Technical - Speaker Cabinet
There are three predominant B-15N cabinet types: the double baffle, the single baffle, and the Theile-Small design. The B-12N cabinets were not scaled down version of the larger cabinets, they had different designs. The somewhat rare B-15NC/BT-15C column cabinet was a different design as well.
Each cabinet has subtle differences in the way that they sound. Each has their fans and they all sound good.
Double Baffle Cabinet
The original B15 speaker cabinet design was a double baffle bass reflex system, incorporating a ported inner baffle - typically with 8 oblong holes - and an outer baffle separated by spacer washers. This design was used from inception through 1966. These cabinets came standard with either a Jensen or CTS speakers stock with JBL D140 available as an upgrade.
The Anatomy of a double baffle cabinet
Trivia question: How many pieces of wood are there in a vintage double baffle B-15N cabinet?
Answer: Let's see if I've got them all: front baffle (1), four spacer blocks around the speaker opening of the front baffle (4), front frame (4), four spacers around the frame (4), rear baffle (1), rear baffle glue blocks (4), cabinet bottom, two sides, and back (4), top front rail (1), top back rail (1), top gasket frame (4), lid (1), amp tray (1) and frame (4), dolly (1). A grand total of 35 pieces.
Hardware includes the blue check vinyl, grille cloth and under padding, the chrome Ampeg logo, a lid handle, an aluminum sheet in the amp tray, four isolation mounts, eight cabinet corners, foot glides: four on the back, three or four on the bottom depending on the model revision, dolly casters: two swivel, two fixed, dolly mounting bolt assembly and rubber disk to seal the inside of the cabinet, four rubber bumpers for the top of the dolly, foam lid gasket, tilt back leg, back panel cup assembly, two leg storage clips, acoustic damping material, four latches and four lid lag bolts, the speaker, speaker wire, wire anchors and ties, speaker connector, and assorted mounting screws, nails, and staples. You could optionally add a protective vinyl cover.
My point is that a lot of material and workmanship go into these cabinets. They are, without a doubt, a work of art.
B-15N 3D exploded view of a double baffle cabinet. Courtesy of Mark Gandenberger.
Variation in the double baffle cabinet's port size
Ampeg flirted with the size and length of the baffles in the double baffle cabinet. Everett Hull wanted to try shorter and narrower, 2" X 1/4", ports and they were in production for a short period of time. Around this same time period Ampeg also experimented with adding foam rubber plugs to some of the ports on some cabinets to alter the voicing. Jess Oliver preferred the sound of the cabinet with the standard 6" X 3/4" ports that were used in his original design. He won out and Ampeg went back to the standard sized ports. The ports are located on the inner baffle. Here is an example of the two port sizes, as well as a photo showing the port plugs.
Single Baffle cabinet
The second cabinet design was a single baffle system, and can be easily identified from inside the cab by the lack of inner oblong ports. The face frame of a single baffle cabinet is also wider than that of a double baffle cabinet. The porting in this cabinet was simplified which made it easier to build. They used spacers to set back the speaker baffle from the cabinet's front frame. There's a gap on each side of the baffle and with the spacers, it forms a port that allows air to pass around the sides of the baffle and out the front of the cabinet.
This design was used on the last year of the blue check cabinets and the black tolex/silver piping cabinets. These cabinets came stock with CTS speakers with JBL and Altec speakers available as an upgrade.
Portaflex amps produced after 1969 incorporated a Thiele-design cabinet which used a shelf port at the bottom which vented out the front of the cabinet. You can see how this works by looking at the Electro-Voice TL-606 cabinet design. This type of design is common in many cabinets produced today. These models were slightly larger in size to accommodate the porting design. The B15S cabinets also utilized this style of porting. These cabinets came stock with CTS or Eminence speakers with an Altec 421 available as an upgrade.
What is a Thiele-Small cabinet?
The term Thiele-Small can be confusing. It simply means that the Thiele-Small speaker parameters for the driver installed in the cabinet were used to design the speaker cabinet dimensions and port size. There are a number of different tunings possible by varying the port. These tunings have different names, such as Butterworth B4, which is a maximally flat amplitude tuning. Thiele-Small cabinets are often referred to as simply Thiele cabinets.
When is a Thiele cabinet not a Thiele cabinet? If you install a speaker with specs that are different than those of the original in a Thiele cabinet, it may no longer be a Thiele cabinet. But if it sounds good, who cares!
Context: Where can I get the T-S Parameters for this speaker?
Description: Loudspeakers are electromechanical devices, or more precisely, electroacoustic transducers. The goal is to convert energy in the form of electrical power to energy in the form of acoustic power. Since loudspeakers have both electrical and mechanical properties, it stands to reason that there are specific mathematical quantities that represent both the electrical and mechanical properties. To design a set of mathematical formulas to combine these quantities and then describe the overall characteristics of the loudspeakers was the goal of two acoustic researchers named Neville Thiele and Richard Small, hence the term T-S Parameters.
They measured a few of the parameters of the loudspeaker such as electrical resistance of the voice coil, free air resonant frequency, inductance of the voice coil, and the cone size. Then, they would mount the speaker in a sealed box of a known volume and remeasure the resonant frequency as well as a few other parameters. Their extensive set of formulas could then be employed to determine and estimate very accurately the remaining unknown quantities of the loudspeaker (or driver), such as the system Q, the volume of air displaced by the cone, voice coil (motor) strength, overall efficiency, and estimated Sound Pressure Level. The resultant parameters are invaluable to speaker cabinet designers, enabling them to design the cabinet, porting, etc. to match and be compatible with the parameters of the loudspeaker. In very simple terms, if you consider the loudspeaker as a signal source with a specific characteristic impedance, by using the T-S Parameters you would be able to design a cabinet that would match the characteristic impedance of the source. This would allow the loudspeaker to operate at optimum efficiency over its frequency and power band. Most manufacturers will provide the T-S Parameters of their loudspeakers and drivers upon request, and some offer examples of designs for both sealed and ported enclosures which will be optimum for their loudspeakers and drivers. By searching the World Wide Web, you can find many examples of cabinets, formulas, and driver data. When you read about a particular guitar amp speaker enclosure and the ad says Thiele cabinet, it means that the designer has employed the T-S parameters of the loudspeaker(s) he or she is using in the design of the enclosure or cabinet. Quoted from: Weber's Glossary of Speaker Terms
B-15NC BT-15C column cabinet
The porting of the B-15NC BT-15C column cabinet that has two fifteen inch speakers is illustrated below. There is a shelf between the two speakers that separates the cabinet into two chambers. Porting is via two triangular cutouts in two corners of each chamber.
Cabinet Tube Chart
A tube charts is glued onto the inside of the speaker cabinet. They identify the model and serial number as well as the tube lineup and their position. Handy when you are changing tubes. Some amps had the pre-amp in the upper section and the power amp in the lower section of the cabinet. The tube chart reflected this. The style of the chart varied over the years depending on the model, an example is shown below. The serial number is usually stamped on, sometimes the tube lineup is hand written. It is interesting to note that, when you examine these labels, the handwriting seems to be the same over the years. I always wondered who that person was.
Ampeg used what are called suitcase latches on the original Portaflex cabinets to secure the cabinet lid in place. Similar latches can be found on some Silvertone amps from the 1960's as well as on Hofner instrument cases. These chrome latches add to the look and character of the vintage Portaflex amps. With extended use, they sometimes need small adjustments to keep them working properly. Details as to how to do this are provided in the Tips and Tricks section .
When Ampeg introduced the Heritage B15 and new solid state Portaflex amps in 2011 they featured cabinets that utilized a draw latch from Southco that was much improved over the original. The new Ampeg cabinets utilize a black powder-coated latch, model #97-50-220-15. If retrofitting a vintage cabinet, the slightly shorter stainless steel version (97-50-110-12) is a better match. It would be paired with the 97-57-105-24 button keeper.
If you have this type of latch on your cabinet and you find that it isn't closing tight enough, bending the curved part inward a little more will tighten the draw of the latch. The metal is spring steel and isn't easy to bend but it doesn't take much to make a difference.
Southco latches can be purchased from D B Roberts.
Plateform or Shock Mounts
Shock Mount Specifications
The shock mounts, also called plateform mounts, used by Ampeg were manufactured by Lord, they had a diamond shaped aluminum base, the part number used was 100PDL-8. Some amps have been found with 100PDL-6 mounts. The difference between the two is the maximum axial rate load at 1/16" deflection, being either 8 lbs or 6 lbs. Replacement Lord type vibration plate mounts are available from Fliptops and McMaster-Carr.
If you look closely, there is a month-year date stamp on the metal support that can be used to date your Portaflex.
Cabinet Shock Mounts
You can get the shock mount from McMaster-Carr , just with the square base instead of the diamond base, P/N 6008K61. You would have to supply your own #8-32 bolt hardware but the shock mount is only $7.58. The new Heritage B15s actually use the square base version. The square metal plate can be trimmed to the diamond shape with a bit of work. Isolation mounts are also available from Fliptops.
Shock Mount Wear
The rubber on the shock mounts that attach the head to the cab lid the rubber can eventually dry out, harden, and crack. The amp can be bounced around while being transported. If they were to fail while the amp is inverted, it could lead to some damage to the speaker and the chassis.
One solution is to replace the shock mounts before this happens, but they are expensive. Another thing you can do, whether you keep the old mounts or buy new, is install a 3/4"-to-7/8" diameter snubbing washer on the underside of the mount. This should prevent catastrophic failure because the washer will bear against the thick part of the rubber if the shock mount gets stretched too far.
Auto parts stores sell rubber seal conditioners which helps prevent the rubber from drying out and keeps it supple. These are liquids that you wipe on. The fluid soaks into the rubber, expanding it slightly and softening it.
B-15N Lid-Plateform Mount-Tray Assembly
The B-15N amp is mounted on an elastic suspension that helps isolate it from vibrations. Four isolation mounts, also called plateform mounts or shock mounts, are attached to cabinet's lid with two or in the case of the Heritage amps, four wood screws. An amp tray is attached with nuts to the mounts. The B-15N amp is attached to the tray with machine screws into threaded anchors.
Lord, the plateform mounts manufacturer, recommends that a snubbing washer be added to provide an interlocking system of metal parts which act to prevent damage from overload or excessive shock impact. The washer prevents the mount from overextending itself. It acts as a stop when the mount receives a large impact. This can happen during transport or if the amp is dropped. When the amp is flipped into the cabinet for storage, the washer prevents the amp from falling if the rubber in the mount fails. Ampeg didn't include this washer in the assembly with their early amps, they added them in the 70's. If you don't have them in place, it's a good idea to have one on each mount. Snubbing washers are available from Fliptops or a hardware store. Any size in the range of 3/4" to 7/8" will work, it needs to fit into the counter-bore hole in the lid. HINT: If you can't find a 7/8" washer, find a 1" stainless steel one, mount it in a drill on an arbor and hold a file against it to file it down to 7/8". Stainless is better than a plated washer, the filing will remove the plating from the edge and the washer will rust.
Diagram and images courtesy of Vintage-Blue.
The Lucite logo
The Lucite or Plexiglass logo slides into the slot in the chassis next to the tube cage and is mounted to the cage with machine screws and PEM nuts. Two lamps mounted under the chassis, illuminate the edges of the Lucite and the engraving. Ampeg offered an engraving service that allowed owners to customize their amps.
The internally threaded broaching nuts are manufactured by Penn Engineering. Two PEM nuts are swagged into holes in the tube cage and serve as mounting anchors for the Lucite logo. It isn't uncommon to see logos that soften and warp due to the heat of the tubes. To avoid this, I like to insert Teflon washers between the tube cage and the logo to allow a bit of space between the two.
These nuts are tiny and they are easily lost. It isn't unusual to find that the PEM nuts are missing from Portaflex tube cages. Consider yourself lucky if your amp has them.
How to remove the cabinet's Ampeg script logo
The original Ampeg logo is made of a brittle pot metal and very delicate. Push a very thin, putty knife under the logo and slowly pry it up a little at a time moving over the surface of the logo to locate the three pins that are holding it in place. If you lift it too much up at one end, the logo will break. Try to keep the logo level as you lift it to ensure that the stress over the logo is even. The three nail heads are embedded into the pot metal. On some logos, the nails are not straight, they are not perpendicular to the logo. This makes removing the logo even more difficult. Try to minimize the stress on the logo. This could require pulling one end out before the other.
How to repair a broken Ampeg script logo
So removing the logo wasn't entirely successful. Repairing pot metal isn't easy. There are pot metal solders available, results can vary. Getting them to work is hit and miss. I've had good results using JB Weld epoxy applied to a fine brass screen used as a backing. I found the brass screen at a hobby train store. It helps to apply the screen and epoxy over a few places on the back of the logo so that it lies flat on the cabinet. If the nails break off the logo, they can be glued back in with some of the screen used to reinforce things. Another approach would be to cut out a brass plate, in the same shape but slightly smaller than that of the logo, so that it can't be seen and use that as a backing plate that the logo can be glued to.
If you have access to a 3D printer, life is a lot easier. You can scan and print a new one.
Installing a reissue Ampeg script logo
The original and new reissue logos both have three pins that are used in mounting the logo. Unfortunately, the reissue logo has one pin, the middle one, in a different location. They did this for a good reason. The old logos had the pin in a place that caused them to break easily. The new logos are a little stronger and work out better. In order to install a reissue logo, you'll have to drill a new hole in the cabinet. The new logo covers the old hole so that isn't an issue. The pins can swage into the cabinet holes. Although not necessary, a dab of rubber cement on the pins can help keep the logo in place. The rubber cement remains pliable and allows you to remove the logo if necessary.
How to strip the old vinyl and prepare the wood for recovering a cabinet
Removing the metal corners can be difficult if the nails that hold them in place are rusted. Rust seems to really grab into the wood. I use a knife blade as a wedge to slowly pry up the nails working from under the metal corner. If possible, you want to keep the original corners and nails. They clean up well when soaked in rust removing solutions. Otherwise, replacements are available from Fliptops.
The original vinyl was affixed with hide glue. This is a natural glue that has been traditionally used in building acoustic instruments such as basses, violins, and guitars. A big advantage is that it softens with heat, steam, or warm water. Start by applying water with a damp cloth at a seam. Then start peeling back the vinyl. It is important to do this slowly so that none of the wood is pulled off. Old dried out cabinets can splinter easily and sometimes you can't avoid it. A layer of glue will be left behind on the wood, don't worry about it at this point. Continue until all the vinyl is removed. Hide glue has a unique smell. I keep the old vinyl if I want to use it as a cutting template for the new vinyl.
With the vinyl removed, the next step involves wetting down the glue and removing it with a scraper. I use a Stanley #9 steel back razor blades (11-515A). The thin blade seems to do a good job for me. It is important to not use too much water on the wood. The old laminated plywood can develop ripples which you want to avoid. It will require several passes to remove all the glue. Any residue left behind will prevent the new glue from adhering as well as it can so do a good job at this stage. Give the cabinet a final wipe with a clean damp cloth at the end.
Any bumps or holes will be visible, even magnified, when the new vinyl is applied so wood preparation is very important. Pay particular attention to the box joints along the edges. They often have gaps that need filling. Sometimes the old wood filler crumbles away and needs to be dug out and refilled. Any good wood filler can be used to repair damaged areas. Some people use Bondo, an automotive filler. I prefer to use a product called Quikwood Epoxy Putty. It dries as hard as a rock and yet it can be quickly wet sanded. Fill any splintered areas, nail and screw holes. Use a nail set to seat any protruding nails. Any ripples in the wood should be filled so that the surface is smooth. Another thing to look for is loose layers and voids in the plywood that can be the source of cabinet rattles. These can be stabilized by injecting a glue such as cyanoacrylate with a needle. Look for voids or loose plys around the speaker cutout. These are common in old cabinets and can be filled with wood filler.
The next step is sanding. This will remove any glue residue and prepare the wood for gluing on the vinyl. I start with 60 grit and a random orbital sander. Use a light touch. I end with 120 grit to leave enough roughness for the glue to adhere to. Be care when sanding the edges, you don't want to change the round over. If you sand too much, the metal cabinet corners will no longer fit tightly. If you take off too much, this is where the Quickwood filler comes in handy. You can rebuild surfaces with it and sand them down to the original dimensions. When done, just to be sure, test fit the corners with a scrap of vinyl under them before gluing down the vinyl.
How much vinyl is needed to recover a cabinet and how should it be cut?
It depends on the layout of the vinyl. Ampeg's original approach with separate pieces for the top, back, sides, and bottom optimized production speed. Using a single piece that wraps around the back and sides, as developed by Mark at Vintage-Blue, looks a lot better. It takes skill to apply the vinyl so that all the checks from the different pieces align. Two layout patterns are below, the original Ampeg layout is on the left, Mark's is on the right. Both use about the same amount of vinyl, 2 yards. If you are also covering the dolly, you'll need to purchase an extra yard of vinyl.
Where can I buy thinner vinyl for the amp tray
What type of glue should I use when recovering a cabinet
Ampeg originally use hide glue to affix the vinyl. The workers had to work fast to attach the pieces before the glue hardened. They only had a few minutes to get the job done. Today there are other alternatives available. There is a product called Tolex Glue, which is a water based contact cement. Water based contact cements are available at hardware stores. These products are non toxic and are easy to work with.
Solvent based contact cements are also used. They provide a stonger bond and can be used for both indoor and outdoor applications. They are less forgiving when applying the tolex. Once the two sides make contact, that's it. No room for adjusting. With the water based products, you can lightly apply the vinyl and pull it around in a limited way before pressing it down.
Non water based contact cement makes a stronger bond. How well does water based contact cement work? I learned my lesson the first time that I had to remove some the vinyl that was applied with water based contact cement. The wood failed before the glue did. Splinters everywhere. It is more than good enough. When would I use non water based contact cement? On a particularly difficult job where the glue doesn't want to stick. Wood preparation before gluing is important. Any residue on the wood from the old glue will prevent the new glue from sticking as well as it could. The solvents in non water based glues tend to penetrate into the base better, doing a better job. The tradeoff is that these glues are toxic to work with and as I mentioned, less forgiving when applying the vinyl.
People also use PVA type glues such as Weldbond or carpenters glue. These glues take longer to cure. The water in the glue can't evaporate through the vinyl so it must do so through the wood and this takes time. The vinyl needs to be taped or tacked down until the glue dries.
What should I use to re-glue vinyl edges and small nicks
Cyanoacrylate (often called CA or super glue) works well and is easy to use. The advantage of this glue is that it holds even the smallest nicks down well and it stands up well with time. Clean the vinyl and the wood as well as you can to remove any old residue before applying the glue. I use a toothpick to apply and spread it. Be care not to use too much. Breath on the area first, moisture helps the glue to set faster.
There are rubber based CA products available (in black only as far as I know) that will fill gaps where the vinyl edges have pulled apart. Shrinkage happens on old amps.
Vintage cabinets used hide glue to affix the covering. It is available at luthier supply and some wood supply stores if you want to keep a restoration authentic. This is a water soluble glue and comes in crystal and liquid form. Different strengths are available but it also be regulated by how much water is added.
Other glues, such as contact cement and carpenter's glue will work as well.
Cabinet acoustic damping
The original cabinets had a yellow spun fiberglass insulation material glued onto the inner rear wall of the speaker cabinet. This type of material is not used in modern cabinets because the fiberglass posses a health hazard and there are better solutions available. The rule of thumb when using damping material is to line a ported cabinet and to fill a sealed cabinet. The Portaflex cabinets are all ported so they should be lined. Ampeg elected to just line the back wall of the cabinet.
What does acoustic damping do? Sound waves are reflected backward from the baffle wall, they bounce off the real cabinet wall and reflect forward. The reflected waves combine with the speaker waves to create an interference pattern which can make the cabinet sound muddy, lacking clarity. Acoustic damping absorbs some of these reflected waves, decreasing their presence within the cabinet. The sound waves are converted to heat and dissipated within the damping material. The damping material's composition and thickness helps determine how effective it performs. In theory, the entire inner surface of the cabinet should be lined but in practice, most people find that lining only the back surface is sufficient.
What material should be used to line a cabinet? There are a lot of options. Anything soft with fibers and air pockets that form a mesh will work. Acoustic foams are specifically designed for lining cabinets. Foam mattress tops or foam padding as well as spun polyester fiber pads available from fabric stores works well. Acoustic insulation sheets are available from hardware stores that look similar to the original fiberglass. Felt insulating padding that is used under carpets works well. A new generation of acoustic damping materials are available that are made out of materials such as recycled cotton. Loud uses a material like this in their Heritage B-15 cabinets.
How much should be used? It depends on the type of material, its thickness, and where it's placed. Depending on the material, the lining can be as thin as 1/8" and as thick as 2" or more. You can tune the cabinet by ear with some experimentation. If it is thin, add a layer and try playing through the cabinet. Add another layer and listen for a difference. If adding more doesn't help, you have enough.
How should the material be attached to the cabinet? Staples, glue, double sided carpet tape, or friction all work. It depends on the damping material, thinner ones need to be fixed to the wood surface. Double sided carpet tape is a nice alternative to glue and staples. The glue covers the wood and can be messy to remove if you have to change the damping material if it get damaged, the staples leave holes in the wood. Thicker foams and more rigid felts can be cut slightly over sized and press fit into the cabinet. They stay in place without any concerns of buzzing or coming loose.
3/4" sonic barrier acoustic sound damping material available from Parts Express
1/4" wool felt available from Aircraft Spruce
Owens-Corning Deco Sky fiberglass board.
Bonded Logic natural fiber Ultrasonic Pro panels.
Bonded Logic Ultratouch natural cotton damping material.
Cabinet Rattle and Buzz Checklist
If you flip your top trying to fix your fliptop, you are in good company. Debugging cabinet rattle and buzz isn't easy.
- Low frequency noises are non directional. I've been fooled by stuff in my room rattling, so if you've totally driven yourself nuts, take it outside away from things that can rattle and see if it's not something in the room trying to throw you.
- Try breaking the problem down. It helps to test each part separately. Remove the dolly, remove the head (use jumpers in the case of an SB-12) and connect the speaker cable to the cab. Test just the cab isolated from everything else first. Tape down the latches so they won't vibrate.
- I've seen layers of the plywood that are loose cause vibrations. These can be easily fixed if you are re-tolexing the cab. Otherwise injecting cyanoacrylate glue with a needle and syringe works to stabilize whatever is loose. Filling any open voids on edges like where the speaker cutout is or ports are is a good idea. There can be loose bits rattling in those voids. If you can vacuum them out then tightly pack in wood filler.
- If there is noise around the speaker, you might need to remove it and test it out of the cab. Otherwise, check that the speaker is tightened evenly. Don't torque the nuts too much, finger tightness plus half to three quarter of a turn. Just use common sense. Tighten opposite nuts in turn so the force is spread evenly, like when putting on a car tire. Check that the speaker baffle isn't warped with a straight edge.
- Check the baffle for tightness. Again even tightness is important.
- Check the speaker connector on the cab. The screws holding the connector should be tight, the speaker wires should not be vibrating on anything. The plastic anchors and ties that hold the speaker wire should be tight.
- The edge of the vinyl near a port can vibrate if the vinyl isn't trimmed properly and a flap is partially covering the port. Even if the tolex is trimmed properly, if it isn't glued down well it can vibrate. Some superglue (CA) can be used to fix this. This problem crops up on double baffle cabs with the oblong ports which are close to the edge where the vinyl is.
- On amps that have the tilt back leg, make sure that the bolt inside the cab is tight. If the leg is clipped on inside the cab, check that the screws and the clips are tight.
- Check that the grille cloth is not flapping. You can very carefully apply some heat to shrink the cloth. You can only shrink it so much. Beyond that, the grille cloth might need to be removed, a felt backer glued on to the baffle, and then re-attach the grille cloth. In some cases the grille cloth will need to be replaced.
- The rubber gasket at the bottom of the cab that covers the dolly mounting bolt needs to be glued all the way around so that there are no air leaks. A superglue applied close to the outer edge works well. Also ensure that the rubber itself isn't punctured.
- Air leaks are often a cause of rattling in those cabs. Between the lid gasket and the dolly screw, there's a lot of room for rattle causing. I've de-rattled three cabs, though, and two were hideously bad when I got them. So don't give up, and if you're frustrated, come back to it in a day or two.
- I'll add checking the speaker's gasket. Have had experience with compressed speaker gaskets due to being torqued down, in 'other' used bass cabs. Speaker was 'riding' on its rim against the baffle.
- I put some of the speaker gasket tape around mine recently. It was a little thick at first but after having the amp on top and clamped down for a few weeks it kind of settled in and flattened out. Definitely a nice seal now.
- Check the screws holding the head into the lid. I have a bad one on my 15S. Try pushing down on variuos spots of the head and see if it goes away.
- I had a rattle pop up in my B-15n/PF115he rig last week. At first I thought it was a tube, but then I screwed the tube cage down tight and it went away. Also, I don't think this has been mentioned, but the grill cloth can be a major PITA with old Portaflexes. It can flap against the bar going across the speaker. Jess originally installed foam to stop that, but it disintegrates over the years, and that's also why so many Portaflexes have gross discoloration of the grill cloth in front of the speaker. Nowadays, they glue black felt to the wood in the grill frame and put the cloth on over it. If yours hasn't been done, I highly recommend doing it, even if you have to take the grill cloth off to do it.
- If the cabinet has a four-pin connector or jack plate, check that nothing is loose and that the mounting screws are tight.
- There are plastic anchors that hold down the wire that runs from the speaker to the connector. Ensure that they are tight.
- There should be a rubber disk covering the dolly mounting hole. Check that it is glued all the way around. It could be vibrating and acting like a reed.
- If everything checks out with the cab, install the amp on the cab. A new gasket is a good idea if the old one looks too compressed. Check that the latches are tight. Sometimes the latches can be bent to provide a tighter lock. Sometimes you just need to change them.
- Remove the tube cage and inspect the rubber gasket that fits into the open end. If it is dried out, a rubber conditioner, available at an auto parts store, can be used to reinvigorate it, or it can be replaced. Reinstall the tube cage and ensure that the cages rubber gasket is tight against the chassis. If not change the gasket. The metal tube cage should not sit directly on the chassis. Check that the two Lucite logo nut and bolts are tight and that the logo isn't vibrating against the tube cage or the chassis.
- Check that the base of the power tube is still glued to the glass envelope. NOS power tubes should be removed from the tube socket by holding the base and gently rocking it back and forth while lifting, never pull the glass envelope. It can break the glue joint between the base and the glass envelope. If you wiggle the base back and forth and it moves, the glue joint is broken. This doesn't affect the operation of the tube but it can be the source of a rattle. A repair procedure is provided in the Tube section.
- Inspect the inside of the amp checking to ensure that circuit board mounts, transformer mounts, pots, switches, and jacks, and tube sockets, the lamp mounting hardware, the handle nuts, the tray mounting nuts, and the vibration isolators are all tight. A faulty tube can also be a source of vibration. Something inside the tube can break loose and vibrate. Sometime putting pressure on a tube can help you locate the source if the tube socket has a problem.
- Check that all the screws on the dolly are tight. If necessary, apply a heavy grease on the castor bearings. This stops them from rattling. Once all the bugs are out, install the dolly and test the assembled unit.
Soldering speaker connectors
Be careful when you are soldering the metal pins on the four-pin speaker connector. Too much heat from the soldering iron and the plastic melts which could leave the pins misaligned. It helps to clean the pins well with alcohol before soldering and use solder flux paste (available at Radio Shack) even if you are using a rosin core solder. The flux cleans the metal and helps the solder flow. This helps the soldering to go quicker and you will need to use less heat. Put the flux into the pin cup and on the wire. For the pin-2 to pin-3 connection, use a solid core as opposed to stranded wire. This avoids having errant strands poking out of the cup which are a problem. Touch the iron to the pin and wire and let it heat for a moment, don't breath the smoke, it is a health hazard, apply the solder. Sometimes I dab the iron in flux, then melt a blob of solder on the tip of the iron first, then apply it to the pin. The solder will wick into the joint, then remove the iron from the pin. This isn't considered good technique but it works when doing connectors. The final joint should be shiny. If it looks dull, you didn't use enough heat and the solder joint is bad. If that happens apply the tip of the iron to the joint, reflow the solder, remove the iron. When done, clean the excess flux up with alcohol or acetone and a Q-tip. It helps to have a small vise or another set of hands to hold the connector when soldering. The pin numbering is on the plastic part of the connector.
The 1/4" jacks are easier to solder. The connections are illustrated below. The standard for speaker wiring is always "+" connected to the tip, return connected to the sleeve. In some cases when mounting the jacks on a metal chassis, insulated jacks are required. This isolates the sleeve from the chassis so that it isn't grounded. Insulated jacks made with a non conductive material are available. Do not use too much heat when soldering these type of jacks as the body is made of plastic. Switchcraft also sells shoulder and flat washers that can be used to insulate their L12A and 12A 1/4" jacks. The "L" indicates that the threaded collar is longer to accommodate the insulating washers. The longer L12A is typically used on amp chassis with the washers although in cases you can get away with a standard 12A jack. The Switchcraft insulating washer part numbers are S1028 and S1029. The shoulder washer normally goes on the inside of the chassis, the flat washer goes on the outside against the chassis, then the metal washer, then the nut. The chassis hole needs to be reamed out slightly for the shoulder washer to fit in the chassis hole. It only takes a few strokes with a round file to enlarge the hole, be careful to only take away what is needed. Try fitting the washer as you go.
Spade connectors are the easiest way to connect to a speaker. The are crimped onto the wire and slid onto the speaker terminal. Some speakers have solder terminals and the wires need to be soldered. Take care not to use more heat than is necessary when soldering. The braided speaker coil wires that are already soldered to the speaker terminals could come loose.
When finished, check all the connections carefully. You can use a continuity tester on a volt-ohm meter to test the connections. It doesn't hurt to do a battery test at this point. Disconnect the speaker cable from the amp. Using a 9 volt alkaline battery and connect the "-" to the speaker return on the connector, momentarily tap (don't hold the connection) the battery "+" to the speaker connector. The speaker cone should move outward. With cabinets that have more than one speaker, all speaker cones should move outward, otherwise you have a phase issue and the leads should be reversed on the speaker that is moving in the opposite direction. If you reverse the battery, the cones should move inward. Note that if your cabinet has a tweeter or horn, disconnect it before performing the battery test as the battery can damage the speaker.
How to install a speaker in a cabinet
Installing a speaker in a cabinet isn't difficult but it has to be done with great care. It's too easy to poke a hole in the cone.
- The first thing to do is to inspect the speaker for any issues. Make sure that the cardboard or cork gasket is in place around the edge of the basket that will go against the baffle.
- Use a straight edge to check that the baffle is flat. A speaker shouldn't be mounted on a warped baffle.
- Inspect the edge of the plywood baffle cutout and look for any loose wood that would vibrate and cause a noise. This is common in old cabinets. Wood filler or cyanoacrylate glue can be used to repair or stabilize the wood.
- If you have an ohm meter, check the DC resistance across the speaker terminals to ensure that it is the correct. A speaker will average 70% less than its nominal impedance. In the case of an 8 ohm speaker, 8 ohms X 0.7 = 5.6 ohms. Typically a reading between 5 and 7 ohms is normal for an 8 ohm speaker.
- If you are not using spade connectors, it is best to solder the wires onto the speaker before the speaker is installed into the cabinet. Speaker wires should be twisted together. Solder them onto the uninstalled cabinet connector first, install the connector, run the wire down the side and along bottom of the cabinet, then up to where the speaker terminal will be. Measure carefully and cut the wire an inch or two longer than is needed. Place the speaker on top of the opening of the cabinet and solder both connections. Be careful not to use too much heat. You don't want to unsolder the braided wire that connects to the speaker coil. Ampeg traditionally used green and black wires. The green wire connects to the "+" speaker terminal, the other end goes to pin-1 on the four pin connector or the tip of a 1/4" jack. The black wire goes to the other speaker terminal and pin-4 or the sleeve of a 1/4" jack.
- Without poking a hole in the speaker, carefully mount the speaker on the speaker studs that are mounted in the baffle. Below is an image of the inside baffle of a B-15 speaker cabinet. The speaker studs can be quite menacing, they are sharp and it doesn't take much to pierce the speaker cone. A good tip was provided by spellcaster which helps avoid damaging the speaker: Push some straws through the frame holes from the back of the speaker frame. Align the straws with the mounting studs, then push the speaker against the baffle. It helps to do the bottom one first, the other ones are easier to see. The straws will pop out the back.
- Tighten the first nut to finger tightness, then install the opposite nut with the same amount of tightness. The nuts should always be tightened in an X pattern (or star pattern if there are more that four mounting points), not in a circular pattern. This helps ensure that the pressure on the speaker's frame is even. Tightening in a circular pattern can warp the frame. By turning the nuts a small amount at a time, the gasket will be evenly compressed. With the nuts against the speaker frame at finger tightness, tighten the nuts an additional 1/4 turn each in the same X pattern. You should need about half to three quarters of a turn and no more than one turn to complete the job. Each nut should have the same amount of torque. Do not use an electric drill to set the torque. Your hands are the best way to feel the amount of torque on each nut. It is better to under tighten the nuts and tighten them as necessary than it is to over tighten them.
- Tie down the speaker wires to the plastic anchors in the cabinet. This helps prevent rattle inside the cabinet.
- Test the installation by playing through the amp. If there is buzzing due to the speaker, tighten the nuts as necessary.
A speaker can be tested by gently pushing on the cone and listening for a rubbing sound which is impeding movement of the cone. Any sign of rubbing is a problem. It could be that there is a piece of debris in the space that the cone travels in. Sometimes a carefully applied shot of compressed air will dislodge it. Rubbing can also be an indication that there is a problem with the coil and reconing is required.
If that checks out, disconnect the speaker and measure the DC resistance across the speaker terminals with an ohm meter. The DC resistance typically averages 70% of the nominal impedance. For an 8Ω speaker (8 X 0.70) this average would be 5.6Ω. That doesn't mean that the resistance can't be higher or lower and still considered normal. If your ohm meter reads infinity, the coil is open which means that it has to be replaced. It is important to note that inexpensive ohm meters do not accurately measure low resistance values. Ensure that the battery in the meter is good if the readings seem odd.
Another test is the so called battery test. This tests if the coil is working and if it is interacting with the magnet properly. Disconnect the speaker and test it by itself. If the cabinet has only one speaker this isn't necessary. If the cabinet has a horn or dome tweeter, disconnect it. Do not perform this test with these type of speakers connected. Take an alkaline battery, a 9V is often used. A speaker typically has a painted dot or a + beside one of the terminals. Connect the - side of the battery to the other other speaker terminal. Connect a wire to the + terminal on the speaker and momentarily touch, on and off, the other end to the battery. The speaker cone should move outward. Reverse the battery and redo the test. The convention is that the speaker should move inward.
If you have more than one speaker in the cabinet, they should all move in the same direction at the same time when the test is performed. This indicates that they are operating in phase with each other. Again, it is important to disconnect any horns or dome tweeters as their coils can be damaged by performing this test. If any speakers are out of phase, the speaker terminal wires should be reversed. Then retest the speaker to ensure that you've got them connected properly.
Torn or punctured cones can be repaired. There are a host of procedures offered online which range from nail polish to carpenters glue with layers of tissue paper, to rubber cement. They all work, some better than others and the extent of the damage to the cone makes a difference. If it is too far gone, the speaker should be reconed.
It is important to not add too much weight to the cone when performing the repair. MG Chemicals sell a product called Speaker Service Cement, which is a rubber based product. Speaker parts companies also sell cement for repairing speakers. Simply follow the instructions.
There are different reasons why a speaker would need to be reconed. The cone or surround may be damaged, worn out, or even the paper may be disintegrating due to time or environmental conditions, the coil may be damaged and not working, or the coil former may be rubbing on the magnet. Sometimes people recone a speaker with a different type of kit because they want to change how it performs or even change the impedance, for example changing it from an 8 ohm to a 4 ohm speaker.
When a speaker blows, it can be repaired to like new condition with a recone kit. The problem with some vintage speakers is that the original repair kits are no longer available from the original manufacturer. Sometimes you can get lucky and find one online. Otherwise, replacement kits or close alternatives are available from speaker parts supply stores. Reconing a speaker isn't difficult but it requires some basic tools and an attention to detail. The recone kit allows a speaker to be rebuilt, changing the moving soft parts. This includes the cone, dust cap, surround, gasket, spider, coil, former, braided lead wires, and coil centering shim. The kit comes partially assembled with the surround glued onto the cone, and the coil and former and the spider are attached squarely to the cone. Glue may need to be ordered separately. Here's an instructional video for those who are interested in DIY. A couple of important points. Take the time to remove all the remnants of the removed parts including the glue and any particles or glue lodged in the voice coil gap. When applying the glue, it helps to use a platform with a lazy susan bearing to rotate the speaker basket to ensure that an even bead is applied. You hold the glue and rotate the basket under it.
Some speaker techs offer to demag and remag of magnets as part of their service. The magnetization can change, for instance, when a Alnico magnet is jarred hard. Over time it can loose some of its magnetization. With the right equipment, it can be brought back to the original specification.
Here is basic explanation of how a speaker works.
Below is a labeled diagram illustrating the various parts that make up a speaker and a three-dimensional cutaway.