Speaker Load with multiple jacks connected and amp power

[ViperDoc]

Apologies for a somewhat rhetorical question, but I want to make sure I understand speaker basics vs. output power.

When using an amp with a 4-8-16 ohm impedance selector switch, the setting applies to both jacks. When inserting a single 16-ohm speaker into the first jack, that's obvious: select 16-ohms. When inserting an additional 16-ohm speaker into the second jack, because these speaker jacks are wired in parallel, you would select 8-ohms on the rotary switch to maintain impedance matching. Right?

I know you can push a 16-ohm speaker load into an amp set for 8-ohms, but not the reverse. But say with an impedance-mismatch when operating the amp, would test-wattages be altered on the power tubes? It SOUNDS like they change to me.

Reason I'm asking is I recently tested an 18-watt Marshall TMB I'd built that was showing 14-watts dissipation on each EL84 tube and wondered if how I'm running the speakers alters that. I have a 16-ohm Celestion Creamback G12M-65 and a 16-ohm Celestion Gold 10 individually plugged into the two speaker jacks at the moment so I could easily study the contribution of each speaker. The switch is set at 8-ohms and it sounds awesome, although with extended playing I'm cooking the EL84s at 14-watts each.

Thanks.

[martin manning]

When using an amp with a 4-8-16 ohm impedance selector switch, the setting applies to both jacks. When inserting a single 16-ohm speaker into the first jack, that's obvious: select 16-ohms. When inserting an additional 16-ohm speaker into the second jack, because these speaker jacks are wired in parallel, you would select 8-ohms on the rotary switch to maintain impedance matching. Right?

Right.

I know you can push a 16-ohm speaker load into an amp set for 8-ohms, but not the reverse.

It depends on the voltage and initial primary impedance. Higher load impedance will be harder on the screens, lower load impedance will be harder on the plates.

But say with an impedance-mismatch when operating the amp, would test-wattages be altered on the power tubes? It SOUNDS like they change to me.

The output power will be different, generally miss-matching in either direction will result in less power out. The idle plate voltage will be the same (at the same idle point), but both the available voltage swing and the current swing (when Vg1 hits zero) will be different.

Reason I'm asking is I recently tested an 18-watt Marshall TMB I'd built that was showing 14-watts dissipation on each EL84 tube and wondered if how I'm running the speakers alters that.

Not much, but lower idle dissipation would let you get by with a bit lower load impedance.

I have a 16-ohm Celestion Creamback G12M-65 and a 16-ohm Celestion Gold 10 individually plugged into the two speaker jacks at the moment so I could easily study the contribution of each speaker. The switch is set at 8-ohms and it sounds awesome, although with extended playing I'm cooking the EL84s at 14-watts each.

You are running an 8Ω speaker load with the impedance selector set on 8Ω, so keep that as it is, just lower the idle dissipation by increasing the cathode resistor value.

[ViperDoc]

Excellent, thanks. Just waiting for a shipment from Mouser and then that cathode resistor will get swapped out. Thank you!

[Roe]

Isnt it safe to run a 16ohms speaker from the 8 ohms secondary simultainously as running a 8 ohms speaker from the 4 ohms secondary?

[martin manning]

Isnt it safe to run a 16ohms speaker from the 8 ohms secondary simultainously as running a 8 ohms speaker from the 4 ohms secondary?

Yes, that'd be the same as two paralleled 16's on the 8, or two paralleled 8's on the 4.

[Roe]

Isnt it safe to run a 16ohms speaker from the 8 ohms secondary simultainously as running a 8 ohms speaker from the 4 ohms secondary?

Yes, that'd be the same as two paralleled 16's on the 8, or two paralleled 8's on the 4.

thanks for confirming. the 8 ohms speaker would get 2/3s of the power, and the 16 ohms speaker would get only 1/3, right?

[martin manning]

Isnt it safe to run a 16ohms speaker from the 8 ohms secondary simultainously as running a 8 ohms speaker from the 4 ohms secondary?

Yes, that'd be the same as two paralleled 16's on the 8, or two paralleled 8's on the 4.

thanks for confirming. the 8 ohms speaker would get 2/3s of the power, and the 16 ohms speaker would get only 1/3, right?

No, they will share the output power equally. The voltage on the 8Ω tap is 1.414x the voltage on the 4Ω tap. Power is V^2/R, so for the 16Ω speaker on the 8Ω tap, the numerator and the denominator are both doubled wrt the 8Ω on the 4Ω tap, and power is the same.