speaker impedence and power tube overdrive

[thickwood]

My question is simply will a 8ohm amp going into 8 ohm speaker overdrive the power tubes earlier than amp 8 ohm going too 16 ohm speaker ?
Some talk of mismatches as a option but what about the tubes , I have run a 8 ohm amp into a 16 ohm speaker with no worries.
I could be defeating the ability to get harmonic power tube OD going there ?

Thanks for any enlightened folks knowing about this .

 

[robdean]

I don't think there can be a simple answer, other than that the maximum power is delivered when impedance is matched. The greater the impedance of a mismatched speaker, the greater the potential for damage to the amp. A factor-of-two mismatch is generally regarded as usually within failure tolerances. It will change the tonality and since it's less efficient when mismatched I guess you'll get power tube distortion 'earlier' in terms of volume (though probably not very significantly so to the ear), whether 'earlier' in terms of power consumption is complicated but again probably but marginally, with tonality more noticeably affected.

This is an informative article: http://blog.hughes-and-kettner.com/ohm- ... impedance/

I'd make a simple bridged-t attenuator as a power soak: 6dB attenuation will likely help more with power tube overdrive than impedance mismatching. Over 6dB I'd include a treble bleed circuit. With my Rebel 30 I have a box offering 6dB attenuation via 100W resistors, then adjustable additional attenuation via a wire-wound L-Pad with treble bleed. The fixed 6dB cut very significantly reduces the power dissipation in the L-Pad (a component that is a bit vulnerable to being fried). Sounds fine to me despite no inductive element to the load.

BTW a bridged-t attenuator presents a fairly fixed load to the amp no matter what speaker is connected to it, so can be used for impedance matching at the cost of a few dB of attenuation.

Of course the harder you drive your power amp, the shorter the life of the power valves and the greater the likelihood of component failure.