Amp boards and high voltage#

Guitar and bass amp boards: complete templates, tubes on sockets that show the tube in 3D, design rules that follow the voltage on every net, checks for the mistakes that hurt in an amp, a tone-stack designer and the calculators. The Amp Designer uses all of this to build an amp from a description; this guide is about the tools underneath, for when you design or edit an amp board yourself.

Amp templates#

Amp › New amp from a template… starts from a complete, placed board that autoroutes with a clean DRC. Amp › Open example amp opens the same boards already routed:

On every board:

• the tubes stand in a row along the rear edge;
• the pots and jacks sit on the front edge, with their shafts through the chassis panel;
• each stage is packed between its tube and its controls;
• the power supply is at the far end from the input;
• transformer, speaker and heater leads land on labelled wire pads (HV1/CT/HV2, AC1/CT/AC2, BIAS/0V, P1/B+/P2, SPK/COM, CHK1/CHK2);
• a ground pour on the bottom layer keeps clear of high-voltage copper.

Tubes on sockets#

Value a tube socket with the tube type, "12AX7", "EL84" or "6L6GC", and:

• the 3D view draws that tube standing in the socket: see-through glass over the plates, the mica spacers, the getter flash, and glowing heaters;
• the BOM lists the tube;
• the amp checks know its pinout, heater current and ratings;
• the simulator plays it (see Running the board).

24 tubes are included:

Amp › Tube pinouts and ratings… lists each one: the equivalents, the base (noval, octal, B7G), the pinout, the heater voltage and current, the maximum plate voltage and dissipation, and the amplification factor.

Ratings are design-centre maximums from the manufacturers' data. Guitar amps often run some tubes hotter, so the checks warn rather than fail.

Sockets#

Pin numbers run counter-clockwise from the gap (noval, B7G) or the key (octal), seen from the component side. The Belton patterns come from Belton's catalogue sheets (BVT9-1, BVT8-1): check them against your sockets.

High-voltage design rules#

Amp › Net voltages and currents… gives each net its DC volts, its AC volts (rms), its current and a mains flag. Boards from the templates and the Amp Designer come with these filled in.

From these:

• Spacing follows IPC-2221B table 6-1, by the peak voltage between two pieces of copper. Pads are component-lead terminations, so they use the lead column (A6); tracks and pours use the conductor column (B2), or B4 for an external conformal-coated board.
• Net classes carry the pad spacing, so a tube socket's own pin pitch is never "too tight". Amp › Apply HV net classes (or the checkbox in the dialog) creates them: HV100, HV300, HV500 and so on by voltage, Mains for mains nets, and Power for high-current nets.
• The autorouter and the pour fill keep live nets apart pair by pair (both ends of an HV winding, B+ next to a heater wire), not just from a single worst-case clearance, and route the highest-voltage nets first.
• Placement puts parts next to the pins they connect to: grid stoppers at the socket, decoupling caps at their stage.
• Track widths come from the IPC-2221 current formula for your copper weight and the temperature rise you allow (10 °C by default).
• Mains to everything else keeps 6.4 mm by default.

The IPC spacings are minimums for functional insulation. Mains wiring needs safety spacing to IEC 62368-1 or UL, which is why the mains clearance is conservative and why mains belongs on the chassis, not the board.

Amp checks#

Whenever a board has amp data, the design rule check (F8, or Amp › Run DRC with HV checks) adds:

• HV spacing from the actual voltage between two pieces of copper (lead or conductor column as above), plus the mains clearance from everything else.
• Track width against the net's current.
• Capacitor ratings: the voltage written in the value ("22uF 450V") against the net's voltage, and reversed electrolytics.
• Resistor ratings: power (V²/R) against the footprint's wattage, and the working voltage.
• Bleeders: every big high-voltage capacitor needs a resistive path to ground, so it discharges at switch-off.
• Tube wiring: unused or internally connected pins used as tie points, 6.3 V against 12.6 V heater wiring, shorted heaters, plate voltage above the tube's rating, and a tube on the wrong kind of socket.

Amp › Heater & HV report summarises the heater load per winding, the hot nets with their voltages and classes, and the amp checks.

The tone stack designer#

Amp › Tone stack designer… solves the classic treble-middle-bass stack as a complex network, with real pot tapers, as you move the knobs.

Presets: Fender Bassman 5F6-A, Fender Blackface (AB763 Twin), Fender Deluxe Reverb (6.8k fixed mid), Marshall JTM45 / 1959, Marshall JCM800 2203, Vox-voiced, a deep bass stack, and two high-gain stacks (a modern 47k slope, and a scooped US stack with a 47n mid cap), or your own values: the treble cap, slope resistor, bass and mid caps, the three pots, the source impedance (about 1k from a cathode follower, about 40k from a 12AX7 plate) and the load.

The plot shows your setting, and all controls at 5 and at 10, with the loss at 1 kHz, the deepest point and the range. Insert tone stack parts puts the stack on the board between the nets you name.

Calculators#

Amp › Track width & HV spacing calculator…:

• Track width for a current, at your temperature rise and copper weight, on an outer or inner layer (IPC-2221).
• Conductor spacing for a voltage, in each IPC-2221B category. For AC use the peak (rms × 1.41); between the two ends of an HV winding use twice the peak.

Amp parts#

All checked against KiCad's datasheet-based footprints where KiCad has them:

Playing the amp#

Amp boards can be simulated and played as a whole amp: the off-board transformer, speaker and supplies wired to the pads are modelled too. See Running the board and Playing guitar through it.

Safety#

Tube amps run at lethal voltages, and the filter capacitors keep them after the amp is switched off.

• The HV checks follow IPC-2221B and common practice, but they don't replace a safety standard (IEC 62368-1 / UL) for mains wiring.
• Keep the mains fuse, the switch and the transformer primary on the chassis, not on the board.
• Check that the bleeders discharge the caps, and measure before touching anything.
• Bring a new amp up on a dim-bulb tester, with the first power-up checklist from the spec sheet.
• If you haven't worked on high-voltage equipment before, learn from someone who has before you build one.

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