The dreaded speaker impedance question - again!

[Fred G.]

I have a Fender Bandmaster (non-reverb) in which I have modded the phase inverter to '59 Bassman values. The power amp component values have been unchanged, although the parts have been upgraded over the years.

I run this amp into the required 4 ohm load.

I'd like to run a 1-12'' extension cab out of the extension jack and tilt it back as a stage monitor for myself. Doing this would result in a 2.67 ohm load.

I have read for years that if you're going to mismatch impedance, it's better to go into a HIGHER load, but I read some posts and articles somewhere not too long ago which said actually the OPPOSITE is true (mismatch into a smaller load than rated if you must mismatch).

Can somebody address this (again) ?

Thanks,

Fred G.

[R.G.]

I have a Fender Bandmaster ...I run this amp into the required 4 ohm load.

I'd like to run a 1-12'' extension cab out of the extension jack and tilt it back as a stage monitor for myself. Doing this would result in a 2.67 ohm load.

It's fine. Do it.

You hear all sides of this. Fact is, mismatch of 2:1 on a tube amp is more of a tone control than anything else.

Tube amps can die from open speaker outputs as a result of some issues I don't feel like typing again. But pentodes and power beam tubes (6L6s) are kind of power limited. Lower load impedances won't usually hurt a tube amp that's not already otherwise on the edge of some cliff. For instance, I would be a lot more cautious about a Marshall, in general.

The power output of the amp will go down a bit when you do this.

[Fred G.]

Thanks, RG. Actually, I seem to remember you may have been the one who had posted this some time ago, or were involved in the thread, maybe? I can't remember where I read it, it was quite an interesting thread/article, though.

If I may ask, why is this a more serious issue with a Marshall?

[Steve Conner]

The output stage in a vintage Marshall is poorly designed and on the edge of self-destruction normally. It doesn't take much more to push it over the cliff.

(As any amp geek knows, Jim Marshall copied the Fender Bassman, but he forgot to increase the screen resistors when he changed the power tubes to EL34s, and he used the wrong impedance of OT. Somehow this became the defacto standard for EL34 amps.)

[R.G.]

The output stage in a vintage Marshall is poorly designed and on the edge of self-destruction normally. It doesn't take much more to push it over the cliff.

(As any amp geek knows, Jim Marshall copied the Fender Bassman, but he forgot to increase the screen resistors when he changed the power tubes to EL34s, and he used the wrong impedance of OT. Somehow this became the defacto standard for EL34 amps.)

Yes~! What you said!!

[MWJB]

To reiterate what Steve & RG say...

Some Marshalls were built with 2.5K primary OTs (lower end of the range for 6L6/EL34), reducing the speaker load, reduced the OT primary further, overstressing the tubes. Your bandmaster will have a primary Z between 3.6k & 4k @ 4ohms, reducing speaker load to 2.67ohms will reduce that to 2.4K-2.67K...which is just what Fender did with the 3x10" tolex Bandmaster. Go ahead.

[Fred G.]

What GREAT info! Thank you all for sharing - cleared up some long-held misconceptions in my head!