Discussion on developing Build Your Own Amplifier Cab

[fender3x]

I plan to make two Jacks although the plans show you how to make it a 210 or in my case a 212. For bass there is no real advantage between 2 110s and a 210, but the former is more flexible.

It's big because it's a semi-horn loaded design. So the box is a big part of maximizing SPL. Even in a normal reflex cab, adding volume to the box can improve the low end SPL dramatically.

My next project after finishing my ESB-4 will is to do two Jack 12 Lites. They're still relatively big for 112s, but they use thin plywood with aircraft bracing to make them light. I plan to make them swith able so that they can be used either for bass or PA....and so that I have the lightest possible cab that can still keep up with a drummer ;-)

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[Simon Barden]

I wish you the best of luck with your build.

[fender3x]

I wish you the best of luck with your build.

Thanks! I had about half the jigs made, the plywood,drivers and tweeters bought...and then I got distracted with a bass kit...

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[Simon Barden]

Better than being distracted by fluffy kitten videos.

[ihasmario]

Ive got a fEarful bass/fullrange cab here

[UpperCaseH]

I'm gonna try to build a small cab to go with my DIY 1 watt amp. I'm also building a different tiny amp, so I kinda need a cab to test them because hooking alligator clips to the speaker in my horrible old no-name practice amp is getting real old.

I've decided to go with a Celestion Eight15, mostly because it was the nicest speaker I could find that wasn't overly expensive. This is more of a test run than anything else. If I enjoy the process and it comes up OK, I'll look into doing it better with a bigger speaker (or speakers).

Going to build an open-back cab for it out of 12mm ply, with pine battens. It will be screwed together and also glued. I'm probably overbuilding, but this is a test run for an eventual 1x12 or 2x12. Plans for finishing are simple: fill screw holes, sand, spray black, clearcoat. I'll get some rubber feet and plastic corners, a grille of some kind, and maybe a handle for the top. Possibly, if the new DIY amp sounds OK, I'll recess a plate into the top and turn it into a combo.

Over the last couple of days I've been trying to learn to use Sketchup to produce plans. There's no indication of where the jack plate will go, but it's gonna be the center of the top part of the back. I've never used a program like this before and I'm sure it shows, but here's what I've come up with:

[Simon Barden]

Looks like a decent design. A bit over-engineered as you say, but you certainly won't get any cabinet resonance!

Have you considered making a spare panel, that either screws or velcros into the rear gap, so you can convert it into what would effectively be a closed-back cab?

Is there any reason, apart from the lower cost, that you've picked an 8" speaker? Unless there's a special reason for choosing an 8" speaker, I'd have picked a 10" speaker myself, even for use with a small amp. 10" is about the smallest size speaker IMO, that you get any decent bass-end from. Looking at the frequency performance graphs on the Celestion website, there's about a 5-6dB difference in output at 80Hz (roughly the fundamental frequency of bottom E on a guitar) between the Eight 15 and the Ten 30. That's almost twice as much output at that frequency. Their outputs are similar once they get to around 150Hz, but that's almost a whole octave where the 8" speaker has quite a lot less bass-end output than the 10".

[UpperCaseH]

Good point on the 8" vs 10+" stuff. That's in line with what I've read over the last couple of weeks. I'll see if I can figure out a better way to cut the ply and if I can get an extra panel then I'll try both open and closed backs.

My eventual goal is to build a tube amp and a 2x12 to go with it, but that's a long way past my current skill set and tool collection. I learn best by doing, but I make a lot of mistakes as I go so I like to try to learn on something that won't break the bank if I mess it all up.

Also this is intended to be a testing speaker for home made amps. I'm sure I'm going to damage it at some point, even with the 0.5-2 watt stuff I'm learning on at the moment. Don't want to waste an expensive speaker on something like that!

[Simon Barden]

When testing out your home-made amps, it may be wise to construct a simple resistor load bank to test the amp's outputs on first, before connecting up to a speaker, especially as you are only dealing with small wattage amps. You can get 5W and 10W rated wire-wound power resistors that come in a range or low value resistances. Some come with a heat sink, others need an external heatsink.

So you could make up a switchable load bank of 4, 8 and 16 ohm resistive loads (or as close to those values as you can get with the resistance values available by using a mixture of series and parallel connections) so that you can test the amp's outputs on at all the available output transformer tappings, and measure the volt drop across the resistor bank to calculate the output wattage being produced, before connecting up to a speaker. Slightly lower DC resistance values than the standard speaker ratings are OK, and most speakers' AC impedance values are lower than the DC resistance value at most frequencies (except around the speaker's resonant peak).

For testing purposes, it would be worth getting a test-signal generator with a variable output level as well, as it's a lot easier than trying to play a guitar whilst measuring voltages! It's also very useful if fault finding signal paths in the amp. And an oscilloscope would then probably be your next purchase. It never ends!

[Simon Barden]

If you were recording with an 8" speaker, you could use the high bass-proximity effect of a ribbon mic positioned right by the speaker to increase the amount of low end in your signal. You can do the same with other sized speakers as well, but you may have to back the mic off a bit as the speaker size goes up, in order to not end up with too bass-heavy a signal.