Technical Information

A Little Background

There are many things that affect the tone of a guitar amplifier, an infinite number really.  It's a difficult and time consuming process to arrive at a combination of elements that embodies a range of tones that the amp designer and musician is seeking.  There are a few common starting points, British Marshall or Vox style, and American Fender "Black Face Twin" style.  Most if not all tube amplifiers on the market today can be traced back to one of these three architectures.    Pages can and have been written about the technical differences and tonal qualities of each style.

Most British amps use EL34 or EL84 power tubes, commonly available in Europe.  The "Twin" style amps use 6L6 tubes commonly availabe in the USA.  There are distinct tonal differences created by the tubes, especially when overdriven.  The British tubes tend to have a "softer" breakup and a "darker" sound.  The American tubes tend to have an "aggressive" breakup and a "brighter" sound.  The differences can be subtle, but most tone-seekers can quickly identify the sounds in a recording.  In addition to the power amp  differences, the two have different tone stack arrangements.  The term "tone stack" refers to the components that make up the treble, middle and bass tone controls.  The schematic diagram of these components looks like a "stack" when drawn on a piece of paper.  The differences in the tone stack has a pronounced effect on the sound, mostly in the mid-range response.  A quick google search will turn up many great places to read more about the history, architecture, and sound of these wonderful creations.

One particularly unique variation of the Fender architecture was implemented by a few innovative amplifier builders in the 70's.  The idea was to take a Twin style preamp and power amp, and put an additional gain stage (preamp tube) in between.  This new preamp stage was used to generate the overdrive tones.  At that time, the most satisfying overdrive sound was created by poweramp distortion.  It sounded wonderful with lots of singing sustain and bite, but for most playing situations, was far too loud.  Fuzz and Distortion foot pedals were available for lower volume, but never really captured the same tone as an overdriven power amp.  They always seemed to have too much "fuzz" and compression, taking away from the guitar's true tone and dynamics.

The addition of the overdrive stage AFTER the "clean" stage inside the amplifier itself is fundamentally different than putting an overdrive pedal in FRONT of the first gain stage.  The overdrive created by this arrangement has a very unique sound, much more life like and open than anything else existing at that time.  Many great players including Larry Carlton, Robben Ford, and the great slide guitar player David Lindley have all used this style amplifier for some of their most famous recordings and tours.  While this idea is has been around for a long time, it has not been widely used in guitar amplifier design.  It's one of the features that give the Double Trouble amplifier it's distinctive sound.

What's Inside a SebagoSound Amplifier

Once the basic configuration of an amplifier has been chosen, then the real work begins.  As we saw earlier, there are a large number of variables that can be used to adjust the tone.  Things like tube choice, transformer choice (most notably the output transformer), tube plate voltages, tube bias points, any of the large number of filter and coupling points, component types and even different manufactures of the same component type will all have an effect on the amplifier tone.  The only way to arrive at a truely unique and satisfying combination of all of the above is to try many different variations, and have as many players as possible evaluate and give their feedback.  This is exactly what we have done at SebagoSound.  Thousands of hours of trial on hundreds of variations of the basic architecture have gone into the component selection for our amps.  Players at every level from local bar-bands to world renowned professional players and many in between have all been involved in the creation of the Double Trouble tone.

All of this testing has allowed us to determine which variables have the biggest effect on the tone, and to create a basic model that contains the best of each.  The standard configuration uses 6L6 power tubes, the standard "Twin" transformers, plate loading and bias points, and the custom SebagoSound tone stack.  Each basic model can be ordered with a 50W or 100W power amp.  The 100W version has a switch to select between 50W (two tubes) or 100W (four tubes).

We offer some selected variations of the basic model to each customer.  You can choose EL34 power tubes, the "jazz" preamp biasing which offers a slightly warmer overdrive at lower volumes.  In our experience with the many players we have worked with, these two options have the most pronounced effect on the tone and are the ones that had the biggest influence on the player's opinion of the amp.

Putting It All Together

Once all of the components have been chosen, they must be assembled into a complete amp.  The assembly techniques have an impact on the tone and the reliability of an amp.  There is usually a tradeoff to be made between lowering the manufacturing cost, and increasing the reliability and improving the tone.  Human labor cost is typically the most expensive manufacturing component; lowering cost means lowering human involvement.  To do this, components are surface mounted to a small number of printed circuit boards and assembled entirely by machine.  This includes the tube sockets, potentiometers (controls), lights and switches.  This eliminates lots of wires (the most labor intensive part of amp building) and makes the amp cheap to assemble.  That may be great for a piece of computer or stereo equipment, but it's not such a great way to build a guitar amp.  It is possible to build a great sounding PCB based amp, but not one that can be manfucactured at the scale of most high production amplifiers in factories staffed by non highly skilled workers.

This is why a "hand wired" amp is such a special piece of equipment.  In a Sebago Sound hand wired amp, all the switches, tube sockets, lights and potentiometers are directly mounted to the metal chassis.  These are then connected to the board containing all the tone shaping components with real wires!  This has two benefits, tone and reliability.  Reliability because mechanical and thermal stress are all absorbed by the wires, nowhere in the system is there a solder joint with any stress applied directly to it.  The wires are free to move and absorb it all.  Tone, because all the wiring, slightly different for every amp since its done by hand, gives it life.  We pay special attention to the wire type, routing and termination to ensure that every Sebago Sound amp is a highly reliable tone machine!

Yes, there is a circuit board inside of the Double Trouble amp.  The board is 1/8" thick fiberglass, the same thickness as a hand-drilled eyelet board.  The components are all inserted into plated through 1/8" holes that are equivalent to punched eyelets.  The leads from the components form the interconnect just as they would on an eyelet board.  The relative placement of the components is the same as in the Dumble amplifiers we modeled the Double Trouble amp after.  There are a few traces on the board, all sized to have the same electrical characteristics (stray inductance and capacitance) as the small number of wires they replace.

We do this for two major reasons, your safety and for reliability of the amplifiers.  Most of the traces on the board are carying the high-voltage from the power supply to the tube plate resistors.  If a wire carrying high voltage comes loose and contacts the chassis, the chassis can be chared to over 400V.  This is normally not a problem if the amp is properly connected through a 3-prong plug and the outlet is properly grounded.  The fuse will blow and protect you from electric shock.  However, if the wiring in the venue is faulty (easily the case if you play in a bar band), then your guitar strings can be become charged with a lethal voltage.  Some amps run the power on wires under the eyelet board, where they can not be inspected for wear or damage.  This is both a safety and reliability problem.  All wires carying high voltage in Sebago amps are highly visable and can be inspected and replaced if there is any sign of wear or damage.