Thoughts / Advice on My 1950's 1X12 'Bell & Howell' Cab Conversion.

[Bone Idol]

First off.... This is for home / light rehearsal use.... I'm not expecting to play the Albert Hall with this thing!.... But I would like it to sound as good as it possibly could, given its limitations... And mine!

 

A while back I was given this Bell & Howell 631 projector speaker cab, complete with its 12" Jenson 'fan-frame' speaker and thought it would make a nice little home bass cab project.

 

So far I have swapped the speaker for a Celestion Pulse 12, added a jack socket and plate, blanked-off the two semi-circular vents to the rear of the cab and added a bit of padding to the inside of the rear door, along with some door seals. I also added some 1960's grill cloth that I had knocking around. (which I might change at a later date).

 

I had to increase the diameter of the apperture in the baffle plate as the Celestion was about 10 or 12mm larger but the baffle retains its felt lining.

I also had to remove the extruded aluminimum grill as it rattled like a snare drum.

Having removed the original grill, I then opened up the lower portion of the front of the cab to match the apperture in the baffle plate as the original design had the bottom of the speaker behind the fascia.

 

 

To port or not to port?... That is the question...

At the moment it's more or less a sealed cab, but quite a small cab, somewhere around 36.3 litres. It's hard to be exact as the sides on the cab taper inwards as they reach the top.

I could possibly fit one or two 2" ports in the the front fascia but 'tuning' them would be another area where I'd need some advice.

 

I will probably 'square-off' the front of the cab's opening (see mock-up photo) as I don't really like the 'smiley face' front it has at the moment.

 

The amp for this cab is a T.C. Electronic BAM200... So nothing too heavy or ambitious.

 

Photos...

1 - How it looked originally (not mine as I forgot to photograph it in its original condition).

2 - As above....

3 - How it looks at the moment

4 - Inside View

5 - Rear View

6 - Mock-up for 'new look' front.

 

So... Your thoughts, comments and advice would be most welcome.... Many thanks.

 

Edited January 14 by Bone Idol
Spelling

[Bill Fitzmaurice]

Porting would give you a bit better result, but not enough to make it worthwhile IMO, the box is too small. You'd need to go up to at least 50 liters not including the space taken up by the port.

[Bone Idol]

11 hours ago, Bill Fitzmaurice said:

Porting would give you a bit better result, but not enough to make it worthwhile IMO, the box is too small. You'd need to go up to at least 50 liters not including the space taken up by the port.

I fully understand that, it's jut a bit of fun for around the house.... As I said at the begining, it will have limitations, I just want it to be the best that it can be.

 

Do you have any advice on size of port or port length?

[bremen]

You've put a BMW engine into a Robin Reliant haven't you 😉

 

Looks great though, why not have some fun with winISD modeling software (free, and easy to use). You can get the driver parameters  from loudspeakerdatabase.com and see what sort of response and power handling you get with various port tunings.

 

I suspect, though, you'll find it's too small to port for anything below middle C.

[Bone Idol]

3 minutes ago, bremen said:

You've put a BMW engine into a Robin Reliant haven't you 😉

 

Looks great though, why not have some fun with winISD modeling software (free, and easy to use). You can get the driver parameters  from loudspeakerdatabase.com and see what sort of response and power handling you get with various port tunings.

 

I suspect, though, you'll find it's too small to port for anything below middle C.

You're absolutely right!

 

I did have a quick look at the 'maths' of it all and it made my head hurt... The software looks like the right way to go so maybe I'll have to take two deep breaths and try again!

[bremen]

I'd keep it as a sealed box. It'll be fine for home use. What have you done with the original driver? Might make an interesting guitar speaker.

[David Morison]

As Bill says, there is a gain to be had from porting, but it's modest - less than 3dB.

 

By the way, how does it sound at the moment?

Are you noticing it being particularly light in the bottom end at all?

Also - how's the back panel & its latch and seal holding up? If you port the cab for more bass, you're also increasing the chance of extraneous noise if the hinges/latch don't hold it closed tightly enough.

 

Anyway, as I am an inveterate WinISD tinkerer, Blue is sealed, Green vented with 2 of these: [url=50mm port]:

 

 

That's theoretical however and there are a couple of caveats:

1; WinISD's port calculation assumes the backside of the port is at least one diameter away from any boundary such as the sides of the cabinet - you will be struggling to achieve that, so the ports may tune lower than predicted. If they were to tune as low as 40Hz rather then the theoretical 46.5Hz, the curve would look like the bold Green trace here - not much different in this case:

 

 

2; Vent Air Speed may be a problem. For best performance, the general rule of thumb is to aim for 17m/s or lower, though some sources do say that for non-hi-fi applications, up to twice that may be acceptable.

At the ~33V your BAM200 can put out, the predicted airspeed is this (again, bold = 40Hz tune - in this case that actually plays in your favour):

 

 

In practice, this is probably less of an issue than the modelling suggests, due to a couple of factors - first, we very rarely play with the amp flat out the whole time and allowing for dynamics in our playing, average power levels should be a lot less than max and second, those graphs assume full power at each frequency, which for an instrument like bass which is rich on harmonics, doesn't happen - our power is divided up over several frequencies for each and every note we play.

 

So, overall, more work for marginal gain - if you're an inveterate tinkerer you may still want to do it but don't expect night & day differences from the current sealed condition.

 

HTH,

Good luck,

D.

 

Edit - link formatting

Edited January 14 by David Morison

[Bone Idol]

28 minutes ago, bremen said:

I'd keep it as a sealed box. It'll be fine for home use. What have you done with the original driver? Might make an interesting guitar speaker.

I've kept hold of it, the cone and coil appear to be in good order (no rips or drag on the coil)... But I think these were made on a budget, hence the lack of a name plate... I also think that it may well be 16 ohm as most of these projector cabs were 16's.

[Bone Idol]

23 minutes ago, David Morison said:

As Bill says, there is a gain to be had from porting, but it's modest - less than 3dB.

 

By the way, how does it sound at the moment?

Are you noticing it being particularly light in the bottom end at all?

Also - how's the back panel & its latch and seal holding up? If you port the cab for more bass, you're also increasing the chance of extraneous noise if the hinges/latch don't hold it closed tightly enough.

 

Anyway, as I am an inveterate WinISD tinkerer, Blue is sealed, Green vented with 2 of these: [url=50mm port]:

 

 

That's theoretical however and there are a couple of caveats:

1; WinISD's port calculation assumes the backside of the port is at least one diameter away from any boundary such as the sides of the cabinet - you will be struggling to achieve that, so the ports may tune lower than predicted. If they were to tune as low as 40Hz rather then the theoretical 46.5Hz, the curve would look like the bold Green trace here - not much different in this case:

 

 

2; Vent Air Speed may be a problem. For best performance, the general rule of thumb is to aim for 17m/s or lower, though some sources do say that for non-hi-fi applications, up to twice that may be acceptable.

At the ~33V your BAM200 can put out, the predicted airspeed is this (again, bold = 40Hz tune - in this case that actually plays in your favour):

 

 

In practice, this is probably less of an issue than the modelling suggests, due to a couple of factors - first, we very rarely play with the amp flat out the whole time and allowing for dynamics in our playing, average power levels should be a lot less than max and second, those graphs assume full power at each frequency, which for an instrument like bass which is rich on harmonics, doesn't happen - our power is divided up over several frequencies for each and every note we play.

 

So, overall, more work for marginal gain - if you're an inveterate tinkerer you may still want to do it but don't expect night & day differences from the current sealed condition.

 

HTH,

Good luck,

D.

 

Edit - link formatting

That's brilliant, many thanks for your advice.

 

To answer your questions...

At the moment it sounds (to me) ok, although I haven't had the chance to crank-it yet.

The bottom end sounds fine but at low volume everything sounds fine!

 

The rear door is great now that I have put some self adhesive door draught-excluder on the joint, the latch & hinges are surprisingly sound.

I am getting sthe odd rattle at certain frequencies which appear to be coming from the wire mesh on the original half-moon vents in the rear door. I did blank these off with hardboard (and sealed with silicone) on the inside... Perhaps the mesh will have to go!

[David Morison]

Cool, well for the time being I'd say leave it as is at least till you've had the chance to turn it up a bit.

The Equal Loudness effect means that as you get louder your perception of bass improves, so even if the low end were to sound a bit light at livingroom volume it might still be viable for your rehearsal use-case, even without adding ports.

[agedhorse]

Enjoy it at low volume as it is. Maybe you don't need to mess with what's already cool?

[Phil Starr]

Nice one @David Morison just the information the OP needs.

 

@Bone Idol The graphs show the advantagesof porting the cab. Crucially the -3db point reduces to around 70Hz which means all the second harmonic down to E is reproduced strongly. You've also got an extra 3db from around 110Hz which is significant. The mixed blessing is that you have roughly a 1db hump in response centred around 120Hz. This will give an artificial warmth to the bass sound which many bassists like and a roughtly half of us don't. The long and short of it is that you are going to get more bass if you port it and it's more than enough for you to hear a real difference. Whether you will prefer the lighter sound of the sealed cab or the fuller sound of the ported version is to an extent a matter of taste.

[Bone Idol]

1 hour ago, Phil Starr said:

Nice one @David Morison just the information the OP needs.

 

@Bone Idol The graphs show the advantagesof porting the cab. Crucially the -3db point reduces to around 70Hz which means all the second harmonic down to E is reproduced strongly. You've also got an extra 3db from around 110Hz which is significant. The mixed blessing is that you have roughly a 1db hump in response centred around 120Hz. This will give an artificial warmth to the bass sound which many bassists like and a roughtly half of us don't. The long and short of it is that you are going to get more bass if you port it and it's more than enough for you to hear a real difference. Whether you will prefer the lighter sound of the sealed cab or the fuller sound of the ported version is to an extent a matter of taste.

More great advice... Many thanks...

I still haven't had chance to wind it up loud, so I won't even consider porting until I know what it sounds like under load... But according to all the advice I've had from this post is sounds like it would be more trouble than its worth, and also I think that a port would not look right in a cab of this vintage.

 

[Bone Idol]

Cosmetically, I'm thinking about changing the shape of the apperture in the front fascia...

 

This is how it looks now...

 

 

 

And this is my latest idea...

It's only a Mock-Up photo but I think it would look more in keeping with it's original form... And would also have the added advantage of providing a place for a single port (if I decided to go down that route).

 

I would very much appreciate everybody's comments on the proposed 'new' shape of the grill.... Cheers.

 

[Phil Starr]

I think going forward in stages is the best strategy. If you like the sound then why undertake surgery on the cab. If you do want to port it then I'd port the back not the front. The cab was originally 'ported' by those cutouts in the back of the cab and personally I'd go for building ports based on extending them if I wanted a reflex cab. You can make ports of any shape with papier-mache

 

I'm conservative with projects like this, a bit like The Repair Shop I don't mind the 'unfashionable' mid century look of the old cutout and I'd be thinking in terms of how I might be finishing the cab before taking a saw to the front. Perhaps ironically it's the look Bugera were going for in your other amp there. The colours of the Bell and Howell and even the grille cloth are a bit reminiscient of the old WEM Dominator amps. What a fun little project!

 

[Bone Idol]

An added bonus....

 

Yesterday, just for the hell of it, I plugged the Bell & Howell cab into my little Bugera guitar amp (the little bugger as we call it), cranked it and put my es335 through it and..... WOW!... I thought that given as the Celestion Pulse 12 is a bass guitar speaker it would sound dull and unappealing with a guitar going through it, but how wrong was I?.... It knocked the Celestion 8-15 that I'd installed in the Bugera into a cocked-hat.

 

I'm really surprised at this...

 

But shhhh.... Don't tell the guitarists or they'll all want one!

Edited Friday at 11:36 by Bone Idol

[ezbass]

5 minutes ago, Bone Idol said:

An added bonus....

 

Yesterday, just for the hell of it, I plugged the Bell & Howell cab into my little Bugera guitar amp (the little bugger as we call it), cranked it and put my es335 throught it and..... WOW!... I thought that given as the Celestion Pulse 12 is a bass guitar speaker it would sound dull and unappealing with a guitar going through it, but how wrong was I?.... It knocked the Celestion 8-15 that I'd installed in the Bugera into a cocked-hat.

 

I'm really surprised at this...

 

But shhhh.... Don't tell the guitarists or they'll all want one!

One of the best guitar amps of all time is the Fender Bassman, but often remains overlooked. Projector amps are a bit of a thing in the US at the moment, with some YouTubers extolling their virtues, you may have got in before the inevitable price increases.

[Bone Idol]

On 17/01/2025 at 10:05, ezbass said:

One of the best guitar amps of all time is the Fender Bassman, but often remains overlooked. Projector amps are a bit of a thing in the US at the moment, with some YouTubers extolling their virtues, you may have got in before the inevitable price increases.

I had a Fender Bassman which I bought for 40 Euros (about £28 in 2010) in a rural junk shop in France. I don't think they knew what it was as it was on a shelf filled with microwave ovens and electric grills. I had no way of testing it, so I bought it blind and had to wait a month until I got home to try it, but it was perfect!... It was too big & too loud for me and I wasn't playing much bass at that time, so I sold it on and made a small fortune! (which makes a bloody change!).

 

When I was given the B&H speaker cab I was also offered the Bell & Howell 631 projector for free... These 'Film-O-Sound' projectors have a 15w all valve amp built into them, which some people seperate for guitar use... But it's a lot of work, and these projectors and their amps run on American voltage and the one offered to me came without the transformer.

 

I'm a terrible hoarder, but even I couldn't justify having the projector!.... But I also have another small projector cab, it's an all aluminium mid 1950's speaker cab called a Danson 540.

The original speaker in the box was dead but being a hoarder, I'd kept hold of an old 8 inch Celestion speaker from a valve radiogram that I'd had in the 80's, which was made in June 1956... The marriage of the two was like it was meant be!

It's no wonder that my wife calls me 'Steptoe'

 

The Danson 540 cab now fitted with a 1956 Celestion speaker.

 

 

Edited Saturday at 19:12 by Bone Idol

[nekomatic]

That Danson cab is gorgeous!

[ezbass]

The boys from Two Rock converted a projector amp into a guitar amp.

 

 

[Bone Idol]

Another weird thing that I'd love to try and use is this valve pre-amp made by Leak, it's called a Varislope III ... It came out of the same 1950's radiogram as the Celestion speaker that I put in the Danson 540.

 

I know it's a 1950's Hi-Fi pre amp but I've always wondered what it would sound like when used with an electric musical instrument... Anyone ever heard or tried one of these?

 

This photo is from the 'net as mine is in the shed!... Unfortunately I didn't think to keep the umbilical lead when I took it from the radiogram, so I have no idea how to wire it up

Edited yesterday at 08:14 by Bone Idol

[bremen]

37 minutes ago, Bone Idol said:

Another weird thing that I'd love to try and use is this valve pre-amp made by Leak, it's called a Varislope III ... It came out of the same 1950's radiogram as the Celestion speaker that I put in the Danson 540.

 

I know it's a 1950's Hi-Fi pre amp but I've always wondered what it would sound like when used with an electric musical instrument... Anyone ever heard or tried one of these?

 

This photo is from the 'net as mine is in the shed!... Unfortunately I didn't think to keep the umbilical lead when I took it from the radiogram, so I have no idea how to wire it up

I had the later version and yes, it made a beautiful fat sound with bass.

 

I have the umbilical and service manual somewhere. It takes 6.3V and 200ishV from the power amplifier, is that something you could provide if I come up with a diagram?

 

If not, it'll get a good price on ebay. Just use the word Mullard and you're away 😄

[Bone Idol]

4 minutes ago, bremen said:

I had the later version and yes, it made a beautiful fat sound with bass.

 

I have the umbilical and service manual somewhere. It takes 6.3V and 200ishV from the power amplifier, is that something you could provide if I come up with a diagram?

 

If not, it'll get a good price on ebay. Just use the word Mullard and you're away 😄

Sounds like it may well be beyond my meagre capabilities

It's a lovely bit of 'objets d'art' (what they'd probably call 'Man Cave' nowadays!)... I was kind of hoping that it would run on mains voltage and I could just 'hook it up', but from what you say it sounds like it originally got its power from the radio's transformer... I did find this schematic on the internet, but it's all a bit Chinese to me!... I've had it since the 80's when I had the whole radiogram and we used the radio section all the time, it sounded great even though Mono amd Medium Wave radio was looked down upon at that time, but you couldn't fault its tone.

[bremen]

That's not the preamp schematic. Looks like a radio with small power amp stage.

 

If you can solder, I could help you make a power supply for the preamp.

[bremen]