I’ve been promising these for a while – it was worth the wait. The clips were recorded using my new cabinet – with the 6 inch speaker – at low volume but you can’t tell.
The two sound clips were recorded using an SE2000 mic into a Focusrite Saffire 6 USB audio interface. The mic was 6 inches (15cm) away from the speaker and right on the axis. The rhythm part was panned hard left and the lead part hard right in order that you can easily separate the individual recordings. The recording level was not changed during the creation of these clips and the relative volumes of the individual recordings was not changed. The clips were normalised after being combined into a stereo mix but that is the only change that was made to the recorded signal.
This was played on a Parker NightFly HSS Superstrat at 0.5 Watt. Gain was set for 1-2 o’clock for both the rhythm and lead parts and the tone controls were left top dead-centre. The rhythm part was played on the neck/middle “inbetween” pickup position – you can hear the amp is right on the edge of break-up. Lead was played on the neck, middle and bridge (humbucker) one after the other in that order.
This was played on an all-mahogany PRS single-cut with P90 pickups. They’re hotter than most other pickups so push the amp hard. The amp was set to 0.25 Watt and a little bass was taken off and a little treble added. Gain for the rhythm part – which was played on the neck pickup – was set at 2-3 o’clock and the lead part – played on the bridge pickup – was on max (of course).
If you’ve been following my posts you’ll know that I’ve spent a lot of time recently researching guitar loudspeakers and their behaviour in different cabinets. What’s most fascinating is that, in contrast to the world of hi-fi loudspeakers, no-one seems at all interested in the acoustic properties of guitar loudspeakers. As David B. Weems remarks in the 3rd edition of Designing, Building and Testing Your Own Speaker System: “The same laws of acoustics prevail for musical instrument speakers as for stereo speakers, but you wouldn’t know it by inspecting many commercial musical-instrument speakers”. He goes on to talk about the apparent preference for open-backed cabinets but he obviously isn’t entirely sure why this should be. It’s a good question: Why do guitarists use open-backed cabinets?
There are problems with open-backed cabinets that the hi-fi designers of today would not tolerate but it was not always thus. Referring to The Why and How of Good Reproduction by G. A. Briggs, it is clear that, back in the 1940s and 1950s, the speaker manufacturers knew very little about the physical properties of a loudspeaker that would influence the cabinet proportions; they knew only the resonant frequency, the frequency response and the impedance. The Thiele/Small parameters, that are taken for granted today in hi-fi circles, would not appear until the 1960s/70s so back in the 1940s/50s creating a cabinet that would allow a speaker to do its job properly was trial and error and the cabinet (probably ported) would have been large. This was the world in which Leo Fender built his first amps.
In the mid 1940s, Leo Fender selected a Jensen speaker for his new amp because he liked the tone it produced. It is important to realise that, at that point in history, all speakers were general-purpose speakers. Jensen speakers from the 1940s were made for hi-fi but ended up in guitar amps because – by today’s standards – they were heavy-duty speakers. Jensen stopped making speakers in the 1960s but the relaunched, Jensen-branded speakers are, by all reports, faithful to the originals so we have a pretty good idea what Leo Fender was working with. Leo’s motivation was to make product that was mass-producible and affordable (i.e. cheap) just like a radio set. If a radio set had a back it would be lightweight and vented to allow the valves/tubes inside to stay cool. Removing the back of a radio would reduce resonances and rattles which would make it sound better (we have Gilbert Briggs word for that). Under these circumstances why would Leo Fender want to put a back on his guitar amp? It would make the amp, larger, heavier, harder to cool and would increase his R&D costs. It was a no-brainer to leave the back off.
So open-back guitar amps/cabinets were produced not because they sounded better than the alternatives but because they sounded good enough and were cheaper than the alternatives. I think that was a good design decision at that time but what puzzles me (and David B. Weems) is why, 60 years later, we are still taking the same approach. That’s a subject for another day.
After much experimentation and extensive listening/playing tests, it became obvious that 12mm plywood is a little too flexible for the larger panels of loudspeaker cabinets and bracing is necessary. You can see the diagonal braces that I added in the picture (one per side-panel); their addition was enough to tighten up the bottom-end.
The carpet underlay that you can also see in the photograph was purchased from the Carpet Underlay Shop.co.uk and has been very effective at cutting out internal reflections and further improving the “punch” of the speaker’s delivery. At a little over £2.00 (ex VAT) per square metre, it’s something of a bargain.
The sound quality is pretty astonishing; it puts a smile on my face every time I play through it. Right now I can’t get enough of it – I really must get around to posting some new sound samples.
The picture shows a Jensen MOD 6-15 speaker in a 50 Litre enclosure. It’s a big box – around two feet tall – but, because the speaker is designed to work in an open-backed cabinet, it’s smaller than it needs to be for maximum bass response (as calculated using the Thiele/Small parameters provided by Jensen). However, the bass roll-off is not dramatic and the bass response is restored by pushing the cabinet up against a wall (that’s the proximity effect at work).
So how does it sound? It’s killer – a joy to play through. There is no lumpiness or bass emphasis as there is with an open-backed cabinet (because of the inherent comb filter). In fact, on paper, the frequency response of this speaker is very similar to the coveted P10R Alnico speaker – a fact that was confirmed by my tests – but it costs a fraction of the price. The only downside is that the MOD 6-15 speaker is 5dB less efficient than its Alnico sibling but that’s not really a drawback when you’re playing at home; 1W has power to spare.
This definitely the way to go for those who want a good sounding setup for home use at spouse- or parent-friendly volumes. I know this sounds unlikely – after all it’s using a cheap and cheesy 6 inch speaker like that in a Champion 600 – but the large cabinet allows the speaker to work to the best of its ability. The rule here is that it’s better to have a cheap speaker in the right cabinet than an expensive speaker in the wrong one.