what size cab do ineed for a 50 watt valve head?

[Zerofret]

Tom - This can unfortunately get complicated. But first, two essentials - the speaker impedance must match that of the amplifier and second, the speaker rating must be in excess of the RMS wattage that the amp is putting out. OK that's out of the way so now the blurry bit. The perceived loudness of a speaker system is largely dependant on the sensitivity of the loudspeaker driver used and this is measured in dB and represents the SPL (sound pressure level). The sensitivity figure tells you how loud a speaker driver sounds for a specified input (assume a loudness factor of 2 for every increase by 3 dB).This figure can be as low as 80 dB for 1W of input and measured at 1 Metre to around 104 dB. The difference of 24 dB doesn't sound a lot but it means that the 104dB rated speaker needs .004 of the power needed by the 80dB SPL rated speaker to produce the same perceived loudness. OK, it's unlikely that you will be comparing speakers at either extreme but just look at what the maths tells us in real life - The first speaker would need a 100 watt amplifier to make it sound as loud as the second speaker running from an amp of just 0.4 watts. So you can see, the speaker and its enclosure is probably more important to 'volume' than you thought!

In the early days speakers didn't handle much input power with and drivers rated at 15 - 25 watts were common place. Granted, bands weren't necessarily as loud back then but some of those old speakers tended to be very efficient and required less wattage to sound loud. by comparison a modern loudspeaker can be designed to handle incredible amounts of input power but can be a lot less efficient. Most manufacturers have gone for reliability as the major factor and one of the problems with speakers in the old days was that we were always replacing them because they were easily blown in spite of their superior sensitivity and loudness. Fortunately there are now modern 'vintage' speakers that are using the old techniques and styles to bring back some of that old speaker magic without the puny power handling that made the originals so vulnerable.

What about the cabinet? Well if you seal a cabinet up completely (an infinite baffle design) the speaker cone is damped and easier to control especially at high volumes. If you have an open backed cabinet, you have very little control over the speaker cone and at high volumes it is likely to cause excessive cone exertion. By allowing some of the air pressure out of the cab (venting) it is possible to get a good compromise between the two extremes of design. Going further, a properly ported design can bring some of the rear out of phase bass, wasted from a speaker cone and port it back in phase from the front of the cabinet increasing the efficiency of the cabinet but, more importantly, improving its bass response. To confuse matters further, every speaker driver has its own inherent resonance (the larger the cone, the lower the resonance) and every enclosure is going to have its own resonance and the idea is to try and keep the resonance out of the playing range which with smaller cabinets is difficult to do. Because the cabinet is also going to depend on the kind of speaker used, it's a good idea to get recommended enclosure plans from the speaker manufacturers themselves.

The Selmer Treble 'n Bass was often used with the Goliath speaker cab of which there were two versions - the '50' with a single 18" driver and the '100' with an 18" and a 12" driver! The Goliath 50 used a Goodmans Audiom 91 rated at 50 watts RMS and had an impedance of 15 ohms. The cab measured 39.75" x 24" x 16" and came up to your waist! I've no idea what the SPL was for this driver but it certainly packed a punch if a little lacking in the mids.

I think if I was in your shoes, I would build a substantial plywood cabinet and fit something like an Eminence 15" Legend 1518 which has an SPL of 102.2dB Apparantly Fane are producing a vintage range of instrument speakers and if you look at their website under 'bass guitar' it says "coming soon" so that might be worth a look.

Steve

[alexclaber]

Lots of good points Zerofret! However...

[quote name='Zerofret' post='275502' date='Sep 2 2008, 03:20 PM']But first, two essentials - the speaker impedance must match that of the amplifier and second, the speaker rating must be in excess of the RMS wattage that the amp is putting out.[/quote]

The speaker impedance should be greater than a solidstate amp's minimum impedance, it doesn't need to match. With a valve amp the speaker impedance should be similar to the output transformer tap impedance - if there isn't a matching one then you should use the next highest tap.

The speaker power rating does NOT need to be in excess of the amp power rating.

Also a ported cab is not a compromise between an open-backed and a sealed cab as at all frequencies above the port tuning frequency the air inside the cabinet is effectively sealed from the outside air.

Balcro, I'm not sure why you think 15"s are less sensitive than 10" or 12" speakers. If all else is equal then sensitivity (efficiency) is proportional to cone area.

It seems to be that using a 50W valve amp for bass is only a wise idea if you like your sound to be dirty and overdriven all the time or you play in a small jazz combo. If not you're so short of headroom that you'll have to cart such a big cab to get loud enough and stay clean. Nice wattage for guitar though, more than enough for anything!

Alex

[The Funk]

[quote name='alexclaber' post='275544' date='Sep 2 2008, 03:57 PM']It seems to be that using a 50W valve amp for bass is only a wise idea if you like your sound to be dirty and overdriven all the time or you play in a small jazz combo. If not you're so short of headroom that you'll have to cart such a big cab to get loud enough and stay clean. Nice wattage for guitar though, more than enough for anything![/quote]

Noone else had the heart to say it.

[Bill Fitzmaurice]

[quote name='Zerofret' post='275502' date='Sep 2 2008, 10:20 AM']I think if I was in your shoes, I would build a substantial plywood cabinet and fit something like an Eminence 15" Legend 1518 which has an SPL of 102.2dB

Steve[/quote] The sensitivity ratings for raw drivers apply to above 100Hz, so that in and of itself says nothing about sensitivity where demands are highest, from 40 to 100 Hz. As for the Legend 1518, it's a guitar driver. In a typical cab sensitivity below 100 Hz averages only 92dB, and its 0.8mm xmax limits its average power handling in the bass range to only 20 watts.
Sensitivity ratings given by speaker manufacturers are outright piffel and are useless for comparison purposes. But the more drivers one uses the higher the system sensitivity, as there are more driver motors utilizing the power. For that reason a lot of tens will give a better result compared to a smaller number of twelves or fifteens in the same overall pack space. With the same input two 2x10 cabs will have 6dB more output than one, and that 6dB is the equivalent of going from 50 to 200 watts. With the price of watts as low as they are today using a small amp with lots of cabs isn't the most size efficient route, but it can work if you so desire.

Edited September 2, 2008 by Bill Fitzmaurice

[Zerofret]

Alex, Thanks for your comments and I would like to defend what I said. Regarding the impedance matching, I was refering to the Selmer Treble 'n Bass 50, the valve amp in question where it ought to be the same (or as close as you can get). I completely agree about solid state amps being different.

It is definitely advisable to have some headroom on the driver's input power as running it close to the edge all the time with bass is not a good idea. I wasn't suggesting using a driver that would handle two or three times the power either, just a bit of headroom is useful for reliability. Anyway I guess it's not a valid argument anymore because you'd be hard pushed to find a 15" driver rated at 50 watts nowadays!!

I didn't suggest that a ported cab was a compromise between an open backed and a sealed cabinet - I said that a vented cab was a compromise. A vented cab is usually a crude generic design where there is generally less science involved but it generally improves the lower bass response. A ported cab is design critical and driver specific - I thought that I got that across by suggesting sticking to a speaker manufacturers published designs but maybe not because I was probably rambling by then.
Steve

[quote name='alexclaber' date='Sep 2 2008, 03:57 PM' post='275544']
Lots of good points Zerofret! However...

The speaker impedance should be greater than a solidstate amp's minimum impedance, it doesn't need to match. With a valve amp the speaker impedance should be similar to the output transformer tap impedance - if there isn't a matching one then you should use the next highest tap.

The speaker power rating does NOT need to be in excess of the amp power rating.

Also a ported cab is not a compromise between an open-backed and a sealed cab as at all frequencies above the port tuning frequency the air inside the cabinet is effectively sealed from the outside air.

[alexclaber]

[quote name='Zerofret' post='275659' date='Sep 2 2008, 05:55 PM']It is definitely advisable to have some headroom on the driver's input power as running it close to the edge all the time with bass is not a good idea. I wasn't suggesting using a driver that would handle two or three times the power either, just a bit of headroom is useful for reliability. Anyway I guess it's not a valid argument anymore because you'd be hard pushed to find a 15" driver rated at 50 watts nowadays!![/quote]

Indeed - in this case you'd struggle to find a driver without much greater thermal power handling. However due to the dynamic nature of music even a heavily clipped amp is unlikely to put out more than 1/3 power averaged over time so thermal power handling headroom is not required. Mechanical power handling headroom (excursion ability) is far more useful as that limitation is one which peaks can reach, it doesn't require sustained power to cause problems, but unfortunately no-one quotes figures for this type of power handling.

[quote name='Zerofret' post='275659' date='Sep 2 2008, 05:55 PM']I didn't suggest that a ported cab was a compromise between an open backed and a sealed cabinet - I said that a vented cab was a compromise. A vented cab is usually a crude generic design where there is generally less science involved but it generally improves the lower bass response. A ported cab is design critical and driver specific - I thought that I got that across by suggesting sticking to a speaker manufacturers published designs but maybe not because I was probably rambling by then.[/quote]

I don't know of a difference between ported and vented, they're synonymous nowadays. I've seen older designs with bizarrely shaped vents and so on I presume they're the generic designs to which you refer. I think most of these designs still work like a ported cab though, with a mass of air (in the vent/port) resonating against an air spring (the air inside the cab) and frequencies above the resonant frequency of that system being kept within the box.

Alex

[Zerofret]

Bill - You're right, I was totally wrong about the Legend 1518 - I meant to suggest the Legend CB15 which is designed for bass and has excellent sensitivity at the lower end. Its 4.8mm xmax is more like it too.

I agree with you on the 4x10 route and it would be my choice as a player but there is still something to be said for that vintage sound from a big cab and a small valve amp that puts a smile on your face! Thinking about it it doesn't put a smile on your face carting it around to gigs and up three flights of stairs though......

Steve


[quote name='Bill Fitzmaurice' post='275595' date='Sep 2 2008, 04:40 PM']The sensitivity ratings for raw drivers apply to above 100Hz, so that in and of itself says nothing about sensitivity where demands are highest, from 40 to 100 Hz. As for the Legend 1518, it's a guitar driver. In a typical cab sensitivity below 100 Hz averages only 92dB, and its 0.8mm xmax limits its average power handling in the bass range to only 20 watts.
Sensitivity ratings given by speaker manufacturers are outright piffel and are useless for comparison purposes. But the more drivers one uses the higher the system sensitivity, as there are more driver motors utilizing the power. For that reason a lot of tens will give a better result compared to a smaller number of twelves or fifteens in the same overall pack space. With the same input two 2x10 cabs will have 6dB more output than one, and that 6dB is the equivalent of going from 50 to 200 watts. With the price of watts as low as they are today using a small amp with lots of cabs isn't the most size efficient route, but it can work if you so desire.[/quote]

[Bill Fitzmaurice]

[quote name='alexclaber' post='275673' date='Sep 2 2008, 01:10 PM']I don't know of a difference between ported and vented, they're synonymous nowadays.
Alex[/quote]
The officlally recognized term for a direct radiator which is not sealed is VB, for Vented Box. Precisely how it's vented is another matter entirely.

[bass_ferret]

There is a guy I have seen a few times on the local pub circuit with a T&B 50 and an ancient 2x15 without a grill so god knows what it us. I have not noticed he was any quieter than the rest of the guys with assorted megawatt amps.

[Balcro]

[quote name='alexclaber' post='275544' date='Sep 2 2008, 03:57 PM']Balcro, I'm not sure why you think 15"s are less sensitive than 10" or 12" speakers. If all else is equal then sensitivity (efficiency) is proportional to cone area.
Alex[/quote]


Hi Alex,
I made the statement because from endless spec checking over drive unit web-sites like usspeaker/madisound etc., I saw a pattern emerging. Of course it is a generalisation but I was also aware from the posts preceding mine, that a developing advice theme of adding more drive units, whatever the size, would assist "tom skool". Hence my remark about yours and BFM's earlier posting. It wasn't a techie scientific assertion. Tom_Skool's original enquiry didn't seem to need any greater depth of drive unit science other than add a drive unit and get 3dB.

Balcro.

[alexclaber]

[quote name='Balcro' post='275774' date='Sep 2 2008, 09:00 PM']I made the statement because from endless spec checking over drive unit web-sites like usspeaker/madisound etc., I saw a pattern emerging.[/quote]

Aha! Presumably because the 10"s and 12"s you were looking at had a higher Fs and lower Qts than the 15"s because they're less aimed at producing lows, thus higher efficiency.

Alex

[tom skool]

blimey you have all been busy!
Much appreciated. Theres some very usefull info in all that
Right then this is what i think.

It appears unlikely that i was putting to much pressure on a 200w speaker so i think i need to get the amp looked at as i dont think i should be getting that kind of distortion so early on in the volume stage. (there is only one volume knob )

In order to get the most volume(loudness) from the amp the cab needs maximum sensativity and this can be obtained by more speakers (the more the better?) and maximum spl(sensativity) of those speakers. So I still think a 4x10 would be best and i dont want a huge monster to lug about.

still not sure about the wattage(sorry!) anything over 50w?

and finally if the amp turns out to be fine i need to get somthing bigger

[bass_ferret]

Watts is (are?) irrelevant when considering cabs. Unless you are buying vintage any modern cab will handle 50 watts. Some low sensitivty, wide frequency cabs need a lot more but you wont be looking at them.

[alexclaber]

[quote name='tom skool' post='275831' date='Sep 2 2008, 10:02 PM']...i think i need to get the amp looked at as i dont think i should be getting that kind of distortion so early on in the volume stage. (there is only one volume knob )[/quote]

Keep in mind that if you have a loud bass and play hard the amp will distort much earlier on the volume control than if you had a quieter bass and a softer touch.

Alex

[The Funk]

From the For Sale forum, a 4 ohm Mesa 6x10" (which Alex points out could be rewired to 16 ohms)...
[url="http://basschat.co.uk/index.php?showtopic=22354"]http://basschat.co.uk/index.php?showtopic=22354[/url]

[Bill Fitzmaurice]

[quote name='The Funk' post='276136' date='Sep 3 2008, 06:30 AM']From the For Sale forum, a 4 ohm Mesa 6x10" (which Alex points out could be rewired to 16 ohms)...
[url="http://basschat.co.uk/index.php?showtopic=22354"]http://basschat.co.uk/index.php?showtopic=22354[/url][/quote]
Rewiring is probably not necessary. Unlike SS tubes are comfortable with lower impedance loads, and uncomfortable with higher impedance loads, than the nominal transformer rating.