I just want to be sure my understanding is correct here.

I have a Blackface BandMaster head and and when I looked up on the Fender Amp. Field Guide it stated that the amp ran 2 speakers in parallel for a total of 8 ohms. Therefore, each speaker would be rated at 4 ohms...Correct ?

Nope! Equal speakers in parallel will work out to half of one - so two 4 ohm speakers in parallel would give a total of 2 ohms. To get 8 ohms you would need two 16 ohm speakers.

Go back and read the Field Guide again, Dave! It says:

"Speaker/Load:

2 x 12"/4 ohms total (8 ohms each in parallel)"

That says you take two 8 ohm speakers, put 'em in parallel and the total load is then 4 ohms.

I can't find any other sources that state that the Bandmaster is rated for 4 ohms impedance (searched the internet).

Hey, the Field Guide told the truth! It's 4 ohms!:smile:

I'm running into a 1 X 12" cab with a speaker of 8 ohms impedance. Therefore, even though this is a mismatch, the only real concern is some loss of volume...Correct?

Correct, but in practice you won't hear it. You'd need a good lab to measure it!

Will I see any change in tone if I match the impedances?

(just curious)

Maybe yes, maybe no! Might sound better, might sound worse! Might not even be noticeable!

The reverse of this is ( i.e., 8 ohms amp impedance into a 4 ohm speaker impedance) harmful to the amp...Correct?

Dave

No, not really. Tube amps can handle most mismatches without really complaining. Worst case would be plugging a 16 ohm cab into a 4 ohm speaker jack. That MAYBE could cause some damage, if you played for 12 hours straight with the amp dimed to 10.

A mismatch is a mismatch and one is not much different from another to the tubes.

The best way to understand it is to think about a voltage transformer. If you have a 12 volt output transformer that is designed to work from the 120 volts from the wall outlet you know that it is a fixed ratio device that works both ways according to its ratio. 120 v to 12 v is a 10:1 ratio. If you fed 12 volts into the secondary it would then step up to having 120 v on the primary.

Similarly, if the input voltage was increased to 150 volts the secondary voltage would also increase by the same 10:1 ratio to give 15 volts.

You can never get anything other than that 10:1 ratio, whether you are stepping a high voltage down or bringing a low voltage up.

Output transformers are exactly the same, only they transform impedances rather than voltage. Impedance is like AC voltage resistance and is also measured in ohms.

A pair of 6L6's usually are run at a 6600 ohm plate load. The OT steps this down to speaker load levels, like 8 ohms. This is also done by a fixed ratio of turns of wire in the primary coil and in the secondary. The exact number of turns is a bit complicated and we need a formula to calculate it. Voltages might use 100 turns on one side and 10 turns on the other for a 10:1 ratio but impedances are more complicated and so the turns ratio would be something not so obvious but still, the important thing is that it will be a fixed ratio. To get 4 ohms, 8 ohms and 16 ohms we use taps on the secondary coil winding to get the right number of turns to ratio that impedance up to 6600 ohms for the 6L6 plates.

So if we plugged an 8 ohm speaker into the 8 ohm jack we get 6600 ohms on the tubes. The same if we put a 4 ohm speaker into the 4 ohm jack. The tap on the secondary coil will adjust the number of turns and thus the ratio to make sure we get 6600 ohms.

Now if we made a mistake and plugged that 8 ohm speaker into the 4 ohm jack the ratio will stay the same but because the secondary load was doubled to 8 ohms the result on the primary will be doubled also to 13,200 ohms!

This looks like a big difference but if you ever look at the data sheets for a particular output tube you will usually see some curves of typical conditions showing power out, distortion and maybe other things against plate load. The power curve is always kinda broad and the tube will put out reasonable power over quite a large range. Interestingly, the points of best distortion and most power output rarely match! Guitar amp transformers usually go for max power and hifi OT's best distortion figure.

Hope this made sense, Dave! If it doesn't, it's my fault for not explaining it better!:smile:

:food-smiley-004: