Troubleshooting/Modding the
Seymour Duncan Convertible 100

I am currently in the process of trying to compile a list of serial numbers and manufacture dates. If you have information to contribute, please send me an email. You can try contacting Seymour Duncan directly to get a date of manufacture, but they are pretty slow getting back to you. So, if you know the year your amp was manufactured, please send me your serial number.

General:

Of course, the first place to look is the Owner's Manual for troubleshooting advice. Most of what is listed in the manual is very basic stuff. But, I have heard of a variety of problems that can occur with these amps that are not listed in the Owner's Manual, most of them having to do with output signal problems. Most of these problems can be easily corrected. Listed below are some suggestions for fixing some of the more common problems specific to these amps. For general info, see the Tube Amp Debugging section at http://www.geofex.com

Tubes:

The first place to start is with the tubes. Convertibles are very picky about tubes. The most noticeable one is the power tubes. On a nice clean, undistorted sound, just about any power tube sounds pretty good. It is when you want distortion that the power tubes really count. For example:

When I bought my amp, there were Sovtek EL34WXT "Reflector" power tubes installed in it. They were terrible! Too much emphasis on the mids and ratty sounding distortion. I put some J&J EL34L's in there and it sounds much, much better. These are much more well rounded.

The rectifier tube is not as critical for the sound of the amp, but very important to the whole power supply! NOS tubes can be had for about the same price as new ones, unlike power and preamp tubes. But according to Seymour Duncan, in their fairly extensive testing of rectifier tubes, they found that the Sylvania 5U4GB is by far the most reliable of all types and brands for this particular application. Most European brands tended to be very unreliable with some failing almost immediately, often taking other components down with them.(!) They still have a limited quantity of NOS Sylvana 5U4's in original boxes that they can offer to anyone that needs them for $12 each. They also have odds and ends of the not-commonly-available repair parts. In near future, they plan on publishing a listing of these items at their website, www.SeymourDuncan.com

In keeping with tradition, the preamp tubes that you use will also make a noticeable difference in the sound that comes out of your amp. You can play around with changing the preamp tube gain factors.

See my Preamp Tube Gain Factor Chart for more information.

Output Problems:

The main complaint I hear of is the amp having fluctuations in volume output level. This seems to be mostly caused by loose or corroded wiring connections inside the amp. The leads from the amp control pots, input jacks and effects send and return lines are all connected to the board via crimped wire connectors which slide over vertical posts which rise up out of the PCB. On the one hand, you could say that is good design because it facilitates easy removal of the PCB. On the other hand, all of the connections have an opportunity to get a little corroded over time, which causes weak signals in the circuitry.

The way to fix this is fairly simple. I removed all of the wiring connections off of the pins, one by one, and gently squeezed the connectors a little bit with some needle nose pliers and reinstalled them on the posts, making sure the connections were snug. I also used contact cleaner (see caution in paragraph below) on them before reconnecting them. Care must be taken to make sure the wires are all reconnected properly as they are in sets of two or three in close proximity to eachother. There is about 30 wires to go through. Some of them were pretty loose. It took me about an hour to do this operation and it resulted in my amp sounding a lot better. Ultimately, one could solder all of these wires to the posts and permanently get rid of this problem. However, it would be quite difficult to get to some of the wires with a soldering iron.

Another thing to look out for is corrosion in the input and output jacks. The jacks in my amp had quite a bit of corrosion in the barrel of the jacks. I used a .30 caliber rifle cleaning rod and brush to clean the inside of all of the 1/4" jacks. Then I cleaned them with a patch of cotton soaked with gun bore solvent. This gets them really clean. I also noticed that the outside of the male 1/4" plug on the speaker cord had just a little bit of rust on the outside, but a lot of rust on the inside! That can mess with the output to the speaker. I replaced the plug and the amp sounds a lot better still.

Also, spraying some contact cleaner on the preamp module sockets may help, too. But here are some important cautions: some "contact cleaners" are conductive, most notably Cramolin. Spraying them on the PCB near the module connectors can cause problems. When selecting a contact cleaner, you should look for any indication that the substance is a "contact enhancer". Some amps have had holes burnt in the PCB because the contact cleaner that was used caused arcing from B+ to ground. B+ and ground are right next to eachother on the preamp modules. So use caution if you decide to use any type of contact cleaner on the module sockets. The power tube sockets are especially vulnerable to this because the voltages are very high. They will start to arc as soon as the power is turned on if a conductive contact cleaner is sprayed on them. (Can you say "Poof"?)

Terrible Sound when using Variable Wattage Control:

One person wrote to me and described this problem to me: "I have recently been having problems with the variable wattage output. If I leave it at 100w, its fine, but if I try to turn it down, it makes a terrible hum/buzz which increases the more you turn it down (5watts is worse than 60watts) Sounds like going between AM stations on an old transistor radio." It turned out that this was due to a faulty 12AU7 preamp tube. What was happening was because of that faulty 12AU7. The 12AU7 is a dual triode and it is connected across the output of the phase splitter. Adjusting the power control varies the DC bias on this triode, turning it on more/less. This acts like a variable shunt. At the lowest power settings, the triode is conducting the hardest and therfore provides a fairly low impedance directly across the phase splitter, allowing very little signal to be developed across the grid resistors of the power tubes. So, with this tube being faulty, it would make your amp seem like it was having a meltdown or something when in fact, it's just a bad tube.

Unwanted Distortion:

I had a problem with some unwanted distortion when I was playing on the clean channel. For example, if I played a major 3^rd interval on the E and A strings, and plucked the strings hard, it sounded like the speaker was distorting the signal. First I determined that all the standard things to check all checked out OK. Upon further investigation, I discovered that this problem had to do with the setting on the wattage selector and the amp being on Triode mode. I had the wattage at 100. I tried turning it down to about 60 watts, and the problem went away. When the amp is in Pentode mode, the wattage selector has the full range available – 5 to 100 watts. When in Triode mode, the selector’s range drops down to 5 to 60 watts. So, why the distortion? This problem stems from having preamp tubes with too high of a gain factor installed in the wattage selector circuit. On the bottom of the chassis, there is three preamp tubes. Looking at the back of the amp, from left to right, you should have a 12AU7, a 12AX7 and a 12AU7 installed in there. If you have three 12AX7’s installed, you’re going to have some problems with unwanted distortion. 12AU7’s have a gain factor of 19. 12AX7’s have a gain factor of 100. I have 12AT7’s in my amp, which have a gain factor of 60, but these are probably still too hot.

Excessive Hum:

This one can be difficult to track down. Generally, hum problems come from a bad ground on some component, or a bad capacitor leaking too much AC current. Sometimes it can come from cheap power tubes. It could also be a preamp module, or a bad wiring connection somewhere inside the amp. If you know it is a specific preamp module, then that’s an easy one. That could be a bad component or a bad solder joint on the module itself. If it’s not a problem with a preamp module, then check all the ground connections to the PCB, and especially the PCB connection to ground. Another place to look is the input jack or speaker jack. These amps should have about as much a background hum as a silverface Twin Reverb or similar amp. My amp also had too much hum on the Effects Loop. The source of this problem apparently was a guitar pedal with an inadequate amount of power filtering when used with a "wall wart" AC to DC transformer. The other thing to check is the connections to ground on the effects send & return. A bad connection to ground or ground loop will also cause this.

Reverb Problems:

The Convertible utilizes a SS reverb recovery circuit. Reverb problems can be traced down to a few possibilities. The most obvious ones are defective RCA cables connecting the reverb tank to the send & return jacks on the back of the amp and the reverb tank itself. Sometimes the tiny wires inside the reverb tank can break loose from the spring assembly, resulting in no reverb. Usually the wire can be re-soldered and the problem fixed.
The other thing that can fail is the solid state IC's (Integrated Circuit) chips. They can sometimes go bad. In such case, they have to be replaced. The reverb send uses a National Semiconductor LM380 audio power amplifier, which has the rather unique ability to be ground referenced. The reverb return uses a TI (Texas Instruments) NE5534 low-noise opamp. If these have to be replaced, I would suggest installing a socket for the chip being replaced. This allows for easy removal if required, and protects the PCB from being damaged by repeated soldering and de-soldering.

RF Interference:

An individual emailed me about this one. He bought a new Convertible around 1985. He had an occasional, but long standing problem with radio stations and similar things getting picked up on his amp. No one seemed to know the cause of the problem. One day he was sitting in an orchestra pit with the symphony, in a hall right next to an elevated interstate highway, and had some trucker's CB just blast through through his amp! That unfortunately ended the amp's gigging career, but, one day he took the chassis out and poked around, and found one resistor that had never been soldered in, but was making enough contact on the PCB for the amp to work. He soldered it in, and the problem went away.

Popping Fuses:

Aside from the standard things to check - filter caps, power transformer, output transformer, rectifier tube, power tubes, etc., there are some things that are not so obvious. My friend Steve brought his Convertible over for me to look at. It kept blowing fuses. He thought it may be the standy switch, but I dismantled it and found that it was still in good condition. I checked all of the caps, tubes, power transformer and they all appeared to be in good condition. When I was putting the amp back together, I noticed that one of the speaker wires was loose, a cold solder joint. An output transformer doesn't like not having a full load on it. So, I resoldered both speaker wires and turned the amp on. The problem went away! Not something you would normally think to look at right off the bat, but it could be something just that simple. In this case, the speaker was recently replaced, and the problem arose not too long after.
If your power or output transformers should go bad, replacements are not currently available. Perhaps someone will step up to the plate and start manufacturing replacements, I hope.

"Improving" Tone:

There are some things you can do to improve/change the tone of the Convertible. There is a definite tone improvement in changing the coupling caps. The original caps used were made by Illinois Capacitor (IC). Changing these to a Sprague/Vishay 715P metallized polyethylene "Orange Drops" will give the amp more of a vintage Fender tone, with lots of clarity and "space" without being bright or harsh . You can also get the Sprague 716P Film/Foil caps, but they typically come only in 600V ratings. They are huge and they may not clear the tube in the module. For a lot more money you can buy Angela-labeled Solen Film/Foil caps. But, in the Convertible, they are rather plain sounding compared to the Sprague Orange Drops. They are also $4.50 each versus $1.10 for a 715P. Another cap to try is Audio Caps which while about $4.50 each, make the amp sound very nice and warm. All this stuff works great in the tone control section, too. You can also replace the caps on the preamp modules as well.
Ignoring the age-old carbon comp argument, one other thing you can do is to try using carbon comp resistors in the signal path instead of the factory supplied metal film type. The theory behind that is that they have low capacitance and inductance and therefore cause little detriment to the tone. It is also believed that they impart some subtle harmonic distortion that gives them character. (Cap and resistor information contributed by fellow amp tweaker Gordon Swanson)

Another easy mod which can do wonders for your tone is to move the tone stack after the preamp, 'Marshall-style'. This will give your Convertible a heavier tone, if you don't care so much for the Fender/Boogie sound. I would recommend just moving the tone stack, and not changing any pot values or adding any components. The best sounds to be had are by simply using unmodified modules in the modified chassis. It's kind of an either/or proposition. You can modify the modules themselves OR mod the chassis to get more Marshall-y, as using both results in too harsh a tone. Here is how to do it:
Remove wires to TC1 and TC2 completely, and jumper those 3 posts together. Run a wire from each MV1 and MV2 white to corresponding center lug on the treble pots, then run a lead from the 0.1uF caps in the PCB, after removing the lug facing the front out of the board with needlenose pliers and a soldering iron, to the tone stack, between the treble and mid pots, right of the 100k resistors. Then you're done. It's a pretty simple mod to perform. I recommend this only for those wanting more of a Marshall/Hiwatt/Orange tone. If you're happy with the Fender stuff, this will probably be too harsh. (Contributed by Michael McBane from information extracted from documentation generated by Kevin Beller in 1993, original documents included in .ZIP file)
Original Marshall Mod as received from Seymour Duncan. (.ZIP file)

Carbon Comp Resistors?:

Gordon Swanson sent me an email about his experiences with replacing the metal film resistors in the signal path(s) with carbon comp resistors of equal value. Here is Gordon's description of what he did:

"I went through the preamp and amp section replacing any 1/4 and 1/2 watt resistors in the signal path with carbon comp resistors obtained through Mouser Electronics. Actually to make life simpler, I used 1/2 watt resistors all around, and if you read the carbon comp article at geofex.com, that is going against preferred theory.
I also updated several modules, again strictly in the signal path, and not bothering with anything going to ground. I suggest extreme caution in working with modules. The single-sided traces are very fragile and prone to lifting. The holes are also quite tight making it hard to get some of the components out. The tube sockets also require patience and three hands to gently de-solder and pry the mounting lugs out. A de-soldering pump and solder wick are highly recommended. While I did not lose any modules, a couple are not too pretty looking.
So... how do they sound? Do they sound different? It's my opinion that they soften up the amp a little bit, giving a little more vintage vibe and less edge. At first it appears to be slightly less high end, but you can boost up the treble without it getting harsh or brittle. While it's hard to describe, compared to a recent Fender type amp, it's a sweeter darker sound, kind of what we are trying to get with tubes in the first place. Keep in mind that I typically use my amp in triode mode at fairly low power, so I tend to favor the soft clipping of power tubes over the more bright distortion of preamp distortion."
Judge for yourself. In my opinion, it is probably worth the effort if you want to sweeten your tone some.

Other Tricks:

Here's something that one individual emailed me concerning some other neat tricks you can do with the effects loop:

"I have had a pair of these amps since they first starting shipping them. Also own one as a spare. EV 12L in each of them. All updated mods done to each, except the active effects loop mod of the 2000 series. I liked the original loop better. Works well with pedals or rack gear.
I use these amps a number of different ways. Stereo, with gain pedals pre input, with delay pedals in loop. Also as the power section only using a Demeter pre amp through a rack and then to the effects return jacks in stereo. There is also something you can do with the old loop that you cannot with the updated loop: Take a cable out of the effects send jack and go into the out jack (amp jack on ernie ball volume pedal) of a passive volume pedal (250K or higher) and the pedal will be a master volume without having to go through it. It acts as a variable shunt to ground. If you are using rack gear in the loop, just Y cord out of the send jack with one end going to your rack input and the other end going to the volume pedal. If you like to do delay swells with your rack gear, it is always better with volume pedal before delay. This also serves as a manual noise gate when your not playing, such as between songs or when switching guitars. What I like mostly about this is that you can leave your guitar volume full up and get maximum drive at any volume level from the pre section. This is a better use of a volume pedal than it is using it as a variable power pedal, since the tone of the amp changes too much when you bring the variable power down too far, and the reverb wash becomes too much."

Preamp Tube Heater Filament Voltage Mod:

If you have studied the preamp schematic a little, you would have noticed that the preamp tube heater filament voltage in the Convertible is 10.6 volts instead of the customary 12.6 volts used by most amplifiers. It was brought to my attention by an individual who emailed me recently that running the preamp tubes on a 10.6 volt supply severely shortens the life of the tubes. I suspect that Seymour Duncan designed it this way because solid state modules were also available, and perhaps they wanted to keep the noise floor level down as much as possible - and preamp tubes were cheaper and more readily available when these amps were new. I am currently working on a mod to increase the heater supply to 12.6 volts and I am also working with an individual now who will have the kits available in the near future.
Disclaimer: This is a mod that should increase the life of the preamp tubes, but it may also result in an amp that is noisier with certain module combinations. This is a mod that is easy to implement, and if it is not to your complete satisfaction, it can be easily reversed.
An individual in Germany recently wrote me and told me that his heater supply measured 12.6 volts. At this point, I am unable to determine if the schematic is wrong, or if there was a design change made by Seymour Duncan at some point in time to increase it from 10.6 volts to 12.6 volts. I have not actually measured it on my amp yet. Hopefully someone will help shed some light on the matter.

Preventative Maintenence:

Of course, the best medicine is prevention! That means replacing things before they blow up and your amp has a melt down. Things like filter caps need to be replaced after a certain amount of time. Since a lot of the Convertibles are nearing twenty years old, that is one item that should be considered.

This amp has a lot of tubes, and they generate a lot of heat. A friend of mine told me how some of his modules got a little melted in his amp from the heat inside. That got me thinking... I seem to be the fan-man, so one thing I did to my amp to help prevent problems was to add an internal chassis cooling fan. I simply installed a little 2" 24VDC brushless fan at the back of the chassis where the vents are. I attached it to the chassis with two long mounting screws. It fits there perfectly. It doesn't take up the whole area of the vents, so it allows the fan to draw some air inside the chassis so the air inside the chassis can mix with a little cooler air from outside the amp creating a convection current of sorts which would theoretically promote better cooling. To power the fan, I simply found a small sized 120VAC to 12VDC power transformer that would fit inside the chassis and wired that to the amp's power switch so that the fan turns on when you turn the power on. To keep the transformer from going anywhere, I attached it to the chassis with some long zip ties, using some holes that were already in the side of the chassis. This really helps the chassis to be ventilated a lot more. I don't feel that the passive vents provided on the bottom and the back of the chassis were adequate and I liked the positive ventilation approach idea better. In the long run, this could help all the components inside the chassis to last longer since they aren't being subjected to as much heat. The fan runs at half the speed it would at 24VDC, and it is very quiet. You can feel heat pouring out the back that would otherwise just be stuck inside the chassis, so it makes me feel better about the whole situation.

NOTE: I have removed the internally mounted fan since I wrote this page because I think the internally mounted fan actually increases the temperature inside the chassis, so I removed it. So maybe the passive ventilation isn't all that bad - as long as the factory mounted fan is in operation. CAVEAT: If you do not have the factory mounted fan in operation, it will drastically increase the temperature inside the chassis. This is the main reason why my friend's modules melted - he disconnected the power from the fan for a couple of hours to make it quieter for a recording session. Little did he know it would melt the plastic enclosures of his preamp modules!

Some Convertible owners have told me that they removed the black fiberboard liner at the module access opening to allow air to move through chassis better, too. Probably not a bad idea...

Eliminating Channel Crosstalk:

Replace both of the 47K mixer resistors with 220K to eliminate channel "cross-talk". This mod works great and can be done from the top if you snip out the 47Ks and use remaining leads to tie to (heavy traces on back stabilize the leads). The 100K pots (master volume) also do allow some bleed across when turned all the way off. You may notice this when using different input load resistor plugs. (Thanks to Don Molenda for this info!)

Repairing / Replacing The Can Type Filter Caps:

Click Here to see what this entails.

Occasionally, I get emails about acquiring replacement parts. Here is a list of currently stocked Convertible 100 amplifier parts. Once these parts run out, they will not be replaced. Contact Scott Miller at Seymour Duncan (Scott@seymourduncan.com) for more information.

The Electric Guitar Amplifier Handbook

Tube Amp Debugging Page

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