Parallel vs. Series Wiring When Mixing SPKR Power Ratings

[ViperDoc]

I'm doing some planning for a High Power Tweed Twin build, 4 x 5881. Essentially stock circuit, no funny business, except I will add a PPIMV (purists, please don't shoot me). I have it on strong recommendation from the owner of a well-known speaker manufacturer that their speaker ratings are posted low, and a certain favorite of mine can handle more than the data sheet says. I have no reason to doubt this, just looking for some opinions.

The stock Fender 5F8A circuit puts out about 80 watts. I know the safe call is to feed that to speakers that total twice the rated power. One of my planned speakers is rated at 80 watts, the other I want to include is rated at 40 watts ("program power" of 70 watts on paper). What would be the actual difference in power handling if I wired the 80 watt speaker in first position in series with the 40 watt speaker for a total 16 ohm load (2 x 8R) vs. wiring them both in parallel for an 8 ohm load (2 X 16R)? I seem to recall the paper handling on either would be 80 watts owing to the 2 x 40W rule in this case.

I guess my question is, when is this rule made to be broken? I recall reading about Brad Paisley preferring a Celestion Gold 40W in series with a Celestion Blue 15W for his 4 x EL84 amps, which are around 36 watts, so that comparison seems somewhat even. Thanks.

[pdf64]

I suggest to build the amp and measure its actual overdriven power output before committing to speakers.
On the face of it, if you overdrive the amp heavily, the 40W speaker will fail eventually.
I tend towards series wiring, though Super Reverbs don’t seem to be compromised by their parallel wired cabs

[Roe]

400v thru a gz34 and sag will give you ca 60w at breakup and perhaps 80w at full power

[pdf64]

How it plays out all depends on the details. eg Fender type PTs tend to be rather less saggy than Marshall type, so the HT can be somewhat stiffer, and the ratio of overdriven to clean output power somewhat greater. So even with only 60W clean, the overdriven power may be near 100W.
That’s how it worked out for one of my 2x6L6 builds (Vibrolux type PT / Bassman type OT); 35W clean managed to kill my Weber 12F150 50W when overdriven.
Serves me right for not verifying the actual overdriven power output. Which turned out to be almost 60W.
Lesson learnt

[ViperDoc]

I was looking for a solution for more GB-style early breakup tone in the face of the HP power amp. An easy solution could be a 410 or 412 cab, I was just trying to stuff it all into a sexy 212 HP combo I had made. My OT has a 16R secondary option for a 8R into 8R series wiring. When is impedance mismatching useful? Like a 16R speaker load into the 8R OT secondary would double the reflected OT primary resistance, yes? Half the wattage then? Not sure if I have that right. Not interested in neutering the amp by design though. Higher rated speakers just might be better in any case.

[pdf64]

… Like a 16R speaker load into the 8R OT secondary would double the reflected OT primary resistance, yes? Half the wattage then? …

Not really. The output impedance of pentode amps is high, especially so for beam pentodes. Even at full overdrive it’s likely to be several times the load impedance. So it’s more akin to a current source than a voltage source.
The point being that unlike classic solid state amps, doubling the load impedance does not halve the power output.
As previously mentioned, the only way to get it right is measure it, making assumptions with this may trip you up.