Mismatched impedance?

[LarryLarry]

Ok, so the TW Express build I just finished is a tad too loud with EL34's into the correct impedance speaker load. It is also (maybe, need to play with the band to tell) possibly too quiet with 6V6's. So, I tried running EL34's into a an 8 ohm load with the impedance switch set to 4 ohms. I lost a bit of volume and/or high end, but the overall perceptable sound seemed to be right in there.

Any risk at running the mismatched load? Doesn't Matchless do this?

BTW, Weber WOT45LHR OT...

TIA

[Cliff Schecht]

The only risk is getting excessive flyback voltages which can cause the transformer windings to arc. Not sure if the protection diodes actually protect from this type of flyback, it's from the interaction between the speaker and transformer. With that said, I've been running my Express at 8 Ohms with a 16 Ohms speaker for over a year at practice and have noticed no ill side-effects. KF even recommends doing this to cut down on volume without too much negative effects on the tonality.

[Super_Reverb]

Ok, so the TW Express build I just finished is a tad too loud with EL34's into the correct impedance speaker load. So, I tried running EL34's into a an 8 ohm load with the impedance switch set to 4 ohms. I lost a bit of volume and/or high end, but the overall perceptable sound seemed to be right in there.

TIA

The effect on your EL34s is higher reflected impedance due to higher speaker impedance, that is, the EL34s are not being required to source as much AC current as when your OT is loaded w/ 4 Ohms. It's the same effect you'd create by removing 2 of 4 output tubes (one on each side of OT ) on a Fender Twin w/o changing speaker load.

Quality OTs are forgiving. It's about sound - if you like it better, go with it. Most amps will operate just fine with a 2:1 impedance mismatch, higher or lower.

cheers,

rob