The Seymour Duncan Convertible 100 Tube Amp

Latest Update 09/18/09
Previous Update 01/22/03
By Paul J. Marossy
Special Thanks to Kevin Beller, Chief Engineer at Seymour Duncan for his contributions to this website. You rock Kevin!


There is a real lack of any information on the web for this amp, so I decided to make this web page. I just bought it on 3/15/02. It was a lot of work to clean it up and get rid of all the dust inside the amp. One of the first things I did is write to Seymour Duncan and requested that they send me some schematics and an owner's manual, and I received photocopies in the mail about a week later. But they did not send me schematics for the modules... Below is some pictures of my amp with attached verbage. I am told by Seymour Duncan that this amp was manufactured in 1986.

This amp is an out of production 100 watt combo amp with 2 channels, reverb and one 12" speaker. It has interchangeable preamp modules, making it a forerunner to the "modeling" amps of today, only this baby is all tubes. I like the way this amp sounds. I would say it sounds most like an older Boogie. It has a tube rectifier instead of a full wave solid state rectifier like my '74 Fender Twin Reverb has. It responds a little differently. This amp has a great tone and has a very nice overdrive sound. All of the controls are sensitive, so a little adjustment can have a very noticeable effect. The reverb sounds pretty nice, too.

After reading in the owner's manual about the input load resistor plugs, (which I didn't have), I decided to manufacture a couple. One of them is a 1M, and the other one is a 10K w/ 220pF capacitor in parallel. I like the second one better because it mellows out the tone and makes the highs a little less harsh. The 1M plug is still too bright for my taste. Another cool feature of this amp.
Click here for more info on Load Resistor Plugs.

According to an article I found, "the Seymour Duncan Convertible 100 amp has a variable wattage output, on amp control pot, or can be controlled with a volume pedal (jack for remote). Selectable: 5 =96 100 watts in pentode or 3 =96 60 watts in triode. Other unusal features, too. Actually not variable power, just a different sort of master volume control. A triode is connected across the output of the phase splitter and varying the power control varies the DC bias on this triode turning it on more or less; thus acting as a variable shunt. At the lowest power settings, the triode is conducting the hardest and therefore provides a fairly low impedance directly across the output of the phase splitter, allowing very little signal to be developed across the grid resistors of the output tubes." The power amp section of the Seymour Duncan Convertible 100 was originally designed by Bruce Kennedy and Kevin Beller. Hi-fi tube guy Roger A. Modjeski, who used to make Music Reference tube hifi amps and sold RAM Labs tubes was hired to review the design and add his input. "The preamp section was done in-house at Seymour Duncan."

From what I have been able to find, there are many who say these amps have serious reliability issues. The biggest complaint seems to be the preamp modules, sound cutting out intermittently, crackling or popping and hum. These amps do need a little more T.L.C. than the average amp. My personal experience has been that this is a reliable amp, as long as you take care of it. Of course you will have problems with crackling and popping if you have 2mm of dust all over the insides... The hum problem, I have found, is mostly caused by cheap power tubes or the wrong preamp tubes in the variable power circuit. You should use (2) 12AU7's, not 12AX7's. These tubes have quite different gain factors, and I have found this to be the source of any objectionable hum. The biggest problem is finding spare preamp modules, although you can build your own. It's too bad Seymour Duncan & Co. couldn't overcome the bad reputation they got as an amp builder, because this is a good amp.

Jeff Beck at one time apparently endorsed the Convertible. Check out
this ad from the January 1987 issue of Guitar Player magazine.

Chassis removed from the speaker enclosure. Chassis is constructed out of what appears to be about 18 or 20 gage steel.

Serial # 0854 8434 2

There is a variable damping output, effects loop and two 120VAC accessory outlets. Massive power transformer. 480 watts total power rating. 700 watts for the accessory outlets. 5 amp fuse.

Top view of chassis showing PCB (Printed Circuit Board). PCB is 1/16" thick military grade. There are five sockets for preamp modules.
Here are the sockets that the preamp modules plug into. They are a lot sturdier than you might think. Filter caps are in the lower right corner.
Here is the front of the chassis showing all of the control knobs. From left to right: Overdrive, Master Volume, Treble, Mid, Bass, Reverb. The one furthest to the right is the wattage selector. It ranges from 5w to 100w. At far left is a phono jack above the input jack for a plug-in input load resistor and the 1/4" jack below the input jack is an auxiliary instrument input. The chassis itself weighs 25 lbs.
This is one of the preamp modules. Several different ones were available. You could have a very versatile amp by changing out these modules. They are no longer made or offered by Seymour Duncan. Modules appear for sale on ebay on a fairly regular basis.
Here is where the modules go when the amp is fully assembled. Each module gets a preamp tube. (Except for SS modules)
Here is the cover that goes over the module access opening.
Here is the Celestion Sidewinder speaker. It is rated for 150watts @ 8 ohms. It has a diecast housing. Sounds pretty good to my ear, anyhow.
Rear view. This amp has four EL34 power tubes, a total of 8 preamp tubes and a 5U4G rectifier tube. Note the large can type capacitors. There is also a triode/pentode switch. Another cool feature. You could also use 6L6GC power tubes, but you would need to change the bias supply resistor to use these tubes. They sound harsher than the EL34's.
This amp has a factory installed fan in it! Now I like that... 120VAC, 0.11 amps. Manufactured by EG&G Rotron.
Speaker enclosure is all 5/8" plywood, covered in black Tolex. Built very sturdily.
Here is the front view. This amp is very heavy for a 1x12 combo. It must around 70 lbs! (Although Seymour Duncan claims it is 57lbs.) Built like a tank... This one is definitely getting casters! Overall, I am impressed with it's features, design and construction. I know an individual that uses one of these amps, and I've always loved the tones he got with it. Now I got me one, too.

Convertible 100 Owner's Manual


Channel Switching Info

Module Information

Troubleshooting & Modding Information

Main PCB Component Layout

Questions/Comments? Email Me...

Additional Information

Convertible 60 watt Page

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If you find this website helpful, please consider a small donation to help keep it going.
It took countless hours and over six months to create and gather the information on this page.
I also purchased several modules to be able to make the information provided as complete as possible as well as some other investments.