Copyright 2011 © Troels Gravesen

I suggest reading the greencone article before reading this, as the use of the resonanz boxes is a key component in trying out these vintage SEAS drivers. The 21TV-G drivers may do well in TQWT cabs too.

Since the article on "world's worst dome tweeter", the SEAS 21TV-G was in the back of my mind, having an overall smooth response and a sound that supersedes the German greencones, thus this article on a Northern equivalent. Let's call them blackcones.

After some "massage" the 21TV-G drivers display a nice and smooth midrange with an upper limit of 5 kHz. Above 5 kHz the roll-off is smooth with a single breakup at 8 kHz. If only all modern drivers would do the same.
The vintage SEAS 21TV-G is a low-ohmic, high sensitivity driver, fitted with a lightweight paper cone driven by a 1" voice coil. The surround is a corrugated extension of the paper cone and coated with some sticky goo. This "goo", coating material, is probably the most consistent coating ever made as its elasticity appears unchanged over decades. I have a suspicion it's a xylene based material and a no-no due to workers' safety. The downside, besides possible toxicity, is that it also attracts dust.
The 21 TV-G is slightly larger than the greencone and due to the 1" voice coil power handling is vastly improved, also helped by an acoustically transparent dust cap, not forming a pneumatic brake as seen on the greencones. Price to pay for high sensitivity - a healthy 96 dB - is low impedance reaching 3.5 ohms. But should I keep a speaker for my 8 watts 300B SET amps, this speaker would be it. The 300B never felt better and finally shows what an excellent bass response the 300B valve can deliver.

Finding a tweeter to match the 21TV-G wasn't easy until Torbjørn of Norway offered me five units of vintage SEAS 9TV-LG tweeters. These are all 16 ohms tweeters and we need two to match the sensitivity of the highly efficient 21TV-G. Several configurations of these tweeters are possible: All drivers on a vertical line or tweeters side by side, either plane with the front panel or slightly angled to enhance dispersion although 9TV-LG dispersion is fairly good considering a 3" cone speaker. Actually one above and one below may be ideal.

1. This speaker may be for low-wattage SET amp aficionados dealing with 2-6 watts. That is, if you're able to find some old loudspeakers fitted with these 21TV-G drivers. My friend Michael found
these at a local second hand store and a few may be available from here.
2. Forget about traditional hifi, we're into a quite different ball game here and getting into resonant boxes may be quite a journey of endless finetuning of resonances, something I didn't even try with my plywood boxes. The long and the short of it is that it is possible to make decent bass from these boxes and due to the lightweight nature, stored energy is low and the trick is to control the resonating panels by degree of bending, thickness (mass), materials (plywood or low density solid wood), bracing, etc. An example can be seen here: View video.
I'm surprised the resonant nature of the boxes apparently doesn't compromise midrange performance. I had expected this.
3. The use of an extended range driver up to 5-8 kHz supplemented by a super tweeter has its shortcomings. Severe beaming is the result with congested treble and increased sibilance. I know this approach is a key feature of e.g. French
PHY-PH systems, but no matter how well drivers are made, this is something we cannot overcome. When the wavelength starts approaching the diameter of the driver, we start having beaming. The single-cap people seems prepared to live with this, but to my ears an 8" drivers shouldn't be used above 3 kHz, some would even say 1.5-2 kHz but it may depend of the tweeter used and slope of the crossover.
4. The final crossover (v3) driven from my 300B amp has given me fine moments with my recordings and should I take it even further I would substitute the two 9TV-LG tweeter with the Audax TW025A tweeter having sufficient efficiency to cope with this remarkable SEAS vintage driver.
Taking the point of crossover down to the 3 kHz region eliminated most of the beaming and made a much more tolerable speaker handling most musical genres equally well. Nothing new under the sun here. This has been confirmed over and over again, not least during the more thorough investigation of the Supravox driver in
DTQWT cabs.

The Drivers

SEAS 21TV-G driver. View data below found on SEAS website.

21TV-G measured TS data. Added mass used.

Five 9TV-LG tweeter in excellent working condition. I'm surprised by the consistency in performance of these tweeters. View measuring data below.
The 9TV-LG tweeter features alnico magnet, paper cone, heavily doped surround and an aluminium dust cap.
Finally it comes with a diecast chassis. Quite an unusual thing for a vintage driver like this.


Left: 21TV-G SPL @ 1 meter, 2.8 volts. Both 21TV-Gs shown.
Right: Five 9TV-LG tweeters; SPL @ 0.25 m normalised for 2.8V/1 meter. This driver more looks like a midrange driver, but high-end extension is excellent.

Left: All five 9TV-LG free air impedance. Right: 9TV-LG dispersion from 0-35 deg. Evenly distributed readings.

SPL @ 1 meter/2.8 volts for 21TV-G (red) and 9TV-LG (blue). As can be seen, two tweeters are needed to match the 21TV-G.

Crossover Simulation

Crossover v2.

Resonanz Box

- exactly the same as for the greencones.

Measurements in resonanz cabs from crossover v.2.

Left: SPL of drivers in cabinet, two tweeters here in parallel. Right: Impedance of drivers in cabinet (no crossover).

Left: SPL from drivers driven from crossover. Right: Impedance of final system.

Left: Step response. Right: Distortion @ approx. 90 dB/1 meter. Purple = THD.

CSD, 25 dB scaling.

Measurements in resonanz cabs from crossover v.3.

Simulation of crossover version 3. Point of crossover is here taken down to ~3 kHz.

Left:SPL @ 2.8V, 1 meter. Overall system sensitivity is around 96 dB/2.8V. Right: Same with 200-20000 Hz scaling.

Left: Response from drivers driven from crossover plus summed response. Right: Final system impedance, minimum 3.5 ohms @ ~250Hz.

A friend of mine wanted the blackcones incl. resonanz boxes for his 2A3 amplifier, so I put together these V3 crossovers, mostly Intertechnik stuff.