Bass amp speaker

[loopyloo]

Went to play a little bass guitar yesterday and noticed the speaker is starting to sound a lot like fluttering paper. You know, like fuzzing not buzzing.
The amp is an Australian made "Stage Musician BS60". You won't find anything on the net about it, I couldn't either.
The speaker cone support ring has a few minor breaks in it, and I guess the papery sound is air pumping in and out through those slits.
Can't afford a new speaker so looking at buying and installing a new foam ring to it.

Anyway that's not the subject here. One thing that prob didn't help it is the thump when turning it on and turning it off, it does thump hard.
I should have looked at it years ago, I assume something is awry in the circuit, bad cap maybe ?
Any ideas before I open it ?
It's actually a really good sounding and powerful amp so worth fixing.
I guess if I can't find the fault I'll have to fit some sort of delay relay to the speaker.
I figure I might as well fix it while I'm waiting for the foam ring to come from china.

[Reschs]

Very surprised to find a foam surround on a Bass Amp speaker.
Foam surrounds are for Domestic HiFi.
If it is a full paper cone, you can use the woodworking glue to repair.

The thump is caused by the charging times of various capacitors in the power supply.
Does it do it with the master volume at zero ?

[Skepticist]

Some amps just thump and others are designed not to. Could be an amp problem depending on the output stage (DC bias out of whack on the output)
What I've resorted to a couple of times is to fit a time delay circuit (actually a small pic chip like a 508 or 509 with selectable time delays via dip switches) that switches a relay to connect the speaker after sufficient delay for the amp and power supply to stabilise. Could get really fancy and sample the mains AC input to de-energise the relay if several consecutive cycles are missing, that way a shutdown thump can be avoided as well by disconnecting the speaker before the main caps discharge.

[loopyloo]

Very surprised to find a foam surround on a Bass Amp speaker.
Foam surrounds are for Domestic HiFi.
If it is a full paper cone, you can use the woodworking glue to repair.

The thump is caused by the charging times of various capacitors in the power supply.
Does it do it with the master volume at zero ?

Yep the thump is irrelevant of where the vol is set.

The surround is paper but the only ones for repair I could find are foam.
I think woodworking glue is not flexible enough, esp since a bass speaker cone can travel far. I've used it before along with tissue to repair cones (and it works well) but not the surround.
I did contemplate trying a rubber glue or contact adhesive, that's pretty flexible but might go hard over time. Still, I'm going to do that for now so I can use it til I get a new surround.

PS when speaker cones get old and furry (which means they're also softer) a coat of shellac does wonders. Nothing like havin' a stiff one

[Reschs]

Yep the thump is irrelevant of where the vol is set.

The surround is paper but the only ones for repair I could find are foam.
I think woodworking glue is not flexible enough, esp since a bass speaker cone can travel far. I've used it before along with tissue to repair cones (and it works well) but not the surround.
I did contemplate trying a rubber glue or contact adhesive, that's pretty flexible but might go hard over time. Still, I'm going to do that for now so I can use it til I get a new surround.

PS when speaker cones get old a furry (which means they're also softer) a coat of shellac does wonders. Nothing like havin' a stiff one

Aquadhere is perfect for speaker cones and is often used to "Dope" the cones to control their performance.
Fitting a foam surround will be difficult and completely change the behaviour of the speaker.

A lot of the older transistor amps thumped on switch on and off.
Unless the cone is extremely distressed, just ignore it.
Silicon Chip Magazine has many speaker protector kits.
Pick one and I will send you the circuit.

[lsemmens]

I have an old Stage Master Bass Amp - vintage, 70s. Also precious little info on that too. Probably a re-badged Elk. Yours may be the same. Given the rarity what's the chances of finding two people with similar amps.

FWIW I've repaired cones with tissue and aquadhere in the past, These days, if I manage to blow a cone, I'd use it as an excuse for a new speaker or amp.

[loopyloo]

I have an old Stage Master Bass Amp - vintage, 70s. Also precious little info on that too. Probably a re-badged Elk. Yours may be the same. Given the rarity what's the chances of finding two people with similar amps.

FWIW I've repaired cones with tissue and aquadhere in the past, These days, if I manage to blow a cone, I'd use it as an excuse for a new speaker or amp.

[loopyloo]

Aquadhere is perfect for speaker cones and is often used to "Dope" the cones to control their performance.
Fitting a foam surround will be difficult and completely change the behaviour of the speaker.

A lot of the older transistor amps thumped on switch on and off.
Unless the cone is extremely distressed, just ignore it.
Silicon Chip Magazine has many speaker protector kits.
Pick one and I will send you the circuit.

Is there a list for those speaker protection kits somewhere ?

[BCNZ]

I bought a couple of these:

They work really well and for $9 including free shipping you can't go wrong.

I built one into my test set for use on the bench when hooking up amps with unknown faults. Stops my speakers from being nuked.

[loopyloo]

I bought a couple of these:

They work really well and for $9 including free shipping you can't go wrong.

I built one into my test set for use on the bench when hooking up amps with unknown faults. Stops my speakers from being nuked.

Hey that's great for the price.
Do they come with any instructions ?
I'll have to check out the amp to see what's coming off the mains transformer.

PS I had a look on ebay, I see you can get them for $4 but they say to use an independent power supply, Wonder why that is ?

[Skepticist]

Independent supply allows monitoring the mains supply so the speaker can be disconnected on power failure before the amp supply caps can discharge. One of those small mains transformers from Jaycar (12.6V AC CT) would be perfect.

[loopyloo]

Independent supply allows monitoring the mains supply so the speaker can be disconnected on power failure before the amp supply caps can discharge. One of those small mains transformers from Jaycar (12.6V AC CT) would be perfect.

Ah I see, so that's handy if they protect with switch off too.

[Uncle Fester]

Went to play a little bass guitar yesterday and noticed the speaker is starting to sound a lot like fluttering paper. You know, like fuzzing not buzzing.
The amp is an Australian made "Stage Musician BS60". You won't find anything on the net about it, I couldn't either.
The speaker cone support ring has a few minor breaks in it, and I guess the papery sound is air pumping in and out through those slits.
Can't afford a new speaker so looking at buying and installing a new foam ring to it.

Anyway that's not the subject here. One thing that prob didn't help it is the thump when turning it on and turning it off, it does thump hard.
I should have looked at it years ago, I assume something is awry in the circuit, bad cap maybe ?
Any ideas before I open it ?
It's actually a really good sounding and powerful amp so worth fixing.
I guess if I can't find the fault I'll have to fit some sort of delay relay to the speaker.
I figure I might as well fix it while I'm waiting for the foam ring to come from china.

OK, I saw the pic of your amp.

Under no circumstances change the suspension of the cone with a foam version.

This is not an airtight hifi speaker cabinet and your cone will end up all over the place, unless you only intend to play at a quiet room volume.
PVA glue like Aquadhere is good for the cone but a quality contact adhesive like Pattex contact adhesive should be used in the suspension(surround).
I saw it at BigW. It will stay flexible enough for quite a while. I have used the yellow one(in Germany) decades ago and it did get powdery after a few years, so I would just go over it again. This new clear version is supposed to stay flexible.
I also used cold tar(bitumen) paste in really bad cases but I don't know if you can get that here. I would warm it in a pot of hot water(DO NOT USE A HOT AIR GUN OR HAIR DRYER - IT IS FLAMMABLE ! ) to make it more appliable and do the whole surround. The speaker must be lying down flat, while the tar is still runny.
Maybe you can find some kind of bitumen paste in Bunnings where they sell roof repair materials.

Now the thump, I would definitely look at the filter caps, try to measure their capacity even just with an Ohm meter if you don't have a cap range or ESR meter and count how long it takes until it goes to 1999 . The idea is to find out if both caps for the + and - power rail still have the same capacity.
If not you must replace the weaker one (or both) because it is not only causing the thump but I am assuming that your fuzzy sound could be coming from one rail going partially AC under load.

Putting a bandaid like a delayed relay to cover up the thump doesn't fix anything, even if it sounds sensible as a speaker protection.

[Reschs]

Now the thump, I would definitely look at the filter caps, try to measure their capacity even just with an Ohm meter if you don't have a cap range or ESR meter and count how long it takes until it goes to 1999 . The idea is to find out if both caps for the + and - power rail still have the same capacity.
If not you must replace the weaker one (or both) because it is not only causing the thump but I am assuming that your fuzzy sound could be coming from one rail going partially AC under load.

Putting a bandaid like a delayed relay to cover up the thump doesn't fix anything, even if it sounds sensible as a speaker protection.

Given the age of the Amplifier, it would be single DC rail and have a output coupling capacitor.
Most of these had a turn on/off thump by default.
Unless the thump is excessive , leave it alone.
Reassess when the speaker is repaired as it's current state may make the thump seem worse.

[loopyloo]

OK, I saw the pic of your amp.

Under no circumstances change the suspension of the cone with a foam version.

This is not an airtight hifi speaker cabinet and your cone will end up all over the place, unless you only intend to play at a quiet room volume.
PVA glue like Aquadhere is good for the cone but a quality contact adhesive like Pattex contact adhesive should be used in the suspension(surround).
I saw it at BigW. It will stay flexible enough for quite a while. I have used the yellow one(in Germany) decades ago and it did get powdery after a few years, so I would just go over it again. This new clear version is supposed to stay flexible.
I also used cold tar(bitumen) paste in really bad cases but I don't know if you can get that here. I would warm it in a pot of hot water(DO NOT USE A HOT AIR GUN OR HAIR DRYER - IT IS FLAMMABLE ! ) to make it more appliable and do the whole surround. The speaker must be lying down flat, while the tar is still runny.
Maybe you can find some kind of bitumen paste in Bunnings where they sell roof repair materials.

Now the thump, I would definitely look at the filter caps, try to measure their capacity even just with an Ohm meter if you don't have a cap range or ESR meter and count how long it takes until it goes to 1999 . The idea is to find out if both caps for the + and - power rail still have the same capacity.
If not you must replace the weaker one (or both) because it is not only causing the thump but I am assuming that your fuzzy sound could be coming from one rail going partially AC under load.

Putting a bandaid like a delayed relay to cover up the thump doesn't fix anything, even if it sounds sensible as a speaker protection.

Often guitar amps are open backed, you can throw the cord and foot switch in the back next to the speaker for storage, but this one, although it may not be actually air tight, it is fully enclosed.
I already gave the suspension a coat of contact cement which did cut the fuzz down a lot but not completely. I figured if the glue eventually goes hard I will have worked out by then where to get the right replacement suspension (or afford a new speaker). I made sure the glue doesn't overlap onto the cone so that removal will be easier.
The turn off thump is a little harsh but the turn on is loud and violent so I will be checking the amp out.

Thanks for the good advice, I'll come back here when I get the time to dive into it. Won't be long.

PS Interesting, I've never heard of Pattex, I'll keep an eye out for it cos I reckon it might have some good uses elsewhere too.

[Uncle Fester]

Often guitar amps are open backed, you can throw the cord and foot switch in the back next to the speaker for storage, but this one, although it may not be actually air tight, it is fully enclosed.
I already gave the suspension a coat of contact cement which did cut the fuzz down a lot but not completely.

That is not really good. Are you really sure there is no opening hidden even just a few cm somewhere?
I have seen the strangest ventilation, just a couple of cut away corners in the back or front.


Generally bass musician speaker cabinets should have tuned ventilation. Correctly done it would substantially increase
efficiency in the low frequencies where you need it most.
Properly tuned and the speaker is hardly moving at all at the lowest frequencies when the enclosure provides the correct load.
No wonder the suspension has suffered.
The volume of that enclosure is also too large to provide proper cushioning for a foam rubber suspension which is generally
reserved for hifi/full range music reproduction where pulse accuracy is required for percussion but that is not required for a bass instrument.

Here is my little bass setup:





The Dynacord is a 200W valve power stage driving a Goodmans Speaker. The amp weighs as much as the speaker.
If you look closely you can see from the angle of the photo through the screen the 15" speaker on the bottom, then 2 smaller holes left and
right that had tubes for tuning and above a larger flat hole in the middle. The suspension of this over 40 year old speaker shows not the slightest
sign of wear. I don't even think it is rated for the 220W when I really overdrive the amp.

On top you see a slab of wood with a couple of valves, which I designed out junk bits for shits and giggles.
It outputs a mere 2W RMS. Connected to that speaker cabinet I can play bass next to a drum kit with no problems being heard:







If you are ever in the mood you could make simply make a 40-50mm hole in the baffle so you can insert a fitting piece of PVC pipe as a port tube.

The hole even without the pipe could already make it sound better.
The way I see it anything would be an improvement because it would stop forcing the air through the gaps in the damaged suspension
causing any fuzzy noise and stressing it.
Example use 42mm PVC pipe. Insert different lengths and play the deep E-sting until it sounds fuller and/or the speaker looks like it barely vibrates.
Then you really nailed it but anything that makes you think it sounds better is already a success. Bass is tricky because room resonance plays a big role too.
You will also need to stand a few meters away and move around to hear it right.
This is always best done through experimentation. If you really can't find any improvement then you can always glue up the hole with
a bit of wood, but I am fairly certain that there must be some improvement possible.


Sorry if I might seem a bit carried away.
I worked part time as a tech in a music store in the 1980's and 90's and I really miss those times for a lot of reasons.
I also built speaker cabinets including fairly large folded exponential bass horns.

[loopyloo]

The box is sealed excepting for the tight hole where the speaker cable passes through.

[loopyloo]

Both main caps are 2200uF 50v and measure very close to that, also oddly enough ESR is 0 ohms each.
The yel, yel, grn wires come directly from the mains transformer.

What are the 2 trim pots for ?

[BCNZ]

Hey that's great for the price.
Do they come with any instructions ?
I'll have to check out the amp to see what's coming off the mains transformer.

PS I had a look on ebay, I see you can get them for $4 but they say to use an independent power supply, Wonder why that is ?

No instructions but you don't need them - you can't go wrong. There's only a few connections to make and if you've got basic electronics skills (which I know you do!)
then you'll be fine. If you get stuck you can always ask.

Both main caps are 2200uF 50v and measure very close to that, also oddly enough ESR is 0 ohms each.
The yel, yel, grn wires come directly from the mains transformer.

What are the 2 trim pots for ?

The trim pot closest to the heatsink will set the idling current in the output transistors, the other trim pot whilst it's not totally clear (and I haven't the time to trace the circuit out)
is more than likely a gain adjustment.

[Reschs]

No instructions but you don't need them - you can't go wrong. There's only a few connections to make and if you've got basic electronics skills (which I know you do!)
then you'll be fine. If you get stuck you can always ask.



The trim pot closest to the heatsink will set the idling current in the output transistors, the other trim pot whilst it's not totally clear (and I haven't the time to trace the circuit out)
is more than likely a gain adjustment.

The trim pot at the front is for DC Offset. With the Input to the power amp shorted to ground, adjust for 0v at the speaker output.
That said, neither should need adjusting.
Look at the preamp section for your problem.
To check, short the input to the power amp board and then switch on and off.