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Hi guys---- A friend has a tube amp with 8 Ohm outputs and has plugged in some sensitive 6 Ohm speakers. He's getting a hum on them but, on 8 Ohm speakers it seems quiet.
What should I tell him to do?

The reason, I think, is in the sensitivity of 6 ohm speaker, that is more than 8 ohm sp.
He can buy speaker with the same sensitivity lake 8 ohm sp., or adjust PS to receive small 100 Hz ripples.

in other words, it's not an impedence mismatch that is the problem, rather, decent speakers exposing limitations in the amplifier/preamplifier....time to open the can of worms, and start dissecting amps/chasing holy grails

I have done some reading and some suggest to put the speakers on a 4 ohm output (if there is one)

Connecting 6 ohm speakers to 8 ohm outputs should not normally cause a hum to suddenly appear, perhaps the low frequency response of the 8 ohm speakers was less efficient, or, conversely, the 6 ohm speakers have better LF response and efficiency.

The only event that should occur, is that the 6 ohm speakers would reflect back to the OPT primary, and cause it to appear as 75% of the original impedance.
I.E., if the OPT is 5K, then it would "appear to be" 3750 ohms. All that should do, is to increase both the power output, and THD slightly.

/ed B

Do you have a scope? Maybe the amp is having an "allergic reaction" to the speaker's lumped equivalent and has gone into oscillation?

Thanks for the information I'll run some checks.