OT primary impedance and secondary impedance recommendation

[jojokeo]

Hi all,
I'm looking for a recommendation & explanation (and a little validation to help clarify a brain fart I'm having with this) for running a combo amp w/ single 8 ohm speaker and adding a second cab w/ single 8 ohm speaker as it relates to the OT's primary impedance. Lets say the amp has a single output 8ohm tap for the OT w/ a primary Z of 6k6 running either 6V6s or 5881s. Would it be best to run speakers in parallel which then reflects the primary Z to ~3k3 or or in series which would reflect primary Z to ~13k? Am I thinking correctly and if so, which primary Z would it be best to run the tubes at when doing this?

[HotBluePlates]

This will just be a temporary situation?

If so, why not follow the example of countless vintage amps (that didn't offer impedance selection), and place them in parallel?

[jojokeo]

Yeah, I know. I was just wondering if I was thinking about this right if it would be better to drop the Pri-z or raise it for these situations.

My thinking is that it would be better to drop it which would provide a better "loading effect" than raising would do. You'd get as good or maybe even a little better tone and response over the other way around.

Would there be any "long term" negative effects from this? Buying two new 16ohm speakers doesn't seem to be warranted, and then the cab-only siutation would always be running the amp at the lower higher Z except when running both 16ohms on occasions. Neither situation is ideal.

Summing up: better to run the primary impedance of the tranny (and tubes) at 3k3 instead of 13k2.

[jojokeo]

*see above modified comment

Impedance is not stationary. It is always changing dynamically along with frequency, termed "frequency dependant".

The worst thing you can do to an OT is run it w/ an open or infinity load. Shorting the tranny actually is less destructive given the two extremes.

There are a few people out there that actually llike to run their output tubes at less than the ideal primary impedance which they claim gives them a better tone. Fender and many others put the extra speaker output jack in parallel with the internal speaker(s) which results in halving the load on the tranny. They are designed to take this w/out any real issues.

[HotBluePlates]

I am not an expert on O/T, but following the rule of thumb, not to plug a 4 ohm load into a 8 ohm output, I would say play it safe and wire the system for series operation and not for parallel operation. 

Almost every blackface Fender has an External Speaker jack. The jack places the added load in parallel with the on-board speaker.

Those amps worked fine for decades with or without the extra speaker attached, hence my original answer.

[jojokeo]

I used to have '69 fastback Mustang built up to the max growing up through high school / college. 1st year MACH 1 body style that the new-age ones are modeled after. Great car and sorry I sold it. But WTF is MTBF and why would you ever want to run such a car at 75mph unless you're in a school zone or creeping home at closing hour after a gig or a bender??

[HotBluePlates]

But WTF is MTBF ...

MTBF = Mean Time Between Failures

And it (operation with an extra speaker attached) generally equals the normal operation of the Fender amps in question.