Can someone explain OT impedance matching/mismatching.

[leadfootdriver]

I read that the express is mismatched. How does that work?

[rdjones]

In order to determine what a "mismatch" is you first must define what constitutes a match.
For the HiFi world it might be lowest distortion (or not).
If your looking for highest efficiency it might be maximum power transfer (or not).
Since these two do not occur at the same impedance there is no universal agreement as to what makes a true match.

In a guitar amp we might not even care about low distortion, in fact quite the opposite.
We do care about power output to a large degree, but even more so about tone.

If the goal of the designer (KF) is a certain type of distortion and less of another, or to acheive a particular tone, then whatever impedance that occurs at would be a correct "match".

In my opinion, the Express is a perfect "match".

YMMV

RedDog Steve

[leadfootdriver]

Is it safe to say that an OT with a higher impedance will be quieter because it has a higher resistance? What in the amp is working harder because of the higher resistance?

The BOM for the Express calls for a 6.6k OT. I have a Hammond 1650KA laying around that has a a PI of 3.4k, so I wonder if it would work.

[LeftyStrat]

Is it safe to say that an OT with a higher impedance will be quieter because it has a higher resistance? What in the amp is working harder because of the higher resistance?

Not really. The tone will change.

The BOM for the Express calls for a 6.6k OT. I have a Hammond 1650KA laying around that has a a PI of 3.4k, so I wonder if it would work.

The tone will be different. But if you double the speaker load you will be at 6.8k.

Transformers are just ratios. Put an 8 ohm speaker on the 16 ohm tao and you reflect half the load.