Pulling two output tubes in a Tweed Twin

[Randall]

Will it hurt anything to pull a 6L6 on each side of a 5F8A Twin and still keep the load at 4 ohms?

[The Dude]

Ideally, you'd use a lower impedance transformer secondary winding to compensate, but since one is not available, go for it. People have been doing this for years. FWIW, it won't make the amp half as loud, which is a common misconception.

[Randall]

I just realized I can disconnect one of the speakers to get to 8 ohms. A customer wants his amp to break up faster and not be as loud, so this is worth a try. I just didn't want to be responsible for giving him advice that may toast his OT.

[Chuck H]

Unplugging two tubes and leaving the load as is would give the same impedance match as plugging another 4 ohm cabinet into the "ext. speaker" jack with the amp as stock. So I suppose that's as alright as Fender ever expected

Removing one of the speakers AND unplugging two tubes would give the stock impedance match and greater volume reduction... Obviously.

[pdf64]

I think it likely that removing 2 6L6 will screw up the amp's dynamic response (because the HT becomes relatively much stiffer), and disconnecting a speaker will thin out the tone (because the mutual coupling of 2 speakers acting in unison has been lost, and because there's now a big hole in the baffle).
To make the amp 'not as loud', I suggest to try a lossier rectifier, eg 5U4, 5V4, 5R4 or add a sag resistor (maybe across standby then it can be switched out), or add a cathode / fixed bias switch.
To get earlier break up, ie increase total available gain so that the amp overdrives at a lower number on the volume control, maybe bypass the 2nd stage cathode?
https://el34world.com/charts/Schemat...5f8a_schem.pdf

[Chuck H]

Great ideas One thing though... Pretty sure the idea about the speakers would be to simply disconnect one rather than remove it. A big hole in the baffle would certainly be a tonal detriment

I just had another idea. Since Twins tend to be rather strident in the HF anyway you could have a switch that disables two power tubes and switches the remaining two to triode operation. There would be a bit of HF loss, power would be reduced and (unless I'm mistaken) two tubes in triode mode played into both speakers should be an decent impedance match.?. Maybe include a fixed/cathode bias switch to put back some sag as Pete suggested. I think I did this to a guys 100W Marshall a long time ago. IIRC I used a rotary switch that went stock/cathode bias/two tubes in triode, cathode bias. I don't generally like the HF loss in triode operated pentodes but in a Twin it might be a benefit.

[Randall]

I like the lossier rectifier idea, that would be easy enough.

[Dave H]

Pretty sure the idea about the speakers would be to simply disconnect one rather than remove it. A big hole in the baffle would certainly be a tonal detriment

Would it help to also short the terminals together on the unused speaker to stop its cone flopping about so much with the drive from the other speaker?

[Chuck H]

Would it help to also short the terminals together on the unused speaker to stop its cone flopping about so much with the drive from the other speaker?

I don't imagine it would be a big deal in an open back cabinet, would it? And would shorting the terminals together actually generate enough EM feedback to stop the cone from responding acoustically?

[Leo_Gnardo]

Would it help to also short the terminals together on the unused speaker to stop its cone flopping about so much with the drive from the other speaker?

Funny, I was thinking of that. Would be better to remove the speaker & block the hole with a plywood square. Still, all in all the overall volume reduction won't amount to much.

[Tom Phillips]

Would it help to also short the terminals together on the unused speaker to stop its cone flopping about so much with the drive from the other speaker?

I don't believe that shorting the terminals would change the overall sound significantly. Going back to the original idea to reduce the overall acoustic source level, I suggest that you try the amp with both speakers hooked up and with just one electrically connected and then decide which way sounds best. The impedance missmatch will not be a problem for the Fender twin.

...would shorting the terminals together actually generate enough EM feedback to stop the cone from responding acoustically?

It does make a big difference in the physical response of the cone of the open circuit speaker but I don't think it would produce a significant sound difference either way for the subject situation. However, there is a situation for which shorting the terminals of the disconnected speaker can be very useful. For example, if you have one speaker exhibiting a bad voice col rub in a multi speaker amp such as a 4x10" Bassman or a Super reverb and you need to temporarily stop the buzz during a gig and you can identify and electrically disconnect the bad speaker, then it helps to short the terminals of the disconnected speaker. The damping effect is enough to stop the rubbing sound that occurs if the speaker is left open circuit to resonate in the cabinet.

[Dave H]

Would be better to remove the speaker & block the hole with a plywood square.

Yes, blanking it off with plywood would be better but that means extra work, something I always try to avoid

Anything has to be better for bass than having a big hole in the baffle right next to the speaker.

[eschertron]

Anything has to be better for bass than having a big hole in the baffle right next to the speaker.

Well, that helps mitigate the treble loss from the triode-connection mod, doesn't it?

[pdf64]

I was thinking that an inactive speaker on the baffle is effectively just a piece of thick paper over its baffle cut out. Maybe a bit better than a completely open hole, but it doesn't seem likely to attenuate the sound much; so it may reduce the bass response.
I guess that shorting out its terminals would mostly help to damp down its bass resonance.

[Malcolm Irving]

Some hi-fi speakers have what they call a 'passive radiator'. It is quite like a speaker cone with no drive to it. I'm not sure if they deliberately add some mass to it to increase the inertia of the cone. It is there to change the phase of the sound that is being 'reflexed' from the rear of the actual speaker.

I think a disconnected speaker would have some effect on the low-mid and bass response of the cab. and that the effect would be changed slightly if the damping of the passive speaker was increased by shorting its terminals.

But guitar cabs are not like hi-fi cabs and tend to be more of an 'anything goes' affair. Perhaps a bass cab would show some significant differences.