How To Test A Vintage Celestion Greenback Speaker

How To Test A Vintage Celestion Greenback Speaker

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:

1. Check all glued areas
2. Inspect cone condition
3. Check for voice coil rub
4. Assess sound quality

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.

Spider Support

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.

celestion spider support

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.

amateur spider repair

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.

damaged celestion gasket

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.

Dustcap

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.

lifting celestion dustcap

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!

Cone condition matters!
Cone condition matters!

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:

celestion cone creases

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.

coil rub check

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.

The amp I am using right now for speaker testing is an old Laney 80w PA with a minimum impedance of 4 Ohm. This outputs 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 nice and high. Most speaker problems do not reveal themselves at low volume, so give it plenty of power.

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 at the high volume required to do it properly.

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:

guitar frequency chart

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.

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!

4 Comments

  1. James-F
    April 26, 2020 / 7:25 am

    Excellent article, I’m bookmarking it for future reference.
    This whole website is a goldmine for Speaker info.
    Thank you very much for this Brian.

    • Brian Harding
      Author
      April 26, 2020 / 8:47 pm

      Hi James, many thanks, glad you like the article.

      regards, Brian.

  2. September 9, 2020 / 6:21 am

    Thanks Brian. I learned so much the past to day, where I have been actively bidding on “the bay”. Though I give a comment on one of the clips, I will mention it here too. I am new to the international community of websites selling speakers, preferably pulsonic Greenbacks and I want to share: don’t be too eager when bidding or buying. Really check the pictures even ask for more elaborated pictures for the sellers before bidding.
    It can really be a problem or a drag to withdraw a bid or a pain in the 💥 to return a bought item, because of male functions.
    Best regards Steffen

    • Brian Harding
      Author
      September 12, 2020 / 6:07 pm

      Hi Steffan, good advice. When I am buying speakers on ebay, if I can’t view them in person before making payment, I will often ask the seller to do a full coil rub check and spider support check as shown on this page. Better any problems are spotted whilst in their hands than in mine. The dispute / returns process on ebay can be long and horrible, not all sellers are the friendly type!

      regards, Brian.

Leave a Reply