Tips & Tricks (including modifications)
How to Date a Portaflex Amplifier
Some companies such as Gibson and Martin have kept meticulous records of production dates and serial numbers for their instruments. Amplifier companies tended not fall into that category so it is harder to date their products. There are different ways to estimate the age of a Portaflex amp, they all provide part of the story. Looking at the serial number is the best way to start. The earlier portaflex amps had the serial number on a label in the cabinet. It wasn't until later that a serial number was also stamped on the left side of the back of the amp chassis. The next step is to look for manufacturing date codes on the components. A date code is stamped into the metal housing of the potentiometers. They were manufactured by Chicago Telephone Supply, aka CTS. Look for a code that begins with 137, for example 1376340. 137 is the EIA manufactures' code for CTS, the next two digits are the year, in this case 1963, and the last two are the week of the year, in this case 40. So this pot was manufactured between September 30th and October 4th of 1963. If the cap can capacitor (the metal tube next to the 5AR4) is original, there is a similar date code, the Lord shock mount has a month-year date code, as does the speaker. Sometimes there's a piece of paper under the transformer with a date code as well. Some tubes have date codes but they are less reliable for estimating the age of the amp.
When you compare the serial number date with the component codes, you find that there was a short lead time between when the components were manufactured and when they were placed into an amp. If you take the most recent date code and add a month or so and you have a good estimate as to when the amp was built. Of course, there is always the possibility that someone sourced a replacement part with an date code that is close but newer but from the same era. For that reason, it is important to consider all the date codes when deciding. Some people go as far as examining solder joints to determine if they are original. This could indicate that a part hasn't been changed. But there are ways to fake solder joints so that isn't always useful. My best advice is to consider the amp as a whole and take everything into account when trying to come up with an estimate.
Excerpt from the B15N owner's manual:
All PORTAFLEX models (except B-12-N) are equipped with detachable shock-mounted dollies. Important: Remove dolly while amp is in use. Because the dolly will absorb vibrations, it will also absorb sound waves. There is a definite gain of power if the amp has a solid footing. You may seemingly get enough power with the amp on the dolly, but the speaker is working harder than it should.
The dollies had two fixed castors and two that could rotate through 360 degrees. This prevented the amp from rolling forward on stage while performing. Otherwise a slight tug on the the instrument cable could move the amp. On smaller amps, the dolly was attached with a bolt with a handle. It is important to not over tighten the bolt. It can cause the dolly to bend in towards the cabinet and this puts a lot of pressure on the bolt. So much so that it can lock it in place and make it difficult to unscrew. Pressing the dolly towards the cabinet can relieve the pressure from the bolt.
Gasket foam tape
Where can I get 'gasket tape' for the cabinet, that is, the rubber seal around the cab opening?
It is a closed cell dense foam 1/4" X 1/4". It is available from Fliptops. You may also find it at a local car parts supplier or hardware store. Depending on what you find, you might need to use a razor knife to trim it to a width that fits in your channel. On vintage amps, the foam was glued into place which makes removing it a bit of work. It is important to remove as much of the old residue as you can so that the new foam will adhere well. Start with a wet rag and finish with alcohol or acetone on Q-Tips. Be careful not to get the area too wet, the blue stain around the channel is water soluble and comes off fairly easily. You might need to touch up the stain if too much comes off.
The purpose of the foam is to create an air tight seal around the lid. With time, the foam becomes less resilient and compresses, loosing it's sealing effectiveness. When this happens, it should be replaced because leaking air could produce cabinet noise and vibrations.
Cabinets that have dolly mounting hardware have a rubber disk glued over the hole on the bottom of the cabinet to prevent leaks. These are commonly missing on vintage amps. The glue fails and the rubber disk pops off and gets lost. Amps that have connector and crossover panels like the B-15R have the panel sealed with a gasket and silicone sealant. Cabinets with 1/4" jacks sometime employ air tight jacks to avoid leaks.
Speaker adapter - XLR to 1/4"
B-15N Preamp Output
On the back of my B15N head, there is an input/output( ? ) that is labeled : " Ext Amp ". What is this exactly? I never noticed it before!
The Ext Amp jack serves two purposes. It's a preamp out that can be sent to another power amp or to a DI. It is also a power amp input. Here is a quote from the mid-60's B15N owner's manual:
If you feel you need more power than your equipment is designed to produce, you might consider obtaining an additional PORTAFLEX. All PORTAFLEX models are equipped with an external amplifier jack. This enables you to hook two complete PORTAFLEX amps in tandem, which, depending on the model, can double your output wattage. This tandem set-up is accomplished by using a regular mike cable with a phone plug on each end, connected to the external amp jacks of any two PORTAFLEX units; or, you may plug the other end of the cable into the input of any amp. If used in this manner, adjust the volume on the PORTAFLEX first, then adjust the volume on the secondary amp, which in most cases will be a very low setting. What happens is this: The signal from the pre-amp on the PORTAFLEX is also sent into the second amp, so you get the output from both units. If the second amp has vibrato or tremolo, it will sound as if both units have it. This is another way to get more power if you ever feel you need it.
The earlier and the later B-15's have different preamp designs. The later ones are better at driving DI units via the Ext Amp output. Some people have reported impedance mismatch problems when using the earlier models. With the DI connected to the Ext Amp jack, the power amp looses signal and the volume decreases. If your DI has a high impedance switch on the input, try engaging it and see if it helps. If it won't work, try plugging your instrument directly into the DI, then plug the thru on the DI into the amp's input. The disadvantage of this is that the DI out will not the the B-15's preamp and tone circuit as part of the signal chain.
Mold growing on speaker and cabinet
I discovered mold growing on the cone of the original speaker in my prized '6(early) B-15-NA. What's the best way of getting rid of it?
Use a soft brush to remove the visible mold but that doesn't stop it from re-growing, it appears to remove it but it doesn't remove the deeply seated roots. I've heard guys use all different types of things, leave baking soda in the box or vinegar in a open jar in the cab while it's not being used. I just brush it off and live with doing it every few months.
Borax Laundry Detergent is affective at eliminating mold. It comes as a powder that can be dissolved in water. Spray or gently wipe the solution on with a soft rag or brush being careful not to allow the paper speaker cone to get too wet. Older speakers require more care as the paper can be very fragile. Let it dry and reapply if necessary. Clean the residue away with just water. For mold on the cabinet, use a more concentrated solution with undissolved borax granules in it. The coarser borax will help to grind the mold away. Rinse with water.
Borax has a secondary effect of removing odors. It helps get rid of that old cab smell. It can be used on the vinyl as well. Just be careful not to get too much water into the seams of the vinyl. If it penetrates deep enough it will dissolve the glue and the vinyl will lift. Then it will need to be re-glued.
There can be lethal voltages stored in the power supply caps within your amp so be very careful if you do any work yourself. Even with the amp turned off, the capacitors can be still be charged. They need to be discharged. The procedure for discharging an amp are described here. If you are unsure as to what you are doing, it is best to leave it to a tech. It isn't difficult, you just need to be careful.
Cleaning the amp should be done as part of a yearly maintenance program. Start by blowing out any dust that is inside the chassis. Clean any dirt that has accumulated. Some amps can be pretty dirty inside. Dirt can act as a conductor or capacitor across traces. This can affect the sound of the amp and even the noise level. Isopropyl alcohol (IPA) can be used to clean inside the chassis. It is good for cleaning up those dark brown or gold colored pools of solder flux. I use a product called Safety Wash which is designed for washing circuits. It is a blend of ethyl alcohol, isopropanol and ethyl acetate. Both products are fairly safe to use but be careful, if you let them soak the components too long, color bands and other identification marking can come off. Do not let these products get onto the white chassis lettering.
Metal surfaces can oxidize. This can affect the contacts. Deoxit is a good product for safely removing this oxidation. There are different Deoxit products, (available at Antique Electronics D100L-25). I prefer the needle dropper one because it is expensive and you can apply just the amount that you want with precision. The 100% concentrate quickly removes the oxidation. You can strip the plastic or teflon sheathing off a wire and attach it to the end of the needle to apply the liquid in hard to reach places like upside down pots.
You apply Deoxit to all metal-on metal contacts. This includes the jacks, tube sockets and tube pins. Follow the manufacturers instructions. Apply a small amount, let it sit. I then scrub with a small inter-dental brush that you can get at a drug store. Be careful to not stretch out the contacts on the tube socket. You want them to have a tight contact with the tube pins. If there looks like there is still oxidization on the metal, repeat the process. Then finish off by applying a small amount again and don't wipe it away. The product has an oil in it that protects the metal. On amps that don't have sealed volume and tone pots like the CTS ones in the Portaflex amps, you can apply a drop inside the pot. There is an opening near where the solder tabs are. Just apply one drop and then move the knob back and forth several times through the full travel of the pot. When you tip the amp back into position, some of the Deoxit may run out. Clean it with a Q-Tip. Deoxit will clean and lubricate the pots preventing them from sounding scratchy when you turn them. The product can be used on your bass pots and jack as well. I don't use it on the wire wound hum pots that are in the back of some amps.
If your amp has a hum pot, adjust it by ear for minimal hum with the amp on and in playing mode.
Inspect the components and tube sockets for any sign of charring on the circuit boards. Look for a black soot. This is evidence of burning resistors or arcing problems. An ESR meter can be used to examine the health of the electrolytic capacitors. If the amp has developed a hum, it might be time to change these capacitors. They have a limited lifetime, even when the amp isn't being used. It is a good idea to turn an amp on in playing mode (not on standby) at least every six months for a half hour or so to help reform these capacitors. It will extend their life.
If your amp's power tubes are fixed biased, the bias should be checked at least once a year. As the tubes are used, the bias that they require will change.
I use an automotive product called Autosol Metal Polish on the chassis and other metal parts to remove rust and leave a protective coat. Any chrome metal polish will work. You have to be very careful though. Don't let any of the cleaner come in contact with the chassis lettering. It will be dissolved and removed in a heartbeat. Start with the most conservative approach and then try more aggressive approaches as required. I start by washing the outside of the chassis with a mild soap and water solution using a light touch over the lettering. The lettering can be easy to scratch off so be very careful. Then use the metal polish on the other areas of the chassis to remove the rust and clean and polish the metal. I remove the screws, knobs, nuts and washers first so that they don't get in the way. They can be done individually.
For badly rusted areas, I use a product called Restore Rust Removal Gel. There are other similar products available. It is a gel based rust remover that sticks to the surface and doesn't run or dry up while you are using it. You can put it on horizontal or vertical surfaces and let it work. It helps to try to pick away and remove any loose rust first. These rust removal products are expensive but you don't need to use a lot so they last.
I remove the metal corners, keeping the nails. I place them in a rust removing soaking solution such as Restore's rust removal concentrate, or a product called Evapo-Rust. The nail heads and corners are polished after the rust is removed.
It is surprising how well the metal can be cleaned up.
When cleaning the vinyl, avoid getting too much water on the seams. Vintage amps used hide glue to affix the vinyl and it's water soluble. This can cause the vinyl to pull away and it will need to be re-glued if it starts to come apart.
I like to start with the least aggressive approach and move up to stronger cleaners in order to avoid damaging the vinyl. Again, I start with soap and water and a soft brush. Someone else's tooth brush works well. I use a medical scrub brush because it has a larger surface area and is gentle on the vinyl. Dental picks are good for getting into cracks and the ridges of the Ampeg blue check. As you can imagine, this is a tedious task.
Additional details related to the vinyl are in this section.
To get rid of smells, I start with a coarse solution of Borax laundry soap. Dissolve the Borax in water and then add some more the isn't dissolved to keep the solution gritty. The grit acts as a mild abrasive for cleaning the tougher areas. Rinse well. Borax works well for removing mold as well. It can be used inside or outside of the cabinet.
For more determined smells that won't come out with Borax, I use a product that is available at pet stores for removing urine, skunk, and smoke. These products use enzymes to break down organic based smells. There are many such products, I use one called Odourmute. Follow the directions and use multiple applications if necessary.
Sometime you just can't get the smell out of the vinyl. When all else fails, the cabinet may need to be recovered.
Those odd looking screws
Ampeg used clutch head screws on their vintage amps and cabinets. Someone there must have been a fan of Batman. You need a special screwdriver to work with them. The 5/32" drivers are available at some hardware stores as well as at Fliptops. These screws and a new chassis are about the only Portaflex replacement parts that aren't readily available other than on eBay. The screws were used in some General Motor vehicles, farm tractors, and recreational trailers but finding them in the correct size can be a problem. Often on vintage amps you find that these screws have been replaced with a more common part.
Fixing a vintage latch
The metal latch that holds the lid to the cabinet can fail. If the metal plate that screws to the wood comes uncoupled from the rest of the latch, it may be able to be fixed. Start by removing the metal plate from the cabinet. There are two fasteners holding it in place. The plate has a ninety degree bend on each side. There are two pins at the top end on the left and right sides that clip into holes on the plate. If one or both of these pins are broken, you'll need to buy a new latch. If the pins are in good shape, then they may have just popped out of their holes. Lie the latch down, with the outside surface against a table. Locate the pin holes on the plate. You may need to use a pair of pliers to bend the metal with the pin holes open a little so you can easily pop the pins into the holes on the plate. Push the spring down and pop the plate pins in place. Use pliers to bend the metal with the pins holes inwards so that the pins stay in place. You don't want to bend the metal inwards too much so that the plate won't pivot freely at the pins. Be careful using the pliers. Where ever you scratch the metal, it is eventually going to rust. Polishing the latches with a wax, such as a carnuba instrument polish or Nevr-Dull, will help keep them looking good. New latches sometimes need adjusting before they are installed on the cabinet to prevent the pins from popping out.
Another failure can occur at the claw. The claw clamps onto the keeper that is attached to the lid and holds it down on the cabinet. If the claw is not holding tight enough, the lid isn't pulled down which could result in an air leak or a vibration. With use the claw can be bent out a bit and the assembly loosens. You can use a pair of pliers to bend the claw inward a little to make a tighter contact. Cover the latch when doing this, I use a piece of rubber. You don't want to scratch the plating on the latch, it will eventually rust. This usually fixes the problem. If the contact still isn't tight enough, sometimes you can loosen the latch's back plate, position it a bit lower, and then tighten the fasteners. Another fix is to tighten the square bolt in the lid a small amount. Turning the point upward by a small fraction makes the latch fit tighter.
If the latch is fine but the lid is loose enough to cause a vibration, the foam seal under the lid that runs around the opening may have become compressed. This can happen with age. Changing the foam can correct this. There is more information here and here.
A better latch
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.
Vintage amp buying checklist
What to look for when buying an amp can differ depending on your needs. The cost of any vintage amp should include a checkup from a tech. A tech can look for any potential issues and bring the amp up to spec. Costs to get the amp back into shape can vary depending on the condition of the amp.
For someone that is looking for a restoration project, you often want to look for all original parts. For someone that is looking for a player, it doesn't matter what has been changed, as long as it sounds good. Many of these amps are in pretty rough shape. Don't let that deter you. The good thing about these amps is that, except for the chassis, all replacement parts are available, even reproduction cabinets. They can be rebuilt to better than new condition from the ground up. Recently the blue check vinyl has become scarce because Loud has moved to black check. Hopefully it will be readily available again in the future. Regular maintenance, including anything that needs to be changed to keep the amp in playing condition should not lower the value of a vintage amp. This includes changing the power supply and bias capacitors, a three conductor power cord, and any components such as resistors. An amp that has recently been serviced is good because it will save you the cost of having it done yourself. SO that should be factored into the cost.
How the amp sounds should be your priority. Start with the tone controls at noon which is flat on these amps. It can be easier to evaluate the amp if you have someone else play while you try all settings on the amp and listen. Slowly move the tone pots through their entire range to ensure that they are working. Try all the inputs. If there are two channels, test both of them. Then play through the amp yourself, interact with it, dial in a good tone setting and see what you think.
Here are some things to look for:
- Ask what the service history has been and if there has been any issues.
- Has a three-conductor power cord been installed. This is a safety issue.
- When were the tubes last changed.
- Has it had the power supply capacitors changed. If the amp is humming loudly, it can be an indicator that they need to be changed.
- At what volume level does the onset of distortion occur. Ten or eleven o'clock is normal. If it is lower, the amp might need a cap job or new tubes.
- Ask if the speaker is original or if it has be reconed. If a speaker is reconed properly, it is as good as new.
- Is the original speaker connector in place.
- Ask if any circuit modifications have been performed.
- Are the knobs original and do all they all look the same.
- Is there a scratchy sound when the knobs are turned. This could simply mean that the pots need cleaning.
- Ask if either of the transformers have been changed. It isn't unusual to find a replacement power transformer. This can be a plus because the modern replacement transformers are better than the original ones.
- Listen for any cabinet rattles and buzzes at higher volume levels.
- Is the logo on the front of the cabinet original and in good shape.
- What condition is the vinyl in. When the dirt is removed, the amp will look a lot better.
- Is there a dolly. Not all Portaflex models came with one.
- Is there a dust cover. The best preserved vintage amps are usually the ones that have covers.
- How much rust is there on the chassis. For a chrome chassis, the less rust, the more valuable the amp. Although rust leaves a scar on the chassis, it can be cleaned up and made to look better. A painted chassis can be repainted. Replacing the silk screened lettering is more difficult. It makes it a lot easier if you can leave those surfaces as is.
Vintage vs reissue B-15 knobs
The reissue knobs are visually close to the originals but are not quite the same. The most obvious difference is that the original knobs on the early 60's amps were made of Bakelite, a hard type of phenolic plastic, the modern replacements are lighter plastic. They feel different, the Bakelite knobs have a rougher texture and feel more solid. The reissue knobs are a darker black color that is smoother.
There have been stories of sellers who advertise the amp with all original parts and then swap out the knobs with new ones before shipping the amp. They then part out the knobs for more profit. If you are buying online, ask for images of the knobs on the amp and ask if they are original. Make sure that you receive what you're paying for.
Vintage vs reissue B-15 transformers
It isn't unusual to find one or both of the transformers, most often the power transformer, have been replaced. New transformers aren't necessarily bad to have. The old ones develop cracks in the protective lacquer as it breaks down with age, high temperatures, and being pushed too hard. This can lead to internal hot spots and shorts. The new transformer have better specs and can better cope with the higher line voltages found today. The new epoxy potting compounds do a better job at wicking away the heat from the transformer which helps them run cooler. The transformers that Fliptops sells are made by Heyboer and are excellent products.
How can you tell if you have an original or reissue transformer. The original transformers have a smoother texture on the metal can. The new ones have a rougher wrinkled texture. It is possible that the transformer was replaced and repotted in an original can. So if you see a smooth can, it doesn't necessarily mean that it is original. If you unbolt the transformer and pull it away from the chassis you can see the potting compound at the bottom of the can. Modern potting compound is usually a hard epoxy, the originals used a wax like tar that melts if you heat it. A good restoration will use the tar but this is very rare to see.
The new transformers have a tougher finish. The texture keeps it looking cleaner.
How can I get more clean volume?
This is an often asked question. While great for recording or small venues, a 25 or 30 Watt B-15 amp needs all the help it can get to optimize it's clean volume level. There are some tweaks that can help the amp is deliver all it can. It can vary but it is normal for the onset of distortion to begin at somewhere between 10 and 12 o'clock with the tone controls set at noon. With some amps, it is later. This assumes that the volume potentiometers in the amp are original.
- Healthy power supply electrolytic capacitors are a must. As capacitors age, their performance decreases, lowering headroom and increasing distortion.
- The B-15's power supply capacitors varied with different revisions. In early models, the first power supply capacitor (Node A) was 40uF but it was lowered to 30uF in later models. Using a 40uF/600V, high current capacity, capacitor helps the low end by providing a better reserve to meet the demand of the amp. This increased headroom and lowers distortion. Flashover in the rectifier tube can be an issue with a higher capacitance, so care must be taken when performing this modification.
- Adding a choke is another way of stiffening the power supply. It also addresses the tube rectifier flashover issue mentioned above.
- A solid state rectifier module can be used in place of the 5AR5 tube rectifier. This will avoid the classic power supply sag and distortion when the amp is pushed.
- Pay attention to speaker specifications. A speaker with a high sensitivity over the output impedance range of the amp will be louder. Other specifications are important as well. Everyone has an opinion as to which speaker is best and Ampeg offered a few different options over the years. There are a number of modern options available. There doesn't seem to be a consensus as to which is best. The Faital Pro 15PR400 looks interesting.
- Use a set of power tubes that are selected to provide a late onset of distortion.
- Not amp related, but if the instrument signal is too strong, it can overpower the the first preamp tube and cause it to distort. Lowering you signal level will help. Results will vary with the bass that you use.
Installing a choke in a B-15
A choke is an inductor which can be used as a filter in a power supply. It acts as a buffer, resisting changes in current flow and thereby stiffening the power supply. This reduces the power supply's ripple voltage and hum in the amplifier. When Loud was developing the Heritage B-15, Jess insisted that the amp have a choke in the power supply. He felt that a stiffer power supply would make the amp perform better so they put one in. They also incorporated one into the newer Heritage B-15N.
The easiest way to install a choke in a B-15 is to replace the resistor between the first and second power supply capacitors (Node A and Node B). This is illustrated below in the B-15NC power supply segment. The 1K 10W resistor is removed and replaced with the choke. An added value of the choke is that it permits the power supply to use higher valued capacitors than the 40uF ones shown. The choke should be rated at around 120mA, the inductance used typically varies between 5H and 15H, they used a 9H (Heyboer HY027707) in the Heritage B-15. Mounting the choke is a bit of a problem. Typically two holes are drilled on the side of the chassis at the end with the power transformer. A further modification that some designers like to incorporate is to use a DPDT switch to select either the resistor or the choke in the circuit. If you removed the ground switch as part of a three-conductor power cord installation, you could install the switch there.
Connecting a DI
A Direct Injection (DI or direct box) device allows you to convert a high impedance line level unbalanced output signal to a low impedance balanced mic level signal. It allows you to tap off your instrument signal to feed to a mixer or some other piece of equipment. The DI buffers the signal so that it can be sent over long runs while minimizing high frequency loss, noise, distortion and ground loops.
There are different ways to connect a DI device to an amp.
- Plug your instrument into the DI and split off the signal to go into the amp. The disadvantage of this approach is that the tone of the amp doesn't get sent to the mixer. This approach is best if you want the cleanest uncolored signal send to the mixing board.
- Plug your instrument into the amp and feed the pre-amp output into the DI. The advantage of this approach is that you have the pre-amp's characteristic sound and tone stage in your DI signal. This approach is good if you want the tone of your pre-amp sent to the mixing board.
- Plug your instrument into the amp and feed the power amp's speaker output into the DI. This is the best approach if you want the full tone of the amp sent to the mixing board. This can only be done with a DI that is designed to be connected to an amp's speaker out. The Countryman 85 and Radial JDI can both do this.
Some people prefer to mix a miked amp with a DI. This allows them to take advantage of the strengths of both techniques.
In the case of an Ampeg amp, the EXT AMP jack on the back of the chassis is a pre-amp out/ power amp in. You can use a short instrument cable to send the pre-amp output to a DI. When you tap off a pre-amp signal, the power amp will still be working so you will continue to hear sound coming out of the speaker.
In the case of a B-15, the revisions are not all designed the same and it is possible that you could encounter an impedance issue when connecting a DI to the EXT AMP jack. Some amps have what is called a cathode follower before the pre-amp output jack. This buffers the signal and provides enough current to drive a DI and the power amp. If you connect a DI and hear a drop in your power amp volume, you have an impedance mismatch which causes the signal loss. The degree of mismatch can vary, depending on the design of the DI. Another make of DI may resolve the problem, otherwise a DI that can connect to the speaker output will get around the impedance mismatch problem.
Miking a cabinet
Getting microphone placement right at the source will help to optimize the sound and minimize the need for EQ. The EQ can be adjusted by moving the position of the mic on the cone. It is important to experiment and find out what placement works best for your cab with the microphone that you are using. You can vary the distance between the mic and the speaker cone, where the mic is positioned on the cone, as well as the angel of the mic.
Common positions or combinations include:
- center close to the dust cap edge which gives you the most high and low frequencies
- center off axis, with the mic tilted 45 degrees or so, which gives you a darker sound
- outer edge off axis with the mic close to the edge of the cone (not the surround), which gives a more even compressed sound balancing highs and lows
- distant center position, one or two feet in front of the speaker. This works well with more sensitive mics.
Cabinet miking positions (Diagram courtesy of Michal Kaszczyszyn)
Microphone preferences vary. I like to use a Neumann U87 or AKG C414 forward cardioid pattern places a foot or two in front of the speaker. Otherwise, despite its lack of low end, the Shure SM57 is commonly seen live. Similar to the SM57 is the Peavey PVM 45. I like the flat grilles on these mics which helps with precise positioning. Some people like to mix in the signal from a DI to help fill out the sound.
Popular favorites for bass include the EV RE20, Heil PR40, Sennheiser MD421, and Beyer M88.
B12N/B15N tone modification
Here is a B12N Tone Modification. This would apply to a B-15N as well as the amp is the same, the speaker cabinet is different. They add ultra high and ultra low tone switches to a 1968 B-12N.
How to build a Baxandall tone circuit
Adam's Amplifiers shows you how to build a Baxandall tone circuit, with the components mounted on the pots. As the tone circuit ages, the components can go out of spec and the amp doesn't sound like it should. If you wanted to rebuild the tone circuit without the PEC module that Ampeg used, this is how to do it. Consult their web page for additional details.
They also have a simple modification for a mid frequency tone shift. It adds a mid range hump when the tone controls are set flat.
PF-115HE Port Modification
Clint Stiles performed some surgery on his PF-115HE cabinet to add a pair of rear ports to the cabinet. He reports that he is pleased with the results. Read all about how he did the modification here .
category: Ampeg Portaflex wiki