Guitarists often go to great lengths and expense in their quest for the ultimate guitar tone. Yet speakers all too often get overlooked. So long as they make a noise they are good right?
Actually, there are many potential problems a speaker can have, from voice coil rub to a torn cone, all will have a negative impact on your tone.
Here is my simple 4 step process to assess the condition of your speakers:
To perfom all 4 tests should only take a couple of minutes per speaker and they are all super easy to do. Apart from the sweep test (optional, but highly recommended), all of these tests can be done with just your bare hands, eyes, ears, and the gear you likely already own as a guitarist. No excuses!
Step 1) Check all glued areas
There are 3 important glued areas on a speaker; the edge of the spider support, the edge of the dust cap, and the outer edge of the cone. For a speaker to function properly and sound ‘right’, all of these areas need to be 100% fully glued down.
To check the glue at the spider support, lightly brush your finger tip against the edge in an upwards motion to check it is still holding. Work all the way around the edge, bit by bit.
It is always tempting to do this in a ‘half arsed’ way, so time yourself. To check it properly should take a full 60 seconds per speaker minimum. Be as thorough as you can.
Use your ears as well as your eyes. Very small areas of failed glue may not be visibly obvious, but if the glue is not holding you will hear it as your finger brushes against it, as in the video above.
Failing glue at the spider support is probably the most common fault to find on old greenback speakers. Therefore it is very important to check, and is always the first thing I look for.
Also watch out for sloppy amateur repairs. A bad repair done with super strong modern glue is often irreversible and the speaker could be ruined with permanent coil rub if the voice coil has not been re-aligned properly.
Cone edge and gasket
Check the cone has not been torn off, or become unglued, at the outer edge. This happens more often than you might think. It is usually caused by someone trying to pull a stuck speaker out of a cab with brute force. The gasket stays stuck to the baffle while the cone rips off.
Brush the edge of your thumb against the gasket in an upwards motion to make sure it is still holding all the way around.
When removing speakers from cabs, always make sure they are free from the baffle 100% before trying to lift them out. Use something thin like a feeler gauge to work underneath the gasket and unstick it from the baffle.
Gaps in the dustcap glue are fairly common and will cause the speaker to buzz. This is not usually a serious problem unless dirt has gotten inside the speaker causing voice coil rub. To repair a dustcap like this neatly, it is best to carefully remove the whole thing and reglue it back on.
Step 2) Assess the condition of the cone
The condition of the cone is important because it will have a direct impact on the quality of the tone it produces.
The best sounding cones are usually the ones that are still near mint – firm to the touch and blue/grey in colour. Cones in poor condition that have gone very brown and soft, had severe repairs, or flood damaged, will usually sound the worst – if they are even fit for purpose at all. However, I do think it is important to let your ears have the deciding vote, some speakers can surprise you!
Also watch out for small tears forming at the outer cone edge, and along the rear of the cone. This is usually the sign of a well used speaker:
Step 3) Check for ‘voice coil rub’
There are many reasons why a speaker can have voice coil rub. From a bit of dust in the coil gap to more serious problems such as a damaged voice coil or a shifted magnet.
To check for voice coil rub you need complete silence, so find a quiet room and switch off any background noise. Then put the speaker on its back, preferably on something soft like a folded blanket then you don’t scratch the plastic cover when rotating it.
The resting position of the cone is roughly the halfway point of its full movement. So it is important to test both the inward movement, and the outward movement.
Check inward cone movement:
Press lightly down on the edge of the cone with both of your hands touching at the index finger, and then let go. Repeat this about 6 times so that you “pump” the cone in and out, slightly off its normal axis. Do not press hard, just use a light pressure.
It should be a completely silent movement. Any scraping or clicking sounds are bad!
Bored already? There are videos demonstrating this process further down the page 😉
You need to do this all around the front edge of the speaker cone. So turn the speaker an inch or two and repeat. The way I usually do it is to imagine the front of the cone cut into 8 sections like a pie, and perform the test in each section. Use the screw holes as a location guide.
Check outward cone movement:
Do a similar thing again for the outward movement. This is a bit more awkward because the chassis legs will be in the way, so you will only be able to do it at 4 locations.
Place your finger tips under the chassis about mid-way up the cone, and press upwards using light finger pressure. Then turn the speaker and repeat until you have done it on all 4 sides.
Again, this should be a silent movement.
How not to do it
Most people check for voice coil rub by pressing directly down on the cone only, not around the edge as shown above. This is nowhere near a sufficient test in my opinion.
Speakers can still sound really horrible and pass that test. Don’t ask me why, but I can only assume that the voice coil vibrates a fractional amount horizontally too when in use. If you watch all the videos on this page you can see this being demonstrated.
I also think it’s important to check the full range of motion of the cone. So always check the outward cone movement as well as the inward cone movement.
Find ‘benchmark’ speakers for comparison
If you’re not sure if you are performing these techniques correctly or maybe just think I am talking a load of nonsense, then for comparison try them on a modern speaker that you know to be perfect. That way you have a ‘benchmark’ for how a good speaker should test. Ideally use a speaker that has been well broken in and has good cone movement.
Likewise it is a good idea to keep a known bad speaker around so that you have an idea how a bad speaker tests. Eg how loud you need to play before the coil rub makes itself known. Every speaker is different of course but having some benchmark speakers to hand will definitely help you when first starting out testing speakers.
Step 4) Assess sound quality
Ideally you first want to test your speakers with an audio generator to listen out for faults, and then with guitar and amp to assess the quality of the tone. However if you don’t have the equipment you can probably get away with just one method or the other.
Click here to skip to testing with guitar and amp.
Test 1 – using an audio generator (sweep testing)
If you do not own a sine wave audio generator, don’t panic, there are various free apps around thesedays such as www.synalski.com tone generator that you can use instead.
If you want to buy an audio generator, check whether it has a good frequency display or not. To match frequencies with the notes on your guitar, you will need to know what frequency you are at.
If you’re interested in what gear I use myself, I had a ‘Tenma 72-455A’ for many years and can recommend them. However I am currently using an older ‘Trio AG-202A’ unit.
One thing you will definitely need is a suitable amplifier. Ideally transistor then impedance matching is not an issue. Also you do not want to kill your speaker. Something that puts out about 15 to 20w is ideal for testing old greenbacks.
Remember that transistor amps only put out their highest power rating when used with the minimum speaker load, usually 4 Ohm. So a 100w amp into a 16 Ohm speaker will probably only be putting out about 25w, I’m reliably informed. However do not risk it and fully dime the amp. Just have it loud enough to get the speaker cone visibly moving.
The amp I am using right now for speaker testing is an old Laney 80w PA with a minimum impedance of 4 Ohm. So in theory, that puts out around 20w with a 16 Ohm speaker.
How to do it
Lie a single speaker on its back, preferably on something soft to cushion the vibrations, then connect up to your audio generator and amp, and power on.
With the frequency dial on the lowest setting (10Hz on mine) turn the volume up. Most speaker problems do not reveal themselves at low volume, so give it plenty of power, but not so much that you are going to blow the speaker.
At 10Hz the cone will start bobbing up and down through it’s full movement. You will learn a lot about your speaker just by running it at 10Hz like this. You should only be able to hear a slight purring noise of the cone moving. Any rubbing, scraping, rattling, or clicking noises are bad!
Next, slowly turn the dial up through to 100Hz and listen out for rattling, buzzing and distorted overtones. A speaker without any issues, and that is nicely broken in, should sound nice and smooth as you move up through the frequencies.
A low frequency sweep like this will weed out 99% of coil rub and other speaker faults in my experience, without needing to sweep the higher frequencies.
To check for cone cry on the higher frequencies I prefer to use guitar and amp (see next test below), because the high frequencies on an audio generator can be very painful on the ears.
About “Cone Cry”
Cone cry can be described as an additional ghost note or an unwanted harmonic. Usually heard whilst bending notes on the guitar.
Pretty much all vintage speakers will have a small amount of cone cry somewhere when tested with an audio generator. Unless the issue is particularly bad it is not something I would worry too much about.
However, if you do find any very troublesome frequencies or severe cone cry, then it is always worth trying to replicate that frequency with your guitar just to see how noticable the issue is ‘in the real world’.
To do that you will need a reference chart like this one to match the note on your guitar to the frequency displayed on your audio generator:
Test 2 – using guitar and amp
You don’t want to overheat your vintage greenback so choose an amp around 15w to 20w that you can run into a single speaker. I’m currently using a Marshall SV20H, but for many years I used an Orange Tiny Terror.
How to do it
Lie a single speaker on its back, preferably on something soft to cushion the vibrations, then connect up to your guitar and amp, and power on.
It is best to use a fairly clean tone, or just on the edge of break up. Too much distortion, or other effects, can mask any sound issues with the speaker.
Get loud! As I mentioned earlier, most speaker problems such as voice coil rub will not reveal themselves at low volume, even severe cases. Also, in my opinion speakers only show their true tonal characteristics when they are made to work hard. Find the sweet spot. My rule of thumb is that if you cannot visibly see the cone moving up and down whilst you are playing then you are probably not playing loud enough. Get it really pumping!
To check the speaker properly you should really do a full chromatic run (play every note) from the low E note (82Hz) to the highest note on your fretboard. Some speaker issues will only reveal themselves when you play certain notes, so try to cover them all, especially if you have skipped testing the speaker with an audio generator. This is the next best substitute.
It doesn’t have to be completely boring though. Have some fun and have a bit of a jam all over the fretboard. Play some power chords and palm muted ‘chugs’ to check the lower frequencies. Check the mid frequencies with some double string bends around the 8th to 12th fret, and check for cone cry in the higher frequencies with some single string bends high up on the fretboard.
Just generally have a bit of fun and listen to what your ears are telling you. Some speakers will disappoint you and some speakers may surprise you. That is all part of the fun of vintage gear. Keep experimenting until you find the really great sounding gems!