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Vintage Ampeg Scroll Basses:
Setup, Maintenance & Technical Issues




Ampeg Scroll Bass Owners: If you have any technical questions or tips from your own experience, I'll be glad to include them here to share with other Ampeg owners.


General Setup & Adjustment Of All Models:

I've never seen any "official" setup specifications for vintage Ampegs. The original instruction sheet refers to the "factory setup", but it probably was a guy sitting at a bench turning the screws until he liked it. I doubt they kept any real record of settings other than notes on a scratchpad on the guy's bench. So, these are my recommendations based on my experience with the instruments in my collection, and instruments of customers and friends that I've worked on.

To setup an Ampeg, I generally start by trying to adjust the action to about the right height, and looking for problems that might need truss rod or neck angle adjustments. I'll fix those problems first before moving on. Next, on the models with the "mystery" pickup, I adjust the tailpiece height to dial in the tone of the pickup. Finally, I fine tune the action and set the intonation.

Adjusting The Action:

The "action" is the height of the strings above the fingerboard, and it is adjusted by raising and lowering the height of the bridge with the vertical setscrews using a 1/8" (3/32" on AMB-1/AMUB-1 models) allen wrench. Action is measured as the gap between the top of the last (20th) fret and the underside of the string, and should be in the range of 0.100" to 0.180". You can measure it by placing the end of a small machinist's ruler on the top of the fret and eyeballing the gap to the bottom surface of the string.

Here's a trick for getting fast, repeatable measurements: I made up a small wedge out of a piece of maple about 2" long by " wide and tapering in thickness from " to 1/32". I used a pair of calipers to find the portion of the wedge where the thickness goes from 0.100" to 0.180" and marked it on the top surface. Slipping the wedge between the string and the fret (or fingerboard on a fretless bass), I tap lightly on the string while moving the wedge until I find the point where the sound stops going "ding" and starts going "click". I can read the action measurement right from the lines on the wedge, while adjusting the bridge.

The Ampeg bridge doesn't have individual height adjustment for each string, so the action is set for the E and G strings. If the A and D strings are off in relation to the E and G, this is corrected by gently filing the string notches in the bridge saddle blocks with the appropriate size small file. Luthier's nut files, in the correct width for each string are best, but small round needle files from a hobby shop will also work. If the bridge is adjusted all the way down as low as it will go and the action is still too high, then either the truss rod or the neck angle needs to be adjusted.

The setting of the action is really a matter of preference. I set most of my vintage Ampegs at about 0.150" on the E string and 0.130" on the G. Setting the action lower means that the fingering is lighter, but you have to pluck the strings gently. Higher action requires more left hand strength, but you can pluck the strings harder and get more dynamic tone out of them.

Neck Relief & Truss Rod Adjustment:

The "relief" is a small amount of forward curvature in the neck that is adjusted with the truss rod. This is necessary to prevent string buzzing and rattling. Don't confuse relief with neck angle, which is described seperately below. Very high action may be caused by too much relief, or an incorrect neck angle, or both. The truss rod will adjust the relief, but will not change the neck angle. Cranking on the truss rod to try to correct a neck angle problem will probably ruin the instrument!

Relief is checked by pushing down on one of the strings (usually the A) at the first fret and the last fret at the same time, and looking at the gap between the string and the frets around the middle of the neck. The greatest gap will probably be around the 10th fret, and is ideally in the range of 0.030" to 0.060" (or about 1/32" to 1/16"). If the gap is much greater than 1/16", it means that the neck is bowed forward and the truss rod needs to be tightened. If there is no gap, or less than 0.030", it means that the neck is bowed backwards and the truss rod needs to be loosened.

Unfortunately, many Ampeg necks have developed serious bows and twists over the years, usually from the instrument being stored for a long time without strings on it. If the neck has 1/4" of relief or 1/8" of back bow (both of which I've seen), this cannot be adjusted out with the truss rod! At this point, you have to decide whether to leave it alone and live with it to keep the instrument original, or perform serious surgery. This usually means resurfacing or replacing the fingerboard, and probably repainting the back of the neck.

The truss rod in an Ampeg is a 3/16" diameter mild steel rod that runs almost the whole length of the neck. It's a classic single acting Gibson-style truss rod installation, with the rod resting in a trough with a curved bottom, and a thin stick of maple glued into the trough on top of it. At the heel end of the neck, the truss rod is staked and welded to a short slug of " steel rod, forming a T-shaped anchor point with the T end perpendicular to the fingerboard. The bottom end of the anchor is exposed on the heel surface, and serves as the contact point for the neck angle adjusting screw in the body.

At the headstock end, the truss rod is threaded 10-32, and the adjusting nut threads onto it through a pocket at the end of the fingerboard. The odd little aluminum piece that sets the spacing of the strings also has a small vertical ear that serves as the washer for the truss rod nut. The truss rod nut has a head similar to a socket head cap screw, with a 3/16" internal hex.

To adjust the truss rod, insert a 3/16" allen wrench into the hex in the truss rod nut. Make sure the wrench is fully seated in there; you may need to pick the gunk out of the hex first. Loosen all the strings about a full turn on the tuners. Looking from the headstock end, turn the wrench clockwise to tighten the truss rod, which will pull the neck back and reduce the relief. Turn it counterclockwise to loosen the truss rod, which will increase the relief. If the truss is already fairly tight, the wrench should only be turned about 30 degrees at a time. The total range of adjustment from a flat neck to maximum relief is only about 180 degrees of rotation of the nut. After making an adjustment, flex the neck gently with your hands, then bring the strings back up to tune and re-check the relief. Take your time; it's a subtle and sensitive adjustment that may shift as the instrument is played.

Warning: Adjusting the truss rod on a vintage Ampeg must be done with caution! It was a weak design to begin with, and if you try to crank on the truss rod to straighten a seriously bowed neck, something is going to break. Many Ampegs have been destroyed or seriously damaged by applying too much muscle to the truss rod. Either the threads strip, or the rod snaps off internally, or sometimes the wood near the headstock splits.

Another common problem is that both the truss rod and the truss rod nut are steel, and they may have corroded and locked together. If, when you try to turn the allen wrench, it springs sharply back, Do Not keep turning it! You are twisting the entire truss rod, and if you keep turning, it will snap off inside the neck, which is a very bad thing. Fortunately, the truss rod nut is hollow down the center, so the threads are somewhat accessible. Stand the instrument straight up and put a couple of drops of WD-40 down into the hex of the truss rod nut, where it will seep down into the threads. Don't just blast the whole area with a spray can of WD-40, because you really don't want it seeping into the wood or staining the paint or the fingerboard. I spray a little puddle into the can's cap, and then use an eyedropper to apply it.

Let the WD-40 soak in for a few hours, and then start gently working the wrench back and forth to see if the nut will break loose. If, after a couple of tries, it's still frozen, then my advice is to leave it alone rather than risk doing permanent damage to the instrument. An Ampeg neck with a frozen or broken truss rod can be rebuilt, but it's a major repair job. It's usually quite difficult to remove the fingerboard without damaging it. If I need to rebuild a vintage Ampeg neck, I'll put in a more robust truss rod system similar to what I use in the new AEB-2, and probably replace the fingerboard.

Neck Angle Adjustment:

Neck angle is the alignment in the vertical plane of the neck in relation to the top surface of the body. That is, looking from the side of the instrument, it's the alignment between the surface of the fingerboard and the surface of the body. The neck angle needs adjustment only if the relief is about right, but the action is still too high with the bridge adjusted all the way down. To correct it, the neck is effectively tipped back slightly in relation to the body to bring the action back into a reasonable range that can be fine tuned with the bridge. Ideally, with about 0.150" of action, there will be about 1/8" gap under the bottom of the bridge to allow a good range of adjustment.

Ampegs are a flat design; the fingerboard is parallel to the heel surface on the underside of the neck, and the bottom surface of the neck pocket in the body is parallel to the surface of the body. Normally, the four neck attachment screws hold the neck seated tightly in the pocket, with the heel surface and pocket surface in solid contact. Ampeg included a neck angle adjusting screw, similar to the system that Fender introduced in the late '60's.

If you remove the black plastic cover plate on the back of the body, there's a small hole in the center in between the four neck screws. Buried in the body is a 10-32 setscrew that, when turned in, comes in contact with the exposed end of the truss rod anchor on the heel surface of the neck. The official procedure is to loosen the two rear neck screws a bit, turn in the adjusting screw with a 3/32" allen wrench to tip the neck back, and then re-tighten the neck screws. It's a very sensitive adjustment, requiring only a turn of the adjusting screw or less to move the neck quite a bit.

My advice is not to use the neck angle adjusting screw; it's a good way to hurt the tone of the instrument. Rigid, solid contact between the neck and the body is critical to the tone of a bass, particularly the clarity of the low frequency overtones. Using the adjusting screw essentially jacks the neck up on one edge and the point of a screw, leaving an air gap between the neck and the body.

If the neck angle needs to be adjusted, the right way to do it is to fit a thin wedge-shaped shim made of a hard material in between the neck and the body. Ideally, the shim is maple, covering the whole neck heel surface and tapering to a feather edge. Hard plastic, brass, or aluminum are okay, but don't use cardboard or paper. Contact me if you want more detail on how to make a wooden wedge-shaped shim, or if you want me to send you one.

Intonation Adjustment:

Correct intonation means that a note played at the 12th fret is exactly an octave above the note of the open string. This is adjusted separately for each string by moving the small aluminum saddle blocks on the bridge, by turning the four screws on the front face of the bridge. This is best done with an electronic tuner plugged into the output and the tone control wide open.

Play an open string and tune it to exactly the correct note. Then play it at the 12th fret, being careful not to push the string sideways at all. If the note at the 12th fret reads too flat, the saddle block needs to be moved towards the neck by turning the screw clockwise (tightening it) with a Phillips screwdriver. If the note reads too sharp, the saddle needs to be moved away from the neck by loosening the screw and pushing in on it. Normally, the G string saddle block will end up closest to the neck, and the E string will be farthest away.


Setup & Adjustment Of The "Mystery" Pickup On AEB-1, AUB-1, ASB-1 & AUSB-1 Models:

Ampeg's "mystery" pickup (that's what it was called in the factory literature) is an unusual contraption with a thin steel diaphragm mounted over top of an epoxy block containing two large magnets and wire coils. Some of the earliest prototypes have a single large coil. The whole reason for the design was to allow the AEB-1 and AUB-1 to be used with gut strings, which being non-metal, wouldn't work with a conventional magnetic pickup. Everett Hull, Ampeg's founder, thought that upright bass players converting to electric would want to use gut strings. That didn't happen, and LaBella flatwounds became the most popular strings on Ampegs. So, although the "mystery" pickup's original purpose was lost, it does provide a very unique tone.

The pickup's output is almost entirely in the 40-300hz range and is extremely warm and muddy. The attack of the notes is a percussive thump that's a good imitation of an upright bass. There's also a ringing overtone at about 1500-2000hz with a very metallic sound. Played through a full range amp with the instrument's tone control wide open, this ringing can be very annoying! Everyone I know who plays an AEB-1 or AUB-1 keeps the tone control on the bass rolled off most of the way, all the time. Also, as suggested by Bob Farace on The Unofficial Ampeg Page, plugging a parametric EQ unit between the bass and the amp, and using it to notch out the ringing frequency helps a lot.

There's a lot of variation in the sound of the pickup from one instrument to another. I think it mostly has to do with differences in the flexibility of the steel diaphragm, and how well it's seated against the wood structure underneath. They all have the ringing overtone, but in different amounts and at different frequencies. It's probably a harmonic in the steel. I haven't figured out how to get rid of the ringing, but it can be subdued with some careful adjustment.

The diaphragm is a 5" square, made up of two layers of 0.018" thick spring steel sheet. A black bakelite block is screwed to the center of it, and the bridge adjusting screws rest in two holes in the block. Under the diaphragm, a large area of the maple of the body is routed away to make a cavity for the epoxy block containing the coils and magnets, and a ring of 1/8" masonite is glued down to make a seating surface for the diaphragm. Four tiny screws attach the diaphragm to the masonite ring. The original screws are "clutch head" style, as were often used on old TV's, to discourage owners from taking the pickup apart. If you see slotted or phillips head screws, then someone has probably had it apart at some time. When it's all assembled, there's a 1/16" gap between the underside of the diaphragm and the ends of the two magnets.

The sound of the pickup can be changed somewhat by raising and lowering the tailpiece, which changes the downforce that the strings put on the diaphragm. There is usually an optimum point which gives the warmest overall sound and minimizes the ringing, and moving the tailpiece either higher or lower from that point makes the tone worse. An easy reference to watch is the gap between the strings and the back edge of the pickguard. On all of my instruments, the optimum setting that gives the best tone results in a gap of about 1/8" under both the E and G strings.

Start by loosening the strings a bit. Using a 3/16" allen wrench, turn the two tailpiece attaching screws through the holes in the top of the tailpiece. Switch back and forth between the two screws, turning each a half turn at a time. Adjust the height of the tailpiece to make the gap under the strings about 1/8", then bring the strings back up to pitch. With the tone control wide open, listen to the sound through a clean amp and try tightening and loosening the tailpiece screws a half turn at a time. The sound of the ringing will change, and there will be some point where the ringing is the cleanest and the quietest. That's about the best sound you'll get. Recheck the action setting, because it may have changed slightly due to the movement of the diaphragm.

If you tighten the tailpiece down too far, the pickup may suddenly go dead and have no output. This is because the diaphragm has been pushed down so far that it has contacted the magnets. If this happens, raise the tailpiece a " or so and it should come back on.

One other important note about the "mystery" pickup: there is a small RCA-style jack and plug mounted inside the control cavity which is used to connect the output from the pickup coils to the main harness, which is attached to the pickguard. On most Ampegs that I've opened up, the jack and plug have become badly corroded over the last 30 years. This causes crackling and distortion in the output, and eventually the instrument goes dead. It can usually be fixed with some scrubbing and contact cleaner, but it's better to just replace the jack and plug. An Ampeg with no output is most likely because of this corrosion, although it may also be a shorted tone capacitor or a damaged output jack.


Notes On The AMB-1 & AMUB-1:

The pickup on AMB-1 and AMUB-1 models is adjusted in height like most other basses, with the two screws on either side of it. I normally adjust the pickup height so there is about 0.100" gap between it and the strings, with the strings fretted at the high end of the neck.

Warning! If you ever open up your AMB-1 or AMUB-1, be very careful handling the wire that comes out of the pickup. The pickup coils are cast into a block of black epoxy, and as the epoxy shrinks a bit over the years, there is no longer any strain relief gripping the jacket of the wire. If you twist or pull on the wire, the leads may snap off deep down inside the epoxy block, and there is no practical way to repair it. If you have the instrument open, I recommend that you put a drop of gap filling superglue right at the spot where the wire comes out of the epoxy, as a precaution.

I'm not sure why Ampeg added the round steel bar that goes on top of the strings behind the bridge. Cranking it down to add more string angle over the bridge saddle blocks doesn't seem to have any effect on the tone that I can hear, and it may even damage the strings. Removing the bar completely also doesn't seem to hurt anything. I guess it's just there to look cool...

If you're wondering about the funny looking screws holding the bridge plate to the body, they're called "clutch head" screws, and were commonly used in TV's and electronic gear in the '60's, usually where they didn't want owners to mess around inside. I don't know why they used them here. They were probably just lying around the amp shop. To remove these screws, you need a #3 Clutch Head or "C-3" driver. You can find them in electronics shops, and many of the sets of hex insert bits for screwdrivers also have them.

On some AMB-1's and AMUB-1's with low serial numbers, the bridge was mounted about 1/4" too far back on the body, and there isn't enough travel in the bridge saddle blocks to set the intonation correctly. Here's a quick fix: The three screws that adjust the height of the bridge rest in three dimples in the bridge plate. Loosen the strings and lift the bridge out, and you'll see them. Rather than move the whole bridge plate, it's easier to make three new dimples 1/4" forward (closer to the neck) of the original ones. Measure and mark the new positions carefully, center punch them, and gently make new dimples with the tip of a drill bit in an electric drill or a drill press. Make them about the same size as the originals.


Notes On The SSB & SSUB:

There's not much to add about these models. The adjustments are similar to the AMB-1/AMUB-1, except the intonation is fixed on the bridge. The two in my collection are very stable and comfortable to play as they are, and I've never had the need to do any work on them.


Care & Handling Of The Pickguard:

The pickguards on all Ampegs are made from a 5-layer laminated bakelite material that's known as "engraver's stock", because it was commonly used to make nameplates and wall plaques. As far as I know, this material is no longer manufactured or available anywhere. The multi-layer vinyl used for the pickguards of most guitars today doesn't have the same look. The pickguard is actually the most irreplaceable part of an Ampeg!

Warning! After 30+ years, this material is brittle, and if you flex it or drop it, it may snap or shatter!! When you have the pickguard off of the instrument, handle it as you would a piece of thin glass. Also, be careful cleaning it, because the top black layer is only about 0.010" thick. Don't use any kind of abrasive cleaner on it, or you may go right through the top layer.

A good way to clean it up is with a light application of 3M's Plastic Polish, buffed gently by hand. This is available in auto parts stores, and is sold for cleaning up the plastic trim on cars. I've found that it's also just the thing for putting the final polish on polyurethane finishes on instruments. It works great as it is on the pickguard, but I also have a bottle of it that I've tinted with black pigment, and it helps hide small scratches and faded areas. Rub it on and buff it with a soft cloth. Use an old toothbrush to clean the gunk out of the Ampeg logo.