by Ty_Bower » Mon Oct 19, 2009 8:06 pm
Let me see... maybe I can reason this one out.
Pentodes mustn't be operated with a load that crosses past the left of the "knee" in their curve. All other things being equal, the line of a higher reflected load will have a flatter slope. I'd think that might bring you closer to the region that should be avoided.
The A470 is specified with a 4.3K ohm primary impedance. On the 4 ohm secondary tap, that's an impedance ratio of 1075:1. The 8 ohm tap would be 538:1. That would mean a 5 ohm load would reflect 5375 ohms if connected to the 4 ohm tap, and 2688 ohms if connected to the 8 ohm tap.
I suppose the 8 ohm tap connection is technically the "safer" connection, as it keeps you far away from the kink in the curve. In this case, it's quite a ways from the designed load. You might be able to extract more power at such a low load, but certainly distortion will suffer. I'd use the 4 ohm tap myself. I can't imagine there'd be any problem with that. Speaker impedance ratings usually lie anyway - they're always lower than they say.
You're almost certainly safe to try them both, and see which one sounds better to your ears. The 8 ohm tap should seem louder (at the same setting of the volume knob) than the 4 ohm tap, but it should clip much earlier.
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