Speaker impedence please??

[johnnyguitar]

I have a combo with a 8 ohm speaker and a extension cab with a 16 ohm speaker...I have a selector switch on the amp 4-8-16 to choose from...anyone know the best ohm selection for the amp..the amp is a Carr..thanks in advance

[John Phillips]

4 or 8 ohms. Either is a safe mismatch - the total impedance is actually 5.33 ohms. I would try both and see which sounds best, and if you can't tell much difference, go with 4 ohms since it's slightly less hard on the tubes.

[johnnyguitar]

Thanks John..you've always been a big help to me and other forum members

[EADGBE]

I wouldn't hook them up together because they're not compatible. You could damage your output transformer.

[John Phillips]

I wouldn't hook them up together because they're not compatible. You could damage your output transformer.
Sorry, this is plain wrong.

With such a small mismatch (only 33% either way from 4 or 8 ohms on the amp) the only way you'll risk damaging anything is if it was on the point of failure anyway. While this might be true for a fragile old vintage transformer with degraded wiring insulation, even then it's very unlikely that a mismatch this small would actually cause it to fail where it wouldn't have sooner or later otherwise.

If you know the rules for combining impedances in series and parallel and stay within 'one step' of the setting on the amp (ie no more than twice or less than half), you can use any combination of speaker impedances you like.

You may need to consider that the power distribution will be uneven too, but that's only a possible hazard to the speakers - in this case 2/3 of the power will go to the 8-ohm speaker, but since it's the internal speaker for the combo anyway this can't be a risk, and the 16-ohm speaker only needs to handle 1/3 of the power.

[Old Tele man]

excepted from ELECTRON TUBE CIRCUITS, 2nd Ed., 1958, McGraw-Hill, by Dr. Samuel Seely...Chapter 10, page 314, Figure 10-2 and text:

...here's the rationale behind the tube amp "rule-of-thumb" which states: "...total speaker load of 'HALF-to-TWICE' (ie: Z/2 to 2*Z) the 'optimum' load is OK."

http://home.comcast.net/%7Eelmccaul/GTRS/Half_Exact_Twice.JPG

...the key words are:

(A) "...the power is at least 88 per cent of its maximum value for values of Rl/rp ranging from 0.5 to 2.0" [emphasis mine]

(B) "...shows that the power loss is less than 2.25dB for all values of R between 0.5*rp and 2*rp." [emphasis mine]

...this means there's only about a 12% power loss when using speaker loads that are either: (A) One-half (Z/2) of optimum; or (B) Twice (2*Z) optimum...but, that assumes you started with a "perfect" impedance match to begin with, which is seldom true...so, the actual power loss is usually higher, probably closer to 15-20%.

...and that means, about 1/5th of the power which SHOULD be transferring to the speaker is staying within the tubes as HEAT...which can eventually cause "red plates" due to over heating (also depends upon how 'long' you're playing at full power, ie: the TIME-duration).

Note: Rl = Load resistance; rp = Plate resistance
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[Damon]

Nice post OTM, thanks.:AOK

[Old Tele man]

...originally posted 9-25-2005.