From London Power Kevin O'Connor FAQ on impedence matching
Q: I thought impedance matching was critical. Some designers say the output transformer must be changed if you want to use different output tubes. That seems awfully expensive.
A: It is awfully expensive, and also a ridiculous suggestion. There are two issues here, though; one is the notion of “impedance matching”, and the other is simple design preference.
As stated throughout our TUT book series, speaker load impedances and reflected loads to the output tubes are all “nominal”. An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.
A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the “power bandwidth”. If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.
The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don’t behave like transistors.
The design issue for impedance matching comes into play when a designer takes the approach that “everything is critical”. In some circuits, this may be the case. Tubes don’t really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says “this is good”, “this is bad” and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objective.
Design approaches are dealt with in our book TUT4.Q: An “expert” suggested that I change my speakers to ones that match the highest impedance tap on my amp. How do I do this and still have the option of using a second cabinet when I play out? I think I would need three cabinets to achieve this.
A: Yes, and what a waste of your money.
Not too surprisingly, this is the same expert as in the tube-pulling/power reduction question. He really should stop talking about transformers.
Rest assured, the impedance taps on your amp are there for your convenience, to use as you will. Connecting the rated cabinet impedance to the identical rated tap selection will get you the rated power bandwidth of your amp into that load. As stated above, any “mismatch” reduces power and bandwidth, and that is all.
If you are using your 4-ohm cabinet and the 4-ohm tap, does it matter if the 16-ohm tap is being unused? Of course not. This subject is explained in detail in our book TUT3, as the “Myth of Encompassment” – a myth created purely to sell speakers and transformers. To unsuspecting players and readers of the “expert’s” column, it is no more than a scare tactic.
Transformer designers take into account the loads to be connected to the device. There is limited space in the winding window for each lamination size, and the designer wants the space to be fully utilized. The percentage of space used is the “build”. Ideally, all windings are used all the time, to keep parasitic effects to a minimum. When there is a tapped secondary, some of the secondary may not be loaded under certain conditions, so those “free” parts of the winding can potentially upset the parasitic balance. The amount of upset is usually so small as to be insignificant, even in hi-fi where such a thing might matter. In MI, there is no concern whatsoever.
In most amps, you can set the impedance selector to whatever sounds best. The one caveat is: NOT in English amps. Having replaced more Marshall OTs than anything else, I would advise that the impedance selector always be set to the rated load, or less.
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