Here's Why Session Amps Are Different

The Power Amp
Left: The power amp stage of a popular British 100W combo. It’s a ‘chip’ output stage intended for domestic applications. Even though the tiny chip is mounted directly onto a huge fan assisted heatsink, the chip is said to overheat after about half an hour of playing at high volumes, which results in a severe drop in loudness or failure.
The chip is directly soldered to a PCB (the green board), which has no visible means of support. Therefore, the leads from the chip have to take the weight of the PCB, the weight of the cables attached to it and the vibration stresses caused by high playing volumes. Those leads are likely to break off and cause a failure... not uncommon with chip power amps.

Left: Session amps employ conventional ‘discreet’ power amp designs employing heavy duty 30 amp power transistors which will give years of trouble free service. The whole PCB is securely mounted by an ‘L’ shaped ‘heat transfer’ bracket onto the robust 2mm thick aluminium chassis - which is seven times better than steel at reducing heat and is more expensive. This bracket ensures that the heat from the transistors is quickly taken away and dissipated into the chassis. Keeping power transistors cooler will extend their life dramatically, as heat is the major cause of power amp breakdowns. Cheap designs allow them to run hot to save on manufacturing costs and improve profits.

Construction & Materials
Left: Session amp chassis’ are built using a ‘modular’ format. That means that there are several PCBs used to enable easy repairs. The ST100R above uses three circuit boards, each of which can be removed and fault found with the amp switched on. This greatly reduces amplifier servicing costs in the future.
Usually guitar amps are built on one big circuit board. This makes it virtually impossible to work on the removed pcb whilst the amp is switched on, and very difficult to remove the circuit board to get at any components that are faulty. It can turn a quick input socket change into a 1.5 hour job.
All inter-PCB wiring is hand soldered for reliability. We do not use connectors anywhere inside our amplifiers. We have found that the contacts of connectors become corroded or tarnished and affect the amps long-term performance. Our amps are used in places where condensation and nicotine are prevalent in the atmosphere, so these components are deemed in-appropriate for mobile applications.

Left: You can see how the preamp circuit board has been removed from the chassis and is still wired up so that the amplifier can be switched on by the service technician, allowing fault diagnosis whilst running. This
saves on time. Components can also be replaced in a jiffy!
Sadly, this style of building amplifiers is bound to make an increase in production costs, however, we’re sure you’d rather pay a little extra at the outset knowing that repair costs could be a lot lower when the amp gets rather elderly.

Left: Every amplifier must have a transformer.
The type we use are known as ‘toroidal’. They are recognised by the circular doughnut shape. Unlike conventional ‘laminated’ transformers, these radiate hum in all directions around it’s 360° arc. This causes most of the electromagnetic 50Hz hum radiations to self cancel - reducing the hum picked up by your single coil pickup equipped guitar by as much a 70%. No amount of cavity screening in your guitar can stop electromagnetic radiation from entering the guitar’s signals... so, toroidal transformers dramatically reduce the cause in the first place.
These transformers are naturally more expensive - but worth every penny.