New project. 1 x 12″ Ported Bass Cabinet, for a 30watt, all-valve, Ashdown bass amp.

I recently got the chance to buy one of the last of Ashdown’s production run of their CTM30, all-tube amps, for a knock-down price. Discounted to an obscene level, and direct from Ashdown. The amp has now been replaced in the Ashdown line by a smaller version called the “Little Stubby” – but I’ve always been a fan of all-valve amps, and of the old-fashioned drophead design generally. There’s an element of old valve radios in the design, which makes me think of the old wireless I had when I was a small boy and, although I’ve probably mostly played through solid state amps in the past – I really do like the way a valve delivers clean, warm tone – and then breaks apart beautifully when driven just that little bit more. I’ve been a fan of Ashdown’s stuff since the days when I used to hang around the old Bass Centre in Wapping. It’ll be interesting to see how the quality of their amps has fared as the company has grown over the years. I think some of the production, at least, has now been outsourced to China – but I’m assured quality hasn’t suffered, in the least. This drophead version of the CTM30 looks to be a slightly redesigned version of Ashdown’s, much praised,  “Little Bastard” 30 watt drophead from 2012. If so – it promises to be an absolute steal for the money.

CabSpec_000

In the Citizens, I used the same Trace Ellliot GP7 for about 15 years. As far as I recall – I only ever had the back off it once – to track down a slight rattle. A rock solid, 4 x 10″ combo – with a separate 15″ enclosure for added whoomph. That’s quite enough for most rehearsals and most venues. It was large enough to use without any PA support for small gigs – and more than enough for the set-up most London venues had – where the bass would, more often than not, be sent direct through the house PA, via a DI box. In such cases, the GP7 and the 15″ enclosure, (lovingly referred to as, “the rabbit hutch”), were more than enough to serve as an on-stage monitor. At decent levels, the bottom 15″ speaker would flap your trousers around your ankles. I was told to turn it down regularly. Towards the end of my time with the Citizens, I started to use a SansAmp Bass Driver as a kind of tone shaper, separate from the Trace’s EQ. The SansAmp gave me an additional DI out – direct from the box, so that I could shape the bass tone for the front of house stuff with that, and then EQ the on stage “monitor” speakers to suit. It was a nice, efficient setup – and sound engineers always appreciate a plug and play bass player. They get more than enough diva treatment from guitarists and singers. Turn up with your own DI setup, get your sound check sorted in less than two minutes, and they might even remember to put you on their Christmas card list.

Of course. Such a setup isn’t exactly practical for home use and the odd, informal, rehearsal / jam session. For that – I need something portable. For the home, I need something which won’t alarm fellow domestic travellers. The CTM30, with it’s 30 watt output should hopefully do the job, and it looks more like a piece of furniture than an alien piece of technology. Beneath the anonymous, tolex covered casing, the design uses ECC83 and ECC82 preamp tubes, together with an EL84 tube power section to deliver the tone. Basic, 3 band eq controls allow a certain amount of shaping, and additional boost switches targeted to “deep”, “mid-shift” and “bright” frequencies allow you to add a little extra detail to the shaping. There’s a DI socket, just in case I ever need to plug into a PA, and even a separate effects loop. The amp has outputs for both 4 and 8 ohm speakers – so, all in all, it looks to be a pretty flexible, albeit basic, tube amp. Just what I’m looking for. Now, all I need to sort is a suitable speaker enclosure cabinet.

Now – in the domestic setting – and so as not to unduly alarm Mrs Garageland, the ideal bass cabinet should be small, non intrusive and capable of expressing itself clearly at, sometimes, very low volumes indeed. (Of course – it also needs to be capable of handling it’s fair share of punch when circumstances do allow). Basically, I need a nice piece of funtional “furniture” which will blend into a living room environment. If I finish it nicely – I might just be able to fit it into a small corner, and replace the small table on which my stereo separates sit.

CabSpec_002

I’m going to try and keep the size of the bass speaker down to a single 12″ which, I’m hoping, will give me the best combination of bass and mid response for my 30 watts worth of investment. For the speaker – I’m looking at an Eminence Basslite S2012. The key to getting the best from the speaker will be in the design and build of the cabinet. I’m going to try and base my construction around a slightly modified design for a 12″ Celestion speaker, which Celestion helpfully feature on their website. I’ll be using that as a starting point, and then using Eminence’s specifications for the bass driver to try and modify the construction, and tune the port.

CabSpec_002a

The construction of a bass speaker enclosure is closely linked to the sound which the speaker is capable of producing. There’s a lot of energy in bass sound frequencies, and the speaker’s response can be helped, or hindered, by the design of the enclosure. For bass speakers, a ported design is often used to help reinforce some of the low frequencies. Not to get too technical – but as well as the air that is pushed into the room by the speaker, advantage can also be taken of the air that is pushed back into the sealed cabinet that houses the speaker. If a small, round port is fitted alongside the speaker, then the moving air emerging from that port can be used in conjunction with the moving air from the speaker itself. If the size of the enclosure and the amount of air moving from the two openings can be tuned and optimised, then the cabinet can help the speaker work at maximum efficiency. That means it can have increased presence, and clarity. Distortions in the cone can be more closely managed, and unwanted overtones and distortion reduced.

Of course, I don’t pretend to be the definitive expert in any of this – and I’m going to have to learn as I go along. But I think I’m OK with the principles. I just need a bit of a hand to fine tune things. Fortunately, there are a few websites out there with some handy calculators, and these will help work out some of the necessaries – like tuning the length of the port tube to the resonant frequency of the final enclosure. Things like this all depend on the eventual final volume of the enclosure, so basic plans may have to evolve as I build. But it’s good to have a solid starting point. The construction needs to be rock solid, air-tight and strong enough to prevent deflection in any of the flat surfaces. The component parts need to be accurately cut on the square – to make sure it all fits together properly. It should be a relatively straightforward job, but a good one to test the accuracy of my carpentry. I think I may finally have to get round to buying a decent router…

CabSpec_003

To keep things tidy, I plan to cover the enclosure in “Elephant” Tolex, before fitting the usual metal corners, rubber feet and inset carrying handles. The interior of the cabinet will be shrouded in dark egg-box foam – to eliminate interior sound reflection as much as possible. In order to protect the speaker cone, and to try to make the cabinet look as much of a piece of furniture as possible – I’ll be covering the front with a grille – perhaps, Fender style, “Oxblood” grille cloth. So, as well as testing my carpentry skills, I’ll have to bone up on my my upholstery skills as well. As the good weather returns, it looks like a good project to get back into the workshop with.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s