Ghetto Thiele cabinet for EVM12L

[Steve Conner]

A while back I got an EVM12L Classic reissue guitar speaker, and I've been looking for a suitable cabinet for it ever since.

I got two 1x12 closed-back cabs cheap on Ebay UK, so I thought I would risk adding ports to one of them to make a kind of low-rent version of EV's famous TL806. At 27 litres they were about the same size as the TL806, so I figured it couldn't go too badly wrong.

I'd used my EVM12L in the closed-back cab for a while, and it had hardly any bottom end. When I simulated it in WinISD, it predicted a low-end rolloff of 250Hz, which I guess means hardly any bottom end, sure enough.

Before starting I measured the resonant frequency of the driver in the closed-back cab (110Hz) and in free air (69Hz, not the 55 quoted by EV, even though it's had a fair amount of time to break in.) I changed the EVM12L model in WinISD to reflect this.

After a bit of messing around, I found tuning the box to 89Hz made the plot look good. It looks more like the TL806's "step-down" tuning with the middle port blocked up, but I wanted to err on the low side so as not to lose too much deep bass, not to mention the worry of unloading too early and damaging the cone if it were ever used for bass guitar.

The biggest bass tubes at my local Maplin were 75mm, and WinISD said I'd need two of these cut to 60mm length to get my 89Hz box frequency. So off I went to get two bass tubes, a hacksaw and a hole saw.

On installing the ports, it worked way better than I expected! Testing with the generator and scope again showed two impedance peaks of roughly equal size at 141Hz and 44Hz. When I tried it on an amp, the thin, weedy bottom end that it had in the closed-back configuration was transformed, it now gives a nice grunt that goes all the way down to low E on the bass. The midrange sounds better too.

I'm not sure why I got such different results to what the simulation predicted, but I'm very happy, since I wasn't expecting it to cover that bottom octave and be usable for bass as well as guitar. Of course, maybe it's just distortion harmonics, but it sure sounds as if it's covering it

I'll post pictures if anyone is interested. Anyone got any "EV Equipped" grill badges?

[tubeswell]

I'm interested in the pics Steve.

BTW How do you measure the resonant frequency of a speaker?

[Steve Conner]

Pix attached :-)

I measure the resonant frequency of a speaker with a signal generator and scope. I just connect the generator to the speaker terminals, and connect the scope across it too. By rocking the frequency around you can find a point where the voltage on the scope peaks, and the speaker gets louder too. That's the resonant frequency.

This method works because the signal generator has a higher output impedance than an amp (mine has 50 ohms) so it doesn't damp the resonance as much as an amp does.

[Bill Moore]

I cut a TL806Q in half.

I don't have the equipment to test the way you do, but I took the TL806Q plans, and cut the cabinet to a 2-12. My wife has played bass through it at practice volumes, and it sounds good. The first set I built, I used for mid cabinets in a 3 way pa system. I built another last year to play steel guitar through, and I really like the full range availlable. (I built mine with EVM-12S speakers), and used 3/4 MDF, really heavy. Thanks again for the info!

[Steve Conner]

Hi Bill,

That cabinet looks neat! I wish EV had kept on putting the logo on the dustcap, I was kind of disappointed when mine didn't have it.

Do EV still make the "S" or did you manage to get some second-hand? How do you find the high end for steel guitar? My L sounds kind of dark, I always end up maxing the treble on my amp.

I got the chance to crank it up full and play some bass through it the other day, and I'm pretty happy with it. It doesn't thump like a subwoofer, but it puts out enough bass to rattle things off shelves, and the speaker didn't show any signs of distress.

[Bill Moore]

Steve, these 12's were NOS. Back in the 80's I managed to keep an EV dealership for a while, and was able to hang on to a few speakers. Every musician in the area had EV speakers back then. The reason I built this cabinet for steel, in the late 80's I was playing drums in a country band, and hired a steel player for a 2 week sit down. He blew the BW 15 in his Nashville, and I didn't have a replacement, since we had scaled back the PA from the R&R 3-way setup to 2-way, I had these mid cabinets. He was blown away by the range, and bottom end(even on the C6 neck) and tried to keep the cabinet in lieu of 2 weeks wages. If he liked them, I decided I would when I started working with the steel. I do have 1 12L left, and am going to try to duplicate your cabinet for 6 string. The S series have a mid freq. boost due to the shorter cone length, and work well with guitar and vocals.