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Driving low impedance speakers with tube amp

P

P. S.

Active Member
Hi,
I recently restored my 1st ever tube receiver - an Allied 333 (12wpc). It has 4x 6bm8 tubes in the power amp.. The manual says it can drive 8ohm and 16ohm speakers.

I have 8ohm bookshelf speakers but wondering if I can safely hook it up to 4ohm paradigm monitor 7 towers. The manual for these speakers says they are "8 ohm compatible" which I find truly misleading as I measured 4ohm at the binding posts (!).

I have a question if the same rule applies for tube amps as for solid state in this case - i.e. if you hook up 4ohm speakers to the 8ohm solid state receiver - you may likely overheat it and strain the power section?
Will sound quality be affected anyhow?
Also - are there any considerations about the fact that Allied 333 is over 50 years old (It got new caps but I still run original matsushita tubes!)

Thank you,
-Peter
 
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You measured 4 ohms DC resistance. That is not the same as impedance of the speaker.

It's not unusual to measure 5-6 ohms DC resistance for an "8 ohm" speaker but there isn't an exact relationship. Measuring 4 ohms DC resistance may indicate the speakers are a little less than "8 ohm" speakers but doesn't necessarily mean so.
 
Measuring the DC resistance of a speaker is fairly meaningless, the AC impedance is what matters. DC resistance is generally quite a bit lower because it doesn't account for capacitance and inductance. It's all kinda BS anyway because the impedance varies widely depending on frequency, trying to reduce it to a single number is more marketing than science.

Regardless, tube amps are fairly forgiving; driving a lower impedance load may increase distortion and decrease power output, but probably not enough for you to notice. Monitor 7 are pretty efficient at 93db so as long as you're not going crazy with the volume you'll be fine. If it starts sounding muddy or distorted, back off!
 
Thank you for feedback!

Actually measured resistance was 3.5ohm. In the past I would just add 20% on top of that value to get rough impedance estimate. Seems like this is a pretty ignorant approach ;-)

Guess I will try it out at low volume 1st and see how it goes. Currently my best match with this amp is Norman Labs Model 8. But - Monitor 7 are my favorite speakers out of the whole bunch I have.
 
Since the year of manufacture was well before the FTC regulations governing manufacturers' advertised amplifier ratings, the "12 wpc" spec might be overstated.

In any case, even 12 watts is borderline for a modern speaker designed for considerably more powerful equipment. It will "work" but should be used at low to moderate listening levels only.
 
If it's the original Monitor 7 ("Series 1") or the Series 2- they're rated at about 90dB at 1 watt.

12 watts will give just over 10dB of volume gain over 1 watt. That means that the speaker will be able to do just over 100dB at clipping. Now, that's a SINGLE channel driving a single speaker- with both playing, you should get a bit more- maybe as much as 3dB- more output level. total (close to 103dB). Maybe marginal- but maybe OK, if they're not in an environment with super-loud background noise.

Measuring 4 ohms DC resistance at the terminals, is pretty normal for some "8 ohm" speakers. The amp should still see the effective equivalent of 8 ohms, once the speaker is playing and back-EMF causes the impedance to rise compared to the DC resistance.

Regards,
Gordon.
 
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