A versatile tube amp, such as Bluetone’s Crossroad, is quite an involved design containing many different components. The exact component layout requires a good deal of planning.
As you can see in the picture above, Matti Vauhkonen has sketched the outlines of all the necessary parts going into the Crossroad onto the chassis’ protective foil. Next he has drilled the holes needed to mount the valve sockets, the circuit boards, the transformers, and the leads going through the chassis.
Designing a tube amp is all about balancing the different requirements of the different parts working inside the amp – you can’t simply throw the components together haphazardly.
The large power transformer mustn’t radiate electromagnetic hum into the audio signal, which is why it is always placed as far as possible from the preamp valves. The power amp tubes, for their part, are best kept some distance from the audio transformer, to avoid hum and buzz induction. The audio transformer also has to be angled correctly in relation to the power transformer to keep things as quiet as possible.
One special feature of Bluetone-amps is their use of (doughnut-shaped) toroidal transformers as power transformers, which is still quite rare in guitar amplifiers.
This is despite the advantages a toroidal transformer offers in comparison to a traditional laminated core transformer. A toroidal transformer weighs less than a similarly spec’ed traditional design, and it also radiates less electromagnetic hum, which makes it easier to find a good spot for the power transformer in the amp.
Bluetone’s Harry Kneckt puts it like this: ”If the toroidal transformer had been available to Leo Fender back in the 1950s, he would have used it in all his amps. I’m pretty sure of this!”
Matti Vauhkonen has first installed the transformers, tube sockets (the white components), and the output jacks (the black parts on the right) into the amp chassis.
The eyelet-boards and PCBs were added for these pictures only to give an idea of their placement inside the Crossroad head. The audio signal in all Bluetone amps runs through a pure, point-to-point soldered analogue-and-valve path, but for modern control features, such as channel switching via MIDI or tube biasing via WiFi, the company naturally uses modern technology.
The next step is the installation of all the front panel parts, such as the control pots, switches, and the pilot light.
Bluetone makes some of their cabinets in-house, depending on their current workload. For the head featured here, Bluetone ordered the arctic birch ply amp cabinet from their subcontractor.
At this moment Matti Vauhkonen is dealing with the time-consuming business of soldering and assembling the heart of the amp (here’s a picture of another Bluetone-model). Hand-soldering a point-to-point valve amp is something you shouldn’t do in a rush, if you want to avoid very time-consuming troubleshooting.