acktchually: A 4 ohm cab on the 4 ohm tap will present the same load to the tubes as a 16 ohm cab on the 16 tap.

Yeah, I'll fix my post.
But there must be more current at the output transformer secondary, otherwise there is no power equilibrium?
If not, where does this extra current comes from? The acoustic graphs align, this means there is equivalent amount of power at the speaker.

 

[Humbug]

Yeah, I'll fix my post.
But there must be more current at the output transformer secondary, otherwise there is no power equilibrium?
If not, where does this extra current comes from?

Yes. The winding ratio does that. Some transformers actually (or ackchually, depending on how pedantic I come across) get noticeably warmer with 4 ohm cabs.

 

[James Freeman]

So the current in the Primary winding is the same because the power tubes see the same load, but the same magnetic flux in the core translates to more current and less voltage at the Secondary winding with the lower ohm tap.

Since the the secondary winding is made of the same cross section, even if shorter in length at lower taps I will still presume that there will be less power losses and heat at half the current at 16ohm compared to 4ohm tap.

 

[Humbug]

So the current in the Primary winding is the same because the power tubes see the same load, but the same magnetic flux in the core translates to more current and less voltage at the Secondary winding with the lower ohm tap.

Since the the secondary winding is made of the same cross section, even if shorter in length at lower taps I will still presume that there will be less power losses and heat at half the current at 16ohm compared to 4ohm tap.

I have not (and will not ) done the math, but I suspect you are correct.

 

[sleewell]

Very interesting posts. Thanks!!

I've always matched at 16 with one cab or switched the amp to 8 with two cabs.

I feel like buying one of those dealies that measures the ohm of the cab would be smart

 

[James Freeman]

Math says the same amount of power loss at the secondary.
P=I^2*R
(2^2)*0.16 = 0.64W
(4^2)*0.04 = 0.64W

But math says nothing about how more current creates larger electrical field hence larger force -> mechanical vibration with neighboring current carrying windings, and mechanical vibration = heat.
Let's just agree not to go deeper than that and say that more current = BAD.

Secondary windings on the outside.

 

[Humbug]

I feel like buying one of those dealies that measures the ohm of the cab would be smart

I don't know, man. It's a slippery slope. One day you buy a cheap multimeter and before you know it you're winding your own inductors in a flat with more equipment than furniture...

...or maybe that's just me.

 

[James Freeman]

...or maybe that's just me.