Bill Fitzmaurice

That cab as it turns out has roughly 50 Hz tuning, which is pretty much ideal. It's a bit small in size, frequency response is better with one driver than with two, but that's pretty common with cabs designed before the advent of Thiele/Small modeling. A moderately priced driver that works well in it is the Lavoce WXF 15.400. The cab being small it won't deliver massive lows without a considerable amount of bass EQ. but between their 400w thermal capacity and 6mm excursion a pair of them will work very well, far better than any vintage driver would have, including the JBL E140.

If anything the laser beam highs from guitar cabs mean they're what needs the most to be in the PA, both out front and in monitors. For all their fussing over gear they tend not to understand how it works very well. No one needs more than a 1x12 combo. But somehow far too many of them think it's still 1965 and they need rigs capable of filling Wembley with no PA.

Not necessarily. Running instruments through the PA in small to medium rooms isn't about volume, it's about dispersion. No matter how loud the drummer plays the higher frequencies will always be coming from his kit and won't fill the room. The same applies to all the instruments. Where levels and balance are concerned leave that to your sound man, because he can hear what's out front, you can't. Most of the time my sound man is me, so the only way I can gauge what's out front is to have that same mix in the monitors. If I didn't have them I'd have nothing to go by. I just high pass them at 150-200Hz to keep the mud out. If you do it right you're unaware of what the PA out front is doing while on stage it sounds just like a good studio session. On the subject of drummers and big monitors, in one of the worst examples I've ever heard the drummer had a 2x18 monitor right next to him so he could feel his kick drum. The trouble was with every kick it fed back though the mains and the ringing boom masked everything else. The sound man was clearly an idiot, having no idea as to what was happening or how to fix it. I left halfway through the show, unable to handle it any longer. Now one could say that wouldn't happen with an experienced drummer who had a clue, but it was Max Weinberg. I'd say it's a safe bet that Bruce's crew does a better job. ☺️

All I need is the exact internal dimensions, preferable as a top view cutaway.

In this case it's not so simple, as the ports are tapered. Basic ported box speaker modeling software can't account for that. You can do it with a more sophisticated tool, like HornResp or Akabak, but they require a high degree of expertise. However I do happen to have that high degree of expertise, and my rate is reasonable. Free. ☺️

I can't imagine not having it.

Not necessarily. If you see aluminum domes they were probably JBL. When Sunn first came out with the 200S one thing that made them better than the rest was JBL drivers as standard, rather than as optional. But that also made them expensive, as JBL cost two to three times what others, like Eminence, did. I still remember buying JBL E-140 for $75 USD in 1970. That would be $700 today. In the early 70s Sunn went to the far less expensive Eminence in most of their cabs, but still offered JBL as upgrades.

Square magnets and small domes were characteristic of early Eminence drivers.

They were pretty good bass reflex cabs. Not folded horns as they claimed, but bass reflex with tapered ports, a seat of the pants design, as there was no folded horn or bass reflex design software then and the designers, Norm (of The Kingsmen) and Conrad Sundholm weren't trained in speaker design. The drivers were Eminence rather than the JBL they used in the earlier 200S and 2000S. Driver sizes are approximate, so it's not unusual to need to do some work on the baffle to get replacements to fit.

Thanks. BTW, the columns I used were Shure Vocal Masters. They were too tall to work well as monitors, which I fixed by cutting them in half. That gave us four of them, so there were no dead spots on the stage. This was 1973-74, when monitors were still almost unheard of, so we were quite pleased to have them. I'm not saying I invented them by any means, but I'd never seen monitors used in clubs prior to that. They came about because the band had the Shures when I joined, while I brought along the JBL 4550 copies that I had built, so it was either figure out a use for the Shures or leave them in the van.

No problem at all, I used to do this when touring in the '70s. Since our gigs were usually a full week I used to hang them in front of the band if there was available rigging.

If my sound is dull or flat it's time to change strings. ☺️

That comes as no surprise. 25x25mm pieces of wood connecting the sides to each other and the top to the bottom is the equivalent of doubling the panel thickness for vibration reduction. Three spaced around the driver hole connecting the baffle to the back does the same. 25 to 50mm thick foam or polyester wadding on the interior stops internal reflections that lead to ragged midrange response.

Lowther, and many Fostex models as well, are primarily aimed at audiophools who are convinced of the need for single driver hi-fi cabs. To extend the highs they use small very light cones. That also gives low Qes/Qts for high sensitivity, but with weak lows. To get the lows up to a useful level they put them into rear loaded folded horns. The high sensitivity is useful with very low power valve amps, over which audiophools also wax poetic. But these are the same blokes who'll pay 100 quid or more a meter for cables. They're easily bested with two or three way ported speakers and modern amplification.

Your current Celestion isn't all that bad, but the port is too short. It should be 13cm long, which will give better lows. The driver xmax is very short at only 2.5mm, which limits it to 50w before reaching the excursion limit. If you're not have an issue with distorting lows it doesn't much matter, but if you are that's the likely cause. Going to the Celestion FTR15-3070C with 5.5mm xmax would cure that. You also want to see if the cab has cross bracing and wadding inside. Fender was notorious for leaving out both.

Lowthers are typically used in hi-fi rear loaded folded horns. They'll go reasonably low but only living room loud. The pictured PA horns use compression drivers, perhaps good to 200Hz. That's fine for that application, but not so much for music. In their case they really are PA, as in Public Address.

The port depth is also required.

Cabinet technology not so much, but driver technology for sure. You may be able to greatly increase performance replacing it. I can make recommendations knowing the exact internal dimensions of the cab and port. An 8 ohm cab is best, allowing you to use two should the need arise, although with a 100 watt amp it probably would not.

The mouths. The throat is the small end where the drivers are mounted. Advances in driver technology allow us to use smaller drivers in horns just as in other cab styles, so where a fifteen might have been required 50 years ago a ten would work as well today. But you're not going to get 130dB at 30Hz from a 4 inch driver, no matter how large the horn.

Said horns were typically loaded with fifteens. For instance the Altec A7, or going back further the Shearer horn.

Not even. There are four inchers that would give good lows if the horn is large enough, but their small displacement would limit their output. Even horn loaded eight inches is the smallest practical size.

That's one factor, but not the only one. Juts as significant is power density, which increases by 3dB (doubles) with each octave reduction in frequency. While our E is one octave below a guitar E most of our content is two octaves below the guitar, for 6dB, or a quadrupling of power. The real question isn't now much power does the bass need, it's how little does the guitar need. If you've ever stood in front of a Vox AC 30 you know it's not 100 watts.

That would be a combination of hearing the subs in a big venue, and boundary reflection cancellations in the rehearsal space.

It does in Texas, but not in the way a Brit would likely imagine. 😁

Don't forget about TC watts. 🤪