Need help selecting replacement speaker for repurposed cabinet.

[Hoochie Coochie Man]

I need help selecting a speaker to repurpose an Ampro speaker cabinet that was originally intended for use with a movie projector. My goal is to remove the 70 year old 16 ohm Jensen PM12 B alnico speaker and install a new 8 ohm speaker.

The removable back of the cabinet is currently attached by way of two latches. My plan is to install a continuous cleat around the perimeter on the inside of the cabinet and screw the back to the cleats. I plan to make an opening in the back that a geometry teacher would tell us is a shape called a "stadium". Stadium -- from Wolfram MathWorld. (I could also just make two rectangular panels leaving a rectangular opening).

I intend to use this cabinet with a few different tube amps in the neighborhood of five watts or so, up to a maximum of maybe fifteen for guitar or harmonica. I am trying for a Fender Champ style amplifier.

The interior dimensions with the back attached are (in inches) 14 X 14 x 8, which I calculate to be 0.91 cubic ft.

I think I may be more successful if I use either an eight, or ten inch speaker rather than a twelve inch due to the volume of the cabinet, but since I don't really know what I am doing here I could be wrong about that.

I have considered using any of these Jensen alnico speakers:
Specification Sheet - P12N | Jensen Loudspeakers, Specification Sheet - P10Q | Jensen Loudspeakers, or P8R | Jensen Loudspeakers.

I need help because I have no understanding of how to specify a speaker that will give satisfactory results given the specifications of the existing cabinet.

I would also consider any other speaker anyone who understands how to obtain good results will suggest. If a ten inch speaker would be ideal I may buy two of them and replace the 16 ohm Celestion in my Ampeg GVT1-10 5w-2.5w combo amp to be able to also use the Ampro as an extension speaker with that amp.

One of the amps I will connect to this is an old Voice of Music amplifier I salvaged from a record player, and I also have a couple other five watt tube amps coming my way soon.

The original Jensen speaker works but since it is 70 years old, I would be concerned that the old adhesives could fail without warning at any time. It has a five digit date code 22068 which I think means it was manufactured in 1956, (or the last digit is missing). It is also stamped with the following code: 16641 PM12 B C4440.

I want to do this because I like the vintage style that you will see in a photo below:
:
Here is the interior. (The two "things" at the bottom of the speaker are 1/4 inch plugs with the speaker cables cut off. I removed them after taking this photo). It looks in the photo below that this may be a double-baffle design based on the small gap around the speaker baffle, but I have not confirmed that.

The cabinet is constructed with 3/8 inch plywood, and if it is not rigid enough I will add internal bracing.

I will add that in another thread I promised @agedhorse that if he would be kind enough to advise that me I will not argue with him.

I also invite anyone else who would like to make suggestions to do so. Any advice would be appreciated, even if it amounts to "What the hell are you wasting your time and money on this thing for when you already own more amps and cabinets than you deserve as a no talent hack with far better gear than you deserve. You idiot".

Thanks in advance to anyone who gives me guidance in this potential folly.

 

[rockear]

Anecdotal, but I have a guitar amp built in 1939 with the original speaker (Epiphone Electar Zephyr with a Rola field coil) and it still sounds great. There's a bunch of little Gibson amps and others from the '30s and '40s still out there being used regularly. I wouldn't necessarily assume that yours is about to fall apart unless you have specific reason to believe that. You might want to give it a chance.

Check out this fella:

 

[agedhorse]

I will have to look at my surplus shelves…

 

[S-Bigbottom]

@basscooker is no longer with us on TB (darn it) They look about the same size, he used cheap MCM 55-3212 for his cabs.
They are not efficient and he always said they were not for giging, just cool at home play. They sounded good using the original Ampro 20 watt tube amp placed into one of the cabs as a head.

In the Search type in Ampro then by member basscooker and find all where he has talked about them.

The Ampro is a bit more involved... Notice the top cab has a jewel cover... That's where the amp was, sort of a "combo" amp originally. I also bought two of the speaker boxes, so three pieces total. I sold all 3 of the old Jensens, and tore up one speaker box to make the head box to match the remaining boxes.... loaded them up with MCM 55-3212 4 ohm drivers and use a series cable I made to show the amp 8 ohms.

His rig.

 

[Hoochie Coochie Man]

Thanks for the link.

Those cabinets look to me like they are older than mine. I think mine may have been made in 1956, (younger than I am). Those look like they were made in the 1940s.

I hope to end up with a tone and look that would replicate the sound I hear listening music made in the early days of Chicago electric blues genre. I don't know how to play blues harp, but I know a couple guys who are very good at it and would like to be able to bring my project along and mic it the next time we provide sound reinforcement at a show for one of them.

I am trying to find the late fifties Voice Of Music record player amp that I have around here somewhere, but if I can't find it or it it proves to be unsatisfactory, I will see what else I can scrounge up. Something no more than fifteen watts, and preferably vintage. I am also looking at an amp, I would never have given a second look at, which is one of the Bugera 5 watt amps someone had posted about in another thread here on TB. I may be asking peoples opinion about the Bugera T5, or V5 amps. Those things sell for very little new, and so it might be worth trying one like this:

https://www.altomusic.com/products/bugera-t5-infinium-5w-cage-style-tube-amplifier-head?variant=47778444542235¤cy=USD&gad_source=4&gad_campaignid=22693423633&gbraid=0AAAAAD194NVzhljaYbcOsCyTLoXh_mddv&gclid=Cj0KCQiA7-rMBhCFARIsAKnLKtCQyCy0wpf-SA_QfuDepxooO185sLvuFES6PuPIpKhhJBhSOXUWqFQaAszAEALw_wcB

I am not a big fan of the parent company of Bugera as far as MI amps go, but I think the look of that amp would fit the vintage style of the speaker cabinet. I would not be afraid to monkey around with the preamp circuit or output circuit negative feedback on something like that to tweak the tone profile if need be.

I retired a long time ago, and I need stuff like this to occupy my time and mind.

 

[JimChjones]

The quick and dirty fix might be to find a modern commercial cab that fits inside, and use the ampro purely cosmetically.

Next up would be to calculate the internal volume of the Ampro, and find a commerccial cab with pretty much the same internal volume. Find a secondhand version of the latter and combine the two. Reckon you're right though, youll have to go smaller than 12.

Anothee thing you might want to consider is whether you can enlarge the ampro with acceptable cosmetics. If you could get a reasonable match on the covering you could extend it a good few inches at the rear while keeping the face the same, which would give you a lot more options about driver choice.

 

[Hoochie Coochie Man]

Thanks for responding.

I have calculated the internal volume to be 0.91 cubic ft. as stated in my first post.

I don't know how to determine the dimensions of open vs closed area on the back.

I thought that it would be safe (based on an uneducated seat of the pants guess) to choose a speaker smaller than twelve inches based on the size of the box, and that guitarists often swap speakers in open back cabinets with little or no understanding or regard of any real engineering that a person who knows how this, would go about selecting a driver to suit an existing enclosure. I would rather not spend time and money on trial and error.

I am considering contacting Jensen, Weber, and Eminence and providing them with the information in my first post to see if they can help.

I am surprised that I have not gotten more responses.

I bet that if I would have asked what automotive market fifteen inch subwoofer driver I should buy to upgrade my Subway 115 to make it "the best cabinet for metal" I would have gotten more responses.

 

[Hoochie Coochie Man]

I have decided to install a four ohm speaker instead of my original eight ohm choice for this project. Here is one speaker that is on my list:

Weber model 10F150: 10F150 – Weber Speakers

It looks like it is a ceramic magnet. I doubt that I could tell the difference in the type of magnet a speaker has by ear.

Weber allows four options in terms of the dope applied to the surround and I think if I go with Weber I will buy the non-doped surround version. They also offer it in twenty-five, and fifty watt versions. Since I intend to use this with low power tube amps I would go with the twenty-five watt version.

 

[Hoochie Coochie Man]

Next speaker under consideration is a four ohm Jensen P10R (alnico):

P10R | Jensen Loudspeakers

The P10R is very important in the history of Jensen speakers. Its shimmering, yet throaty and vibrant tone is one of the signatures of the Tweed era and inspired the start-up of the British tones. It was the first speaker reissued by Jensen, presented to a restricted panel of partners at the...

www.jensentone.com

I looked briefly at the Eminence web site, and did not see anything that struck me as a good choice.

At this point, I am thinking about buying the Jensen, and the Weber.

I could use one for the Ampro, and try the other in my little Ampeg GVT 5 combo.

I hope I am not turning into an idiot guitar playing, speaker swapping tone chaser by doing this. If I start buying and selling effects pedals and changing pickups such, I am done for.

Since I can't find the Voice of Music amp I was planning to use for this, I have now just ordered one of these:
Bugera | Product | T5 INFINIUM to play around with in addition to the V5 "chop" combo amp that a previous owner converted into a head box amp by sawing off the bottom part of the combo (that I got for 100 bucks).

This long cold winter (snowing as I type) is driving me nuts and is causing some kind of insanity.

 

[JimChjones]

It looks as if you could probably get Eminence' BassLite S2010 Small Vented, 1x10" Bass Guitar Cabinet design within your box. Worth investigating?

https://cdn.shopify.com/s/files/1/0270/8665/1462/files/Basslite_S2010_cab.pdf

 

[Hoochie Coochie Man]

Thanks for the link. I have not looked at it yet but I will. I am trying to get a cabinet that will work for guitar or harmonica that would be good for what I will call a Chicago blues sound.

I have to admit that all this Thiel and Small graphs, and cabinet design stuff is beyond my capabilities. I also think that for an open back guitar cabinet driven by a five watt amp T&S design design principles are not really applicable. I could be wrong (as I often am), so that is why I appreciate all knowledge, and wisdom anyone here has to offer.

Thanks!

 

[Passinwind]

I have decided to install a four ohm speaker instead of my original eight ohm choice for this project. Here is one speaker that is on my list:

Weber model 10F150: 10F150 – Weber Speakers

It looks like it is a ceramic magnet. I doubt that I could tell the difference in the type of magnet a speaker has by ear.

Weber allows four options in terms of the dope applied to the surround and I think if I go with Weber I will buy the non-doped surround version. They also offer it in twenty-five, and fifty watt versions. Since I intend to use this with low power tube amps I would go with the twenty-five watt version.

IMO the folks at Weber might be a better resource than most of us here, as blue harp is a very different use case than bass guitar and wimpy old Jensens (which I love for appropriate uses) are just the sort of thing many of their drivers are meant to replace and/or replicate. But hopefully someone like Andy who has a big parts stash may have something workable for you. I'll run it by the last local repair shop guy too, old blues music is his first love.

Old school public address systems from companies like Bogen, Dukane, and even Altec used to be used to be a cheap and easy donor source for low wattage tube amps, but I think the cheap part is probably gone by now for the tube stuff. This was a 30 watter I found in a used tool shop in Seattle many years ago:

And it worked just fine, doing hundreds of gigs in a friend's guitar rig. Eventually I reacquired it and gave it to a young TB'er to help him explore tube amp building. It was an actual point to point build for the most part, which is extremely demanding to work on if you want to preserve the old look.

Plenty of low wattage kits and designs out there for DIY'ers though.

 

[Lo-E]

I don't know how to determine the dimensions of open vs closed area on the back.

…guitarists often swap speakers in open back cabinets with little or no understanding or regard of any real engineering that a person who knows how this, would go about selecting a driver to suit an existing enclosure.

Open backed guitar cabinets are all pretty close to an open baffle. Close enough that putting a stadium-shaped hole in the back will likely not sound a whole lot different than just leaving the back off entirely. There’s really not that much engineering involved in those cabs and just picking a speaker you like the sound of is perfectly fine in that context. In your case I think I would stick with a 12” speaker for the blues sound you’re after.

I have one of those earlier, 1940s Ampro cabs and I put an old Jensen Vibranto 12 in it; a cheap replacement alnico speaker that Jensen marketed in the 60s. It sounds fine. It sounds nice for harp or blues guitar. It doesn’t sound great at really high volumes but that’s fine for me; I have other cabs for that.

Ted Weber left us years ago and I haven’t tried one of their speakers since he died but I really, really like the Weber speakers that I got back when he was still running the company. If their quality has remained the same, I recommend them.

The Jensen reissues are pretty okay. Nice. Not amazing. But nice.

Anecdotal, but I have a guitar amp built in 1939 with the original speaker (Epiphone Electar Zephyr with a Rola field coil) and it still sounds great.

Me, too! Not to derail the thread but do you have a schematic for it? Mine was “modified” (molested) when I got it and I was never able to find a schematic. I went with a best-guess circuit based upon common practices at that time and it sounds nice but I’d love to return it to the correct circuit.

 

[Redbrangus]

...for an open back guitar cabinet driven by a five watt amp T&S design design principles are not really applicable.

This is correct; T/S is all about predicting the performance of a driver in a vented, bass reflex enclosure. Pretty much anything would work well enough for this application. There are countless 'guitar' speakers out there, with a wide variety of tones; this is a question that would get better answers in a gee-tar forum.

 

[Hoochie Coochie Man]

Thanks for the reply Mr Escher.

I have a nice collection of potential donor amps for conversion projects. Most of them are not appropriate for this one.

Here are a couple PA amps that if I live long enough I hope to do something with.

Rauland Borg 2135, and Rauland Borg 1960A:

The bottom photo shows just the empty chassis, but I do have the complete amp. Both are in working order.

Also, much thanks to everyone who contributes to this thread!!!

 

[Hoochie Coochie Man]

Regarding speaker diameter. I am not trying for maximum SPL. If smaller works better and is less volume, I am happy with that. I assume though that there would not be a large difference in SPL between a ten and a twelve, but there may be other advantages of one versus another. I don't know. That is why I ask.

 

[Passinwind]

I have one of those earlier, 1940s Ampro cabs and I put an old Jensen Vibranto 12 in it; a cheap replacement alnico speaker that Jensen marketed in the 60s. It sounds fine. It sounds nice for harp or blues guitar. It doesn’t sound great at really high volumes but that’s fine for me; I have other cabs for that.

I had four of those drivers, with matching cabs like so:

Two 115s and then two smaller trapezoidal 112s that stacked on top. I'm sure the guys I sold them to blew them all up right quick, IIRC even a tiny Bogen solid state amp took one out when my electric violin player got rowdy.

 

[Redbrangus]

There are no particular advantages to any particular speaker diameter. Make it easy on yourself and put a 12 in that cab. And the shape of the opening in what is essentially an open-backed cab would make no difference in performance or how it sounds.

 

[rockear]

Me, too! Not to derail the thread but do you have a schematic for it? Mine was “modified” (molested) when I got it and I was never able to find a schematic. I went with a best-guess circuit based upon common practices at that time and it sounds nice but I’d love to return it to the correct circuit.

I don't have a schematic unfortunately. I do have a bunch of before and after photos that were taken when the previous owner went through it.

They said all they did was replace faulty components, but I'm not qualified to confirm that. The story as told to me was that I'm the third owner, and the first never did any work to it, so the before pictures should be the original configuration if that's true. (Actually looking at these pics now I don't think there are any truly "before" pics, which is a bummer. The grounded power and a few non-original looking caps are present in all photos. But there are some pretty detailed shots and annotations of component values and such, so maybe they could still be helpful to someone who understands this stuff (not me).)

 

[agedhorse]

Never trust the previous work to be either correct or accurate.