BigMacAms

Hi,

I'm new to the forum and have enjoyed all the posts so far.

I have a question that's been bugging me and this is probably the best place on Earth to ask.

I have 2 JTM30's (1x12's) which I love to bits. I thought I'd like to get a bit more total volume out of them by adding extension speaker cabs. The matching JTM C12 cabs are usually too expensive (like usually the same price as a complete amp) for me if they ever come up second hand.

As there are not so many 16 ohm, 70 watt cabinets regularly stocked by my local store I decided to try out an Epiphone Valve Junior extension cab (see link below) . It looks nice and the price was right.

The problem is that when I plug in the extension cabinet, the sound is fine, but the total volume is not noticeably louder. Plugging in the extension makes the volume of the amps internal speaker go down enough that the total volume is about the same as without the extension speaker. I wasn't expecting the sound level to double with the extension speaker, but I did expect some kind of boost.

Can anyone with a JTM30 + JTM C12 (or anyone who knows the theory better then me) let me know how their sound level increases when adding the extra speaker ?

Thanks in advance,

BigMacAms


Valve Junior Extension Cabinet

Wilder Amplification

Well...the amp will put out its max rated power regardless of how many speakers are hooked up to it. However, the more speakers you have hooked up, the power splits between each speaker, hence the drop in volume per speaker. In other words, if you have 30 watts of power being consumed by 1 speaker and you hook a 2nd speaker up to the amp, each speaker will now only consume 15 watts of power, for a total of 30 watts.

As far as output power vs perceivable acoustic volume, other factors come into play with this such as speaker sensitivity, cabinet design, room acoustics, etc etc. This is why I constantly tell people that in the world of guitar amps, power does not equal volume. If you had a 50 watt amp and put it up against a 100 watt amp, there will only be a 3dB increase in perceivable volume from the 100 watter over the 50 watter, and it takes 10x the power to get 10dB more perceivable acoustic volume, which is what the human ear perceives as being "double the volume".

__________________
Jon Wilder
Wilder Amplification

sales@wilderamplification.com

tech@wilderamplification.com

BigMacAms

Hi Jon,

Thanks for your explanation, though I'm still left with the nagging thought:

- An extra speaker does not lead to an increase in perceived volume

- It makes sense that due to sharing of power between 2 speakers, each speaker is driven by 15 watts, and that the volume of the combo speaker drops when plugging in an external speaker.

Given these two points then why then does the combo even have an extra speaker output ? And why did Marshall go to the bother of making an extension cabinet especially for this amp ? If an extension speaker does not make it louder, is their another reason ?

Is the same principle true for a stack ? Is a half stack just as loud as a full stack due to the power being shared between the two cabinets ?

Any thoughts greatly appreciated !

BMA

Wilder Amplification

Reason for the extra output? Speaker placement. You can run a speaker on both sides of the stage.

Now in the case of the half stack vs full stack...

In both scenarios you have double the speakers working. However, in the case of your amp, you're gong from 1 speaker to two speakers, whereas with a 1/2 stack versus full stack, you're going from 4 speakers to 8 speakers. However, again speaker placement comes into play here. If you run a full stack in the traditional sense with the cabs stacked, it sounds louder because you have a cab aimed right at your head. However, take that same stack and run it split mono (one cab on each side of the stage) and all of a sudden it doesn't sound any "louder" than a 1/2 stack, it just sounds "fuller" due to having your sound coming off of both sides of the stage rather than just the one with a cab aimed at your head.

Speaker placement plays a huge role in the tone/volume that you hear.

Also, most combo amps are in an open back cab, which makes the cab "omni-directional". You also get some cancellation as some of the sound off the back of the speaker couples with the sound coming off the front of the speaker (sound off the back of the speaker is 180* out of phase with the sound off the front of the speaker). Whereas with a 4x12, they're closed back. This makes them directional and minimzes sound cancellation, which makes them louder than an open back cab on top of the fact that they have more speaker surface area moving air than a single or 2x12 cab. Cabinet design plays a HUGE role in the sound you hear.

__________________
Jon Wilder
Wilder Amplification

sales@wilderamplification.com

tech@wilderamplification.com

BigMacAms

Aha - Speaker placement, I think I get it at last.

Thanks Jon for your answers

Jim