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SB-12 Cabinet Restoration - part 2


Nov 11, 2016
SB-12 Cabinet Restoration - part 2

  • Wood Preparation (top)


    Wood preparation is one of the most time consuming steps in the restoration process. A number of issues need to be dealt with and how well they are addressed will determine how good the cabinet will look when it is recovered. Any flaws in the wood such as bumps or indentations due to ripples in the plywood surface or other flaws in the surface are magnified when the vinyl is applied. This is the time to correct them.

    Glue Removal and Cabinet Sanding (top)


    The hide glue is water soluble and is softened with Warm water. Apply the water with a damp cloth or sponge. Remove the glue with a scraper. Too much water can cause ripple in the surface of plywood and this should be avoided. So don't soak the wood. Use only as much water as is necessary. When you start working, it becomes apparent what works best. Finish by wiping down the cabinet with a Clean damp cloth. You want to remove the glue, not spread it over the cabinet so keep rinsing the cloth and wringing it out.

    Once the glue is removed, it is time to start sanding. The objective is to even out the surface by removing as little of the wood as possible. The surface of some cabinets is fairly flat. Other times the surface is wavy and has ridges. These cabinets require more attention.

    If you look at the grain of the plywood, there are light and Dark areas. The light areas are softer and wood is more easily removed from those areas. Sanding improperly can lead to a more uneven surface so be careful. If you press down on the surface too hard, the Dark areas can remain mostly in place and the light areas are eroded to form valleys. Making the problem worse. You want to remove the hills and even out the surface. I find that a light touch with a random orbital disk sander works best. It is important to work in a well lit space so that you can closely monitor how much wood is being removed. Looking under a straight Edge for light can help you identify the low areas.

    The Grit of sandpaper used will vary depending on the wood. Some plywood is harder than others. I find that the darker the grain, the harder the wood. Plywood with an even coloring is usually easier to work with. If the cabinet is in rough shape, I usually start with 100 Grit sand paper and move to a progressively finer grits of 320 or 220. You don't want it to be too smooth, there has to be some roughness for the vinyl glue to Bite into. When using the random orbital sander, I usually don't go finer than 320 grit. As a guide, move your hand along the surface of a new piece of Baltic Birch plywood. You are pretty much there when it feels the same. In the case of new plywood, a light sanding is necessary to ensures that any Loose fibers and dust are knocked off.

    If you don't have a random orbital sander, a large sized sanding block works well to Cut off the top of the hills. You don't want to sand the hills and valleys in the wood surface equally. That just thins the wood. This can happen if the soft grains are very soft. If that is happening, stop sanding. If the hills prove to be difficult a metal hand scraper is the best way to go. Scrape perpendicular to the hard grain, following the lines. You can see what is being removed and control how the surface is being leveled. If the surface is proving difficult, the next step is to use a wood filler. This will be discussed later.

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    First pass.


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    Second pass.


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    Third pass.


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    Cleaned and sanded.


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    Random orbital disk sander.


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    Steel hand scraper.


    Care must be taken when sanding the cabinet edges. You don't want to change the Round over along the Edge. If you do, the metal corners will not fit snugly over the vinyl and the edge won't look correct. Also, when the vinyl is wrapped around the back and sides, variations due to the distance around the cabinet will affect the position of the checks on the left and right face frame.

    Avoid sanding the edges as much as you can. Unfortunately, the edges are where the finger joints are and there is usually some damage there such as laminate that requires re-gluing or holes that need filling. So some sanding is usually necessary. The Round over on smaller amp cabinets like the B-15N and SB-12 is 1/4", on larger sized cabinets such as the SVT and V4B, the round over is 3/8". I find it helpful to make a template of the original edge with a Thin piece of cardboard. Templates made out of stainless steel are commercially available. You can move the template along the edge periodically to use as a reference as you sand. Any light shining through and you have a low spot. That can be fixed with some filler.

    Making a round over template is fairly easy. Examples for 3/8" templates that are used on SVT and V4B cabinets are shown below. One is made out of paper, the other is made from epoxy putty. For the paper template, start with a Thin piece of cardboard and trace out a 3/4" diameter circle using a penny as a template. A compass or circle drawing template can also be used. Then extend two right angled lines on the edge of the drawn circle. It sounds more complicated than it is. I then carefully Cut it out with a razor knife. The epoxy template was easier to make. Using the paper template as a guide, I found something in my place that had a 3/8" round over. Turns out that the frame on my Apple computer monitor was perfect. It turns out that the edge of a Leslie organ cabinet also has a 3/8" round over so I used that. I cut a two inch length of QuikWood epoxy putty, protected the cabinet with a small piece of kitchen parchment paper taped down to hold it in place, put on a pair of latex gloves, kneaded the epoxy putty and spread it over the round over. I held it in place for a few minutes until it had hardened and I had my template. I sanded down the edges to square it off and make it look a little nicer.


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    QuikWood epoxy stick


    Here is an example of how the template is used. In this case a V4B cabinet's front top edge had been over sanded when it was built. It didn't look quite right so it was decide to rebuild the edge with epoxy and make it right. An advantage of the epoxy is that it is harder than the wood so it will be less prone to damage once it is covered.


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    The round over of the edge has been built up with QuikWood epoxy and sanded. The paper template is used as a guide to bring the round over down to 3/8". Note that the round over for a B-15 and an SB-12 is 1/4".


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    Round over is being refined with an epoxy template. Sight along the edge, light under the template indicates a low spot which needs to be built up with additional epoxy. As many layers as is necessary can be laid down and sanded. I prefer to take the slower approach and hand sand edges. A machine can be too fast and too Aggressive in removing the material.

    Wood Filling (top)

    There are many good water soluble wood filler products on the market. They Dry hard, are machinable, and they don't emit toxic fumes. Different types of fillers can serve different purposes. A regular wood filler can be watered down and injected deep into the wood with a syringe. Filler can be used to even out ripples in the wood surface that can't be corrected by sanding. When areas that need to be built up such as damaged edges or corners, I like to use a heavier epoxy filler such as QuikWood. This is a two part stick that you knead together and then apply to the wood. It sticks like nothing else that I've used and dries very hard. You can use it to rebuild corners for example and sand them down to restore the surface. Bondo, an automotive body filler, also works well. It has a thinner consistency but it can also be used to build up damaged areas.


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    Wood putty filler.


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    Water based filler can be diluted with water to the consistency that works best.


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    Syringe for injecting the filler into nail holes.


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    Syringe loaded with filler.


    Repairing Issues and Wood Blemishes (top)


    Dings, splinters, wavy surfaces, holes and any other damage can be repaired with fillers. They allow you to correct and flaws in the wood that will be magnified when the vinyl is applied to the cabinet. The cabinet should be Clean and dust free. A tack cloth can be used to wipe the cabinet down prior to applying the filler. The wood can be wiped down with a solvent. This will ensure that the bond to the wood is optimal.

    The first step is to glue down any delaminated wood. I use carpenter's wood glue, cyanoacrylate (also called CA or crazy glue), or a liquid epoxy. CA can be injected inside the plywood to stabilize anything that is Loose. This can cause a cabinet vibration and it is best to take care of these sort of issues now. Sometimes loose plys or voids are visible around the speaker cutout of the baffle. These voids are easy to fix with glue or a filler. Sometimes these are missed on the baffle so take the time to do a careful inspection. Another area of concern is along the finger joints. Check for loose laminations and missing bits of wood. Often filler was applied at these joints during manufacturing and it may be soft or have crumbled. In these cases, you need to pick out the bad filler. Again, you can use CA to glue down anything loose and fill any voids. Any rotten wood needs to be removed and built up. Mold needs to be killed and removed. I find that a grainy solution of borax (laundry cleaner) works well on wood surfaces. Any protruding nails need to be reseated below the surface of the wood. Go through the checklist at the top of this section and repair anything that needs to be addressed.

    The next step is to apply the filler where ever it is needed. Spread it on with a putty knife, pressing it hard into the wood so that it extends into the wood fibers, then wipe the excess away with the putty knife. The more that you can remove at this stage, the less sanding there will be need later. Make sure that valleys and holes are well packed. Take special care in areas that are high visibility. You want to ensure that they look perfect as that's where your eye is drawn.


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    Applying the filler.


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    Scraping the filler away with a putty knife means less sanding after it dries.


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    After sanding.


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    Finger joints typically need a lot of attention. Delamination and missing pieces of wood are common.


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    Face frame repair.

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    Front near the highly visible Ampeg logo.

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    Repairing Nail Holes (top)


    It is important to fill the nail holes where the hardware is mounted. This will ensure that the new nails will stay in place when the cabinet is reassembled. An easy way to do this is to use wood filler. In this case, a water soluble filler was diluted just enough so that the liquid could be loaded into a syringe and applied deep into the holes. The more water that you use, the weaker the filling so add water sparingly. You want the entire hole to be filled. It sometimes helps to use a tooth pic to ensure that the filler is packed fully into the hole and that there is no trapped Air, then apply more filler to top up the hole. When applying the filler, the more that you can clean away, the less sanding there will be after it dries. Apply the filler, scrape away the excess with a putty knife, when Dry sand it lightly.


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    Repairing Wood Rot (top)


    Some cabinets suffer from wood rot. It can be on the outside of the plywood but often it is between the plywood layers. The wood is typically very light and crumbles to a powder when you push on it. Sometimes the wood layers separate and you can see cracks along edges. It this happens, all is not lost.

    The first step in repairing this sort of problem is to remove the damaged wood back to where the wood is Solid. A dental pick or small screwdriver can be useful for gouging out the rotted wood. Go as far back into the plywood as you can because you want a sold base.

    The next step is to use a wood repair product. A variety of products are available at woodworking or hardware stores. I have had good results with the products made by PC-Products. A wood stabilizer such as PC-Petrifier is a water based adhesive that soaks into the wood and solidifies and strengthens it. Once dry, the wood is more Solid.

    If you are repairing a surface, an epoxy wood filler such as PC-Woody or Qickwood could be used. You build up the surface with the epoxy, when dry, it can be sanded or machined. Quikwood is a harder product. It's good for building up edges and corners which need a surface that can take a lot of abuse. PC-Woody is a paste which has the consistency of peanut butter.

    When repairing delaminated plywood, I find that a liquid epoxy such as Clear Liquid Epoxy works very well. The epoxy needs to be injected between the layers and the wood clamped. This can get messy as the injected epoxy will seep out when pressure is applied with the clamp. Be prepared to catch the drips and clean up what you can before it cures. Again, when it dries you can sand or machine the edges.

    These epoxies are stronger than the wood when they have cured. There are a variety of YouTube videos and tutorials available that demonstrate how to use these products.


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    Wood stabilizer


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    Liquid epoxy for lamination repairs


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    PC-Woody for surface repairs


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    QuikWood epoxy stick

    This is an example of how a badly damaged cabinet can be restored. There was delamination, the center plywood layers had gone to a powder. The front and side had sections missing. When repaired, the wood is hard, stronger than it was originally, and with the vinyl covering you would never know that it was damaged. The soft powdery wood was removed. A stabilizer was applied to firm up and seal the wood.

    • 1: A liquid epoxy was pumped into the delaminated layers and the voids were filled. The wood was clamped until the epoxy had cured.

    • 2: The side surface has been built up with QuikWood expoxy and sanded smooth. The surface s checked with a straight edge to ensure that it was flat. The thickness of the wood was checked with an outside caliper.

    • 3: The front edge has been built up and the round over restored.


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    This corner section of the cabinet was severely damaged. Here it is repaired and ready to be recovered.


    Baffle Interior Finishing (top)


    With most amps, you never see the interior of the cabinet. That isn't the case with a Portaflex, you see the inside every time you flip the lid. Cabinet interior finishing adds a nice touch to the overall look. The finish shouldn't interfere with the acoustical properties of the wood. It should be thin and not highlight imperfections in the wood.

    The color of the inside of the Portaflex cabinets varied over the years from a redish-brown to different shades of walnut which is much darker. What you use comes down to your personal preference. The darker stains are good because they look cleaner and show dust less. The lighter colors highlight imperfections in the wood. I've seen some cabinets where a thin lamination of furniture grade veneer was added to make the cabinet look better. It comes down to what sort of issues you have inside your cabinet.

    The procedure used in this restoration is quite simple but has to be done quickly because it involves several steps that need to be completed before the finish dries. There are a lot of options available when it comes to finishing products and excellent results can be obtained with different approaches. The wood can be stained and then coated with a finish, a tinted finish can be used, or something in between. I experimented with varnish and a semi-matte acrylic, but the best results were obtained with a gloss acrylic finish.

    The following steps were followed:

    • Prepare the wood surface by washing it with a damp cloth to remove any dirt and loose fibers. Sand it lightly and then use a tack cloth to remove any dust.
    • Apply a thin layer of semi matte acrylic and let it dry for a few minutes in the shade. This acts as a sealer to prevent the stain from penetrating too far into the wood.
    • Apply a thin coat of black matte paint or stain and allow it to penetrate for a few minutes.
    • Rub the surface with a cheesecloth (or oakum) soaked in thinner to blend together and remove most of the two applied finishes. The rubbing will allow you to adjust the color balance and thickness evenly over the wood surface. This allows you to control how much of the grain is seen and allows you to cover any imperfections in the wood or filler. As a side note, similar techniques are used by luthiers when applying hand rubbed stains to instruments. It is a very nice way to obtain a sunburst, for example, where the colors seamlessly blend into one another. The same thing is done here to ensure that the stain is even and that some areas don't soak up more color than others. At this point, the finish will have a matte appearance with a very small percentage being color. Blend it so that it looks good.
    • Apply a layer of semi matte acrylic and let it dry for a few minutes minutes. This acts as a top coat and give the finish a bit of shine.
    • Using a semi-damp with solvent cheesecloth, remove as much of this layer only to give the finish the desired look.
    • You can always backtrack and reapply the color to obtain the desired balance of grain, color, and finish, ending with a thin top coat.


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    Original stained baffle.


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    Raw wood with original stain, prepared for finishing.


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    The stain is applied.


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    Wiping off the applied color with oakum lightly soaked in solvent.


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    The applied stain has been partially removed.


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    The top coat applied waiting to be wiped down.


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    The final thin stained finish.

    Cabinet Interior Finishing(top)


    The same process is used when finishing the interior of the cabinet. These images show how you can mask of the vinyl covering to protect it when refinishing the interior. Normally the refinishing is done before the vinyl is applied but if the interior needs to be repaired, masking is a good idea.


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    Hardware Restoration (top)


    The metal hardware takes a lot of abuse over the years. The corners and feet are there to protect the amp so it makes sense that they get damaged. Scratches in the protective plating rust and they can look pretty beat up. Restoration can take many forms. A simple cleanup and polishing with a protective wax works when there isn't a lot of damage. When a lot of rust is present, the parts can be soaked in a product such as Evapo-Rust which chemically removes the rust but leaves scars in the metal. In many cases this works quite well and you have to look closely to see the imperfections. In cases of more severe damage, the parts can be re-plated. In this case, it was decided to re-plate the hardware.

    The plating process will vary depending on the material. The first step is reverse plating to remove the old material. Then the metal is filled and polished to remove imperfections. This is a labor intensive process. Like woodworking, the better the preparation, the better the final finish. It is even more important when it comes to metal work. This is where you have to ask yourself, is it going to be more cost effective to pay to have the parts repaired or to replace them with new ones. You can't just re-plate scratched parts, they aren't going to look very good. The damage needs to be addressed. Once the parts are ready for plating, sometimes an undercoat such as a copper alloy is plated on first. This provides a better base for the top layer to adhere to and will prevent peeling down the road.

    Chrome can have different colors and different types of reflection. It can range from a satin yellowish plating to a mirror like highly polished reflective surface. The thickness of the plating also makes a difference. A thicker plating looks better. In this case, some of the original hardware was kept. The corners, and latches were replace with reproduction parts and all the hardware was re-chromed to improve the look and to keep the color consistent. A good quality chrome plating can vastly improve the look of the metal.


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    Original rusted hardware.


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    Parts are tied together with wire, they are dipped into an electrolyte solution and a current is passed through the parts to plate them.


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    Original chrome on the left, re-chrome on the right. What a difference!


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    Original chrome on the left, re-chrome on the right. Note the different reflective properties of the surfaces.


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    Before re-plating on the top, after restoring the finish on the bottom. A vast improvement.


    Lucite Logo (top)



    The Lucite Ampeg logo is a rectangular piece of plastic with the Ampeg logo engraved on it. A post card was included with the amp that would allow the owner to specify what they wanted engraved on the plate.

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    With time, the Lucite logo can become Dull. There are plastic cleaners available at some automotive stores that can breath new life into the logo. One example is Meguiar's/PlastX clear plastic cleaner and polish.

    Start with soap and water, try Windex with soft cloth, then if necessary, try a more Aggressive cleaner. It's easy to scratch the surface of the plastic when you are trying to clean it. If it happens, it can be buffed out.

    Sometimes the heat from the Tubes can melt the plastic and warp it. If this happens, the plastic can be softened in an oven and straightened. The temperature required will vary depending on the logo's material (Lucite, Plexiglas, Acrylic, or some other type of clear plastic). Place the logo on a flat pan in a pre-heated oven at around 250 degrees Fahrenheit. When the logo becomes soft, gravity will it flatten out. Remove the pan and let it cool to room temperature before touching the plastic. If a higher temperature is needed, pre-heat the oven to a slightly higher temperature but don't go above 350 degrees. Guidelines for softening plastics are available on the internet. Be careful, plastics are flammable.





    Before Cleaning (top)

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    After Cleaning (top)

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    Tube Cage (top)

    Painting a tube cage involves a lot of work. Traditionally, you would use a paint stripper. A tool like a dental scraper is needed to get into each hole to remove the paint from the inner edge. A labor intensive process to say the least. Then an undercoat is needed and finally the paint. Because the tube cage gets Hot, a paint that can withstand high temperatures is needed. Some high temperature paints, such as those intended for wood stoves remain soft which means that the paint is easily damaged. I've had good results using a zinc chromate undercoat followed by VHT high temperature engine paint.

    The best way to paint your tube cage is to send it out to be sandblasted and then have an electrostatic coat of paint applied, also called powder coating. Electrostatic paints start off as a dry powder that is sprayed on electrostatically. A charge is applied to the metal cage that attracts and holds the paint particles in place. It is then baked in an oven, the paint melts and bonds with the metal. The result is an even, hard, durable coat. It can be expensive, show around, but it's the best way to do these cages. This is what was done in this case.

    Before Powder Coating(top)



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    The paint was thin and uneven, there are chips and rust spots.


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    After Powder Coating (top)



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    A thin, even, hard, durable, mat black coat.


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    Spectacular results!
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