⚡ Tube Amp CAD — Manual

Everything the application does, how to use it, what every value means, and how the simulator gets its numbers. Hover any label in the application to see the same explanations as a hint.

1. What it is

Tube Amp CAD designs and simulates vacuum-tube amplifiers (and hybrid and solid-state ones) in the browser. You draw a schematic; a circuit simulator works out the operating point of every part and the waveforms with a signal applied; bench instruments show the result as a real oscilloscope, spectrum analyzer, Bode plotter and curve tracer would. Then the same file is laid out as a printed circuit board.

• Nothing to install, no account, no server. Everything runs on your computer. The circuit is autosaved in the browser and can be saved as a JSON file.
• Windows. The Circuit CAD (schematic editor) is the centre. From its Windows ▾ menu you open the Oscilloscope, Spectrum analyzer, Bode plotter, Curve tracer and Board layout, each in its own browser window, so they can be arranged across several monitors. The CAD broadcasts every simulation result; the other windows listen and update by themselves.
• Browsers. Chrome, Edge, Brave, Vivaldi, Opera and Safari are tested. Firefox is not tested.

2. Your first amplifier in ten minutes

A common-cathode 12AX7 stage, simulated and measured. Each step names exactly what to click.

1. Start a sheet. File ▾ → New…, choose A4 landscape, give it a title, Create. You get an ISO drawing frame with a title block.
2. Place the tube. Type 12AX7 in the palette's search box and press Enter, then click on the sheet. The tube appears as VL1.1, with its pin numbers.
3. Place the resistors. Type 100k, Enter, click above the anode: that is the plate resistor. Type 1.5k and place it below the cathode, and 1M from the grid to ground (the grid leak). R rotates a part while you place it.
4. Bypass the cathode. Type 47u to get an electrolytic and place it across the cathode resistor.
5. Power and ground. From Sources place a B+ supply (set it to 250 V in the inspector), and from Ground & connectors the Ground symbols.
6. Signal. Place a Signal generator (1 kHz, 0.1 Vpk) and a Capacitor (100n) from it to the grid.
7. Wire it. Click a pin, click the corners, and finish on another pin or on a wire. A wire that ends on a wire joins it with a dot. W starts a wire anywhere; Esc stops.
8. Read it. With Live on, the circuit simulates after every edit. Every net shows its DC voltage and every wire its current. Click the tube: the inspector shows Va, Vg1k, Ia, Pa, and with the signal on the plate swing, the gain and the THD.
9. Look at the waveform. From Instruments place an Oscilloscope; wire CH1 to the grid and CH2 to the plate. Its screen is live on the schematic. Double-click it for the full oscilloscope.
10. Frequency response. Windows ▾ → Bode plotter shows gain and phase from 1 Hz to 1 MHz with the −3 dB points.
11. Check it. Click an empty spot: the inspector shows the circuit's Checks: anything beyond its ratings appears there in red.
12. Save it. Ctrl+S.

3. The Circuit CAD window

Top to bottom: the toolbar; the palette (left), the sheet (centre) and the inspector (right); the panel under the sheet (when open), with two tabs: the bill of materials and the Checks; the status bar.

3.1 Toolbar

On narrow screens the toolbar drops the product name and then the file name, so it stays one row from 1000 px up. At 900 px and below the inspector becomes a drawer over the right of the sheet: it slides in when you select something and out when the selection is cleared, and the Inspector button in the toolbar shows or hides it at any time.

3.2 Palette

• Groups: Passive, Transformers, Sources (DC supplies and AC sources), Ground & connectors, Semiconductors, Instruments, Document, Tubes. Click a group's title to fold it; the browser remembers which are open.
• Search box. Finds parts by name; part numbers (2N3904, IRF540, 1N4742A, 125ESE, 373BX); tubes (EL84, 6N2P, 300B); and values: 47k offers a 47 kΩ resistor or pot, 100n a 100 nF capacitor, 47u an electrolytic, 5H a choke, 300V a supply. Enter picks the first match.
• Click a part, then click the sheet. Shift-click places several. Esc stops placing.
• The Tubes group shows the popular tubes first; the button under the list shows all 99.

3.3 Inspector

The right-hand panel shows whatever is selected.

• One part: its designator, every value (typed with SI prefixes: 4.7k, 220n, 1M), then its operating point and, with a signal, a With signal section; buttons to rotate, mirror and delete. Values beyond a rating are shown in amber (above 85 %) or red (above 100 %), with the share of the rating on a second line.
• Several parts: how many are selected.
• Nothing selected: the Circuit summary: parts, wires and nets; the power balance (power from the supplies, into the speakers, in the tubes, transistors and resistors); the Checks; and Renumber designations.

Hover any label for its explanation. Every field and readout is described in Parts reference and Readouts.

3.4 Status bar

Left: the simulation state: "Simulated in 300 ms · 2 ms window, 16 cycles to settle", a progress bar while running, warnings in amber and errors in red. Right: the pointer's coordinates, or what you are hovering (a wire's voltage, AC swing and current; a sheet connector's partners).

3.5 Resizing panels

Drag the edge of the palette, the inspector or the bill of materials to resize it; double-click the edge to reset it. The instrument windows' side panels work the same way.

4. Drawing schematics

4.1 Placing and editing parts

• Move: drag a part; its wires follow. Drag a wire segment sideways to move it.
• Select several: drag a box over empty space, or Shift+click. Ctrl+A selects everything.
• Rotate R, mirror M (left to right), duplicate D, delete Del. Rotate and mirror also work while placing. The same commands are in Edit ▾ and on the right mouse button over a part or wire (a right-click while placing or wiring still cancels it).
• Mirror works for every part that can rotate, so a complementary pair or a tube driven from the right is drawn without crossing wires. The oscilloscope mirrors too; its screen stays upright.
• Copy and paste: Ctrl+C, Ctrl+V.
• Undo Ctrl+Z, redo Ctrl+Y or Ctrl+Shift+Z.
• Pan: Space+drag or middle-drag. Zoom: the wheel, at the pointer.

4.2 Wiring

• Click a pin, click each corner, and finish on a pin or a wire. A wire ending on another wire joins it with a junction dot; wires that merely cross are not connected.
• W (or Wire) starts a wire anywhere, not only at a pin.
• Esc ends the wire you are drawing.

4.3 Sheets, frames and connectors

• File ▾ → New… starts a circuit on a sheet with an ISO 5457 frame, a reference grid (columns 1, 2, 3…, rows A, B, C…) and an ISO 7200 title block, from A4 to A1, landscape or portrait. Add sheet… adds further sheets.
• Frames are locked so they are not moved by accident; unlock one in its inspector. The title block's fields (title, number, revision, author, approver, dates, status, language) are edited in the frame's inspector.
• Sheet connectors with the same signal name are one net, on this sheet and on others. Each shows where its partners are by zone: "A3,5 B2,5,8", on several sheets "1/A3,5 2/C1,4". A name used five or more times (B+, a bias line) reads as a supply rail and shows no list. Hover a connector to see all its partners in the status bar.
• A circuit with several sheets opens on sheet 1. The sheet list, or F twice, shows them all.

4.4 Drawing standards

• Symbols follow IEC 60617; drawings follow IEC 61082-1: labels to the left of vertical symbols and above horizontal ones.
• Designations use the classic letter codes (as in ГОСТ 2.710): R resistor, C capacitor, L choke, T transformer, VL tube, VD diode, VT transistor, HL lamp/LED, SA switch, BA loudspeaker, G generator/source, P instrument. A dual tube's sections are VL1.1 and VL1.2.
• Renumber designations (inspector, nothing selected) numbers every part in reading order, sheet by sheet.
• Resistors carry their power rating inside the symbol per ГОСТ 2.728.
• Tube pin numbers are drawn beside each terminal.

5. Parts reference

Every part in the palette, what it models, and every field in its inspector. (This section is built from the application's own part library and hint texts, so it always matches the version you run.)

5.1 Transistors, MOSFETs and diodes

• NPN: 2N2222A, 2N3904, BC107, BC547B, BD139, MPSA42, MJE340. PNP: 2N2907A, 2N3906, BC177, BC557B, BD140, MPSA92, MJE350.
• Drivers and power output pairs: 2SD669A/2SB649A, 2SC4793/2SA1837, MJE15032/MJE15033, TIP41C/TIP42C, 2SC5200/2SA1943, MJL3281A/MJL1302A, MJL21194/MJL21193; in TO-3 MJ15003/MJ15004, MJ15024/MJ15025, 2N3055/MJ2955.
• Temperature (every bipolar transistor's inspector, °C, 25 by default): the junction temperature the part is simulated at. IS follows SPICE's law with the band gap (0.67 eV germanium, 1.11 eV silicon), so VBE falls about 2 mV/°C; hFE rises as (T/298 K)^1.5; germanium leakage doubles every 10 °C. Set it to see how a fixed-bias germanium stage drifts on a warm stage or next to a tube: an AC128 on fixed bias (1.5 MΩ from −9 V, 4.7 kΩ collector load) sits at VCE 5.2 V at 0 °C, 2.9 V at 25 °C, and is saturated by 45 °C. The SPICE export carries the part's temperature (temp=), the temperature terms of the model (EG, XTI, XTB) and the leakage as a B source.
• Germanium: USSR MP39B, MP40A, MP41A, MP42B, MP25B, MP26B (PNP), MP38A (NPN), GT402G/GT404G, P213B; Western AC127/AC128, AC187/AC188, OC71, NKT275, 2N404, AD149, AD161/AD162. USSR types are written in Latin letters (MP39B = МП39Б); the search finds them either way. The parts list has PNP, germanium (AC128), NPN, germanium (AC127) and PNP, germanium (USSR) (MP39B) under Semiconductors → Transistors; any NPN or PNP transistor's Type list has the rest under its Germanium heading. The models give germanium's low VBE (about 0.2 V at 1 mA), the datasheet's hFE, and its slow transit (fT 0.15–4 MHz) as a base-emitter capacitance, so the treble roll-off of the old parts shows. Leakage is modelled: each germanium part has a typical ICBO at 25 °C (AC128 4 µA, MP39B 5 µA, AD149 100 µA, P213B 300 µA) flowing from collector to base, which the transistor amplifies to about (hFE + 1)·ICBO; it doubles every 10 °C. The inspector shows it under Leakage, in amber when it makes up half of IC or more. Pinouts of germanium cans vary between makers and years: check the part you have.
• N-MOSFETs: 2N7000, BS170, IRF510, IRF540, IRF820, IRF840, IRFP240, IRFP250, IRFP460, laterals 2SK1058 and ECX10N20, depletion DN2540 and LND150. P-MOSFETs: BS250, IRF9540, IRF9610, IRF9640, IRFP9240, laterals 2SJ162 and ECX10P20.
• Diodes: 1N4007, UF4007, 1N4148. Zeners: 1N4733A … 1N5388B (5.1–200 V, 1 W and 5 W). LEDs: red, yellow, green, blue.

Their inspectors show Vce, Ic, hFE and dissipation (bipolar), Vds, Id and dissipation (MOSFET), with checks against Vceo, Ptot and the zener's power.

5.2 The tube library

99 tubes: the Soviet receiving tubes from Katsnelson & Larionov, «Отечественные приёмно-усилительные лампы и их зарубежные аналоги» (1981), and Western tubes from their makers' datasheets. Each has its Koren model fitted to datasheet operating points, its ratings, its socket and pinout, its heater, and its interelectrode capacitances. Search or sort the table:

Ratings shown blank are not given by the source and are not checked. Capacitances in pF: triode Cgk / Cga / Cak; pentode Cin / Cg1a / Cout. "≈" marks typical values for the kind of tube where no datasheet value exists.

5.3 Your own tube models (SPICE import)

Import SPICE model… in a tube's inspector adds any tube from a SPICE model: Norman Koren's, Ayumi's, Duncan's, Derk Reefman's or a maker's library. Paste the .SUBCKT … .ENDS text or open the file.

• It runs as written. Behavioural sources (B, E/G VALUE={…}, linear E/G) are evaluated directly, with .PARAM, .FUNC, PARAMS:, continuation lines, unit suffixes and the usual functions (exp, ln, pwr, pwrs, uramp, limit, if, atan, tanh…). Internal nodes such as Koren's grid-current resistor and diode are solved too. Capacitors in the model are left out; the tube's capacitances come from the tube it is "like".
• Pins are read from the model's comment (; P G K, A G2 G1 K…). Without one, the assignment that behaves like a tube is chosen. You can change it; the preview curves show at once whether it is right.
• "Like": the imported tube takes its socket, pinout and heater from a tube you choose, so the board footprint and the checks work. Set Pa max and Va max in the dialog.
• Where it is kept: in the browser and inside every circuit file that uses it, so the file opens on another computer. The curve tracer shows it, and the SPICE export writes the subcircuit as imported.

6. Simulation

6.1 What is simulated

Every run computes:

1. The DC operating point: every node voltage and every part's current at rest.
2. The steady state with the signal: the waveforms once every capacitor and choke has settled, over a window that is a whole number of signal periods (and of mains periods in a mains-fed circuit).

The oscilloscope can also run the power-on transient: the first seconds after switch-on, with every capacitor empty.

The Bode plotter runs a separate small-signal (AC) analysis in the background.

6.2 Live and on demand

• Live on: every edit triggers a run. A run that does not finish in a moment continues in the background; the inspector shows a spinning percentage until it is settled.
• Live off: nothing runs until ▶ Simulate or Ctrl+Enter.
• Either way, slow power supplies (large reservoir capacitors behind a choke) are run until fully settled. Press the button again to stop a long run.

6.3 Mains-powered circuits

Draw AC mains → power transformer → rectifier → reservoir capacitor → choke or resistor → filter capacitor, exactly as built. The simulator runs the rectifier cycle by cycle, so ripple, sag under load and the rectifier's peak currents are real. Voltages and currents on the schematic are averaged over the settled window, as a meter reads them.

6.4 When a run fails

• A missing ground: the checks report it; place a Ground symbol.
• Unconnected pins: the checks name them. Every node also needs a DC path to ground: a grid fed only through a capacitor needs its grid leak.
• A shorted winding is reported instead of stopping the solver.
• It does not settle: a circuit that oscillates (a feedback loop with too little phase margin) never reaches a steady state. Look at the Bode plot for a peak, and at the power-on transient.

6.5 Tolerance runs

Real parts are not their nominal values: resistors are ±5 %, electrolytics ±20 %, and two tubes of one type can differ by 20 % or more in plate current. View ▾ → Tolerance runs opens the Tolerance tab under the sheet, which simulates the circuit once as drawn and then once per run with every part varied, and shows how far each value moves.

• Runs: how many varied circuits (each settles fully, as ▶ Simulate does). 20 shows the spread; 100 or more shows the rare corner.
• R ±, C ±: the tolerance of every resistor and pot (both halves of a pot move together, so the wiper stays put) and of every capacitor and electrolytic.
• Tubes ±: every tube draws that much more or less current than its model at the same voltages, as a stronger or weaker tube does. Rectifiers stay as they are.
• Each value is normally distributed about its nominal with the tolerance as two standard deviations (about 95 % of parts are within it), and never beyond it. Seed: the same seed repeats the same runs.

The table lists each tube's Ia, Va and Pa, each transistor's current and dissipation, each speaker's output and each scope channel's AC (Vrms): the circuit as drawn (Nominal), the Min, Mean and Max over the runs, the Spread (two standard deviations against the nominal) and in how many runs the part was over its rating (Pa and Va for a tube, Ic and Ptot for a transistor). Runs that do not settle are left out and counted. Press ▶ Run again to stop; an edit marks the table as the old circuit's.

7. Measuring on the schematic

• Voltages: each net carries a sticker with its DC voltage in yellow. A net with AC on it adds the AC in cyan as ~20.1Vrms (the rms of the wave about its mean, over the settled window); a net with no DC, such as the input or a signal after a coupling capacitor, shows the AC alone. The wires of the nets that carry the signal glow cyan, so the signal path stands out from the DC supply and bias nets. A rail with only some ripple on it (AC under 1 % of its DC) shows the ripple in the sticker but does not glow. Hovering a wire reads the same in the status bar: DC, AC in Vrms and Vpp.
• ∿ Animate (toolbar, next to Simulate): plays one cycle of the signal slowed to 3 seconds. Dots run along each wire with its AC current, reversing every half cycle; brighter dots carry more current. Each node's wires turn orange as it swings above its DC level and blue below, so the inversion through a stage, the signal returning through the supply and a bypass capacitor's work are all visible. Each wire and node is scaled to its own peak, so a grid's microamps move as clearly as a plate's milliamps. Click again to stop.
• Legend: a key in the bottom-left corner of the sheet explains the marks that are showing (the signal glow, the DC and AC stickers, the currents). View ▾ → Legend hides it; the browser remembers.
• Currents: each run of wire carries its DC current and direction ("↓1.38mA"). Currents come from every part's terminal currents and are passed along the wiring, so a junction shows how the current splits.
• Stickers go where they cover no text, symbol or other sticker; if there is no room, a sticker is left out. Toggle them in View ▾.
• Hover a wire: its voltage, AC swing and current appear in the status bar.
• Oscilloscope parts show their two channels live on the schematic.

8. Readouts and what they mean

Every value the inspector, curve tracer and instruments show, with its meaning. The same text appears as a hint when you hover the label.

9. Checks and ratings

With nothing selected, the inspector's Checks list every problem in the circuit. Each tube's inspector shows its values as a share of their ratings: amber above 85 %, red above 100 %.

9.1 Circuit checks

• Unconnected pins, shorted parts, a missing ground, sheet connectors without a partner.
• Resistor dissipation against its power rating (rate a resistor at about twice what it dissipates).
• Output-transformer primary DC current against its rating (single-ended cores saturate above it).
• Transistor Vce and dissipation; zener power.

9.2 Tube ratings

Ratings come from Katsnelson & Larionov for the Soviet tubes and from the makers' datasheets (RCA, Philips, Mullard, Western Electric) for the others. A rating a source does not give is not checked.

10. Instruments

All instruments read an Oscilloscope part in the circuit: place one and wire its CH1 and CH2 where you want to measure and its COM to the reference (usually ground). With several scope parts, choose which one in the window's Scope part list. The scope part's inspector has Oscilloscope ↗, Spectrum ↗, Bode ↗ buttons that open the window on that part.

10.1 Oscilloscope

• Two channels, DC or AC coupled; 10× (10 MΩ) or 1× (1 MΩ) probes that load the circuit like real ones.
• Auto volts/div with offset; the timebase locks to the measured frequency; edge trigger on either channel, rising or falling; X-Y mode.
• Scale labels on every axis: time below, CH1's volts left, CH2's right, on round values that follow zoom and position.
• Readouts: Vpp, Vrms, dBV and DC per channel, and the CH2/CH1 gain and phase.
• Channels with no probe wired are greyed out.
• Analysis: Steady state (the settled circuit, one window repeated as a triggered scope shows it) or Power-on (the first 0.1–2 s after switch-on, with an overview strip of the whole record).
• Autoset resets the view.

10.2 Spectrum analyzer

• The FFT covers an exact whole number of signal cycles, so there is no windowing error and no leakage.
• Harmonic table H2…H10 with frequency, level and percentage, and the THD.
• Other components: anything that is not a harmonic: mains hum and its harmonics, intermodulation products.
• Levels in dBc (relative to the fundamental: harmonics read directly as distortion) or dBV (absolute).
• Linear or logarithmic frequency axis: on the log axis 50/100 Hz ripple and the audio harmonics read in one view.

10.3 Bode plotter

• Gain in dB and phase from 1 Hz to 1 MHz at the scope part's channels.
• Traces: CH1, CH2, or CH2 ÷ CH1, the gain between the two probes (one stage of a chain).
• Gain: re the generator (absolute), or re 1 kHz (normalised, to read the shape).
• Response table: gain and phase at 1 kHz; the −3 dB points either side; the level at 20 Hz and 20 kHz; any peak above the 1 kHz gain (an output transformer's leakage resonance, a feedback loop short of stability).
• Generator: which signal generator drives the sweep; the others are switched off.
• It is the small-signal response: every part linearised at its idle operating point (a mains-fed supply at its averaged steady state). Large-signal effects (clipping, transformer saturation) show on the scope and spectrum instead. It reruns after every simulation while the window is open.
• Tubes include their interelectrode capacitances, so the Miller effect sets each stage's top corner:

f(−3 dB, top) ≈ 1 / (2π · Rsource · (Cgk + (1 + A) · Cga))

10.4 Curve tracer

• Select a tube in the schematic (or pick one in the library) to see its plate curves with: the operating point; the curve at the circuit's bias; the simulated load line (the last whole signal cycle, an ellipse for a reactive load, with arrows for direction); the Pa max hyperbola and the over-Pa area; 70 % of Pa max.
• For pentodes the curves are drawn at the circuit's screen voltage (or one you choose).
• Right panel: ratings, the Koren model coefficients, the operating point, and with a signal the plate swing, gain, THD and average Pa.
• Left panel: the tubes in the circuit, the whole library (search; filter triodes / pentodes / popular), and the socket pinout drawing with heater notes.
• Tube model & fitter: the LTspice .SUBCKT of the tube (copy or download). To model your own tube, upload measured points as CSV rows Va, Vg, Ia_mA[, Vg2]: a Nelder–Mead fit of the Koren parameters is drawn dashed with its R² and RMS error.
• PNG saves the chart.

10.5 Markers and zoom (every graph)

• Click to place marker A, click again for B; drag to move, double-click to remove. Scope: time and voltage with ΔT and ΔV. Analyzer and Bode: frequency and level. Curve tracer: Vg, dissipation, gm, rp and µ at a point, and the load line through A and B.
• Wheel zooms at the pointer (time/div on the scope, the frequency axis on the analyzer and Bode plot, both axes on the curves); Ctrl+wheel zooms vertically; right-drag or Shift+drag moves the view. The Zoom button resets it.

11. Board layout

Windows ▾ → Board layout opens the PCB of the circuit. Parts and connections come from the schematic; the layout is saved inside the circuit file.

The toolbar is one row: undo and redo; the tools (Select, Route, Via, Text, Hole, Pour, Keep-out, Measure); the active copper layer and the grid; View ▾ (grid lines, dots or none, axes, the drawing sheet's format, Lock sheet, the 3D view); Fit; then Update (amber when the schematic has changed), Board ▾ (Board setup…, Design rules…), Export ▾, help and the way back to the schematic.

11.1 Footprints

Every part gets a through-hole footprint chosen from its values: resistors by power (10.16 mm at ¼ W to 50.8 mm at 20 W); film, ceramic and mica capacitors by value; radial, snap-in and axial electrolytics by size; DO-35 to DO-201 diodes; 3 and 5 mm LEDs; TO-92 to TO-264 transistors with each part's own pin order (TO-3 gets wire pads for a chassis heatsink); 9/16/24 mm pots and a 3296W trimmer; real tube sockets (B9A, B7G, Magnoval, octal, UX4) drawn from the component side, dual tubes sharing one socket; EI30–EI96 board transformers; toggle switches, screw terminals, RCA and jack sockets. Chassis parts can get wire pads or a terminal instead. Change any footprint in the inspector.

11.2 Placing and routing

• New parts wait in a row below the board: drag them on, or Place all on the board, which puts each part where it sits in the schematic (the schematic's extent mapped onto the board), big parts first, each in the free spot nearest its place, electrolytics 10 mm from hot parts: the input ends up on the left and the output on the right, as on the sheet. Output and power transformers and filter chokes get wire pads by default (they sit on the chassis); a board-mount EI footprint is an option in the inspector. R rotates, M moves to the bottom side. A banner offers updates when the schematic changes; placed parts keep their spot.
• Route X: click a pad, click corners (45° bends), end on a pad, track or via of the same net. V changes layer with a via; / puts the diagonal first; Backspace takes back the last corner. Width comes from the net's class. A halo shows the clearance; it turns red and refuses the click when too close (Shift+click forces it).
• Edit tracks: drag a segment sideways (45° corners stay 45°). U selects the whole connected track.
• Several at once: box-select left-to-right (inside) or right-to-left (touching); Shift+click adds. Arrows nudge (Shift: ×10). Align and distribute in the inspector. L locks.
• Copper pours (P: click the corners, double-click to close; or from the board inspector + GND plane (bottom) and + GND pour round the parts; or, with several parts selected, + GND pour round these parts). A pour keeps every other net's clearance (by voltage too), joins its own net's pads by thermal reliefs and drops islands, so it never shorts anything. It is a shape you edit: select it, drag a corner to move it, drag the + in the middle of an edge to add a corner, double-click a corner (or drop it onto its neighbour) to remove it, drag its outline or inside it to move it all; the copper refills as you drag. Round the parts and Board rectangle in its inspector reshape it at once. The board outline cuts it, round corners and edge clearance included.
• Keep-out areas (K): forbid tracks, vias, pours and optionally parts on either layer (under a mains transformer, along heater wiring).
• Measure (D) snaps to pad centres, track corners, board corners and edges.
• Nets tab: every net's class, DC current, routing left and length; click one to highlight it.
• Cross-probing: selecting on the board selects in the schematic and back.
• Comfort: rest the pointer on a pad, part, track or via for a card with its net, class, voltage and current; while routing the card shows the net, width, layer and length, and W steps the width. Right-click opens a menu for what is under the pointer (turn, flip, lock, route from a pad, a track's width or layer, a pour's shape, a via or hole right there); right-drag still pans. + / − and the zoom bar zoom, Z zooms to the selection, Ctrl+F finds a part or net, \ hides the side panels. The checks and the layers fold away and stay folded; long check lists are grouped by kind. Values sit inside a part when they fit and never cover a reference.

11.3 Design rules and checks

Board ▾ → Design rules… sets clearances (with a separate high-voltage clearance above a threshold voltage), the edge clearance, track widths, via and drill sizes, annular ring, solder-mask opening, and the allowed warming of a track at its DC current. Net classes (Signal, Power, HV) are set automatically from the simulated DC voltage, or by hand; heater nets are Power.

Clearance by voltage. On top of the classes, two nets keep the gap IPC-2221B table 6-1 asks for the largest voltage between them. The voltage is the worst case of their simulated DC levels and their swing in the simulation; the anode end of an output transformer's primary counts at twice its DC level, which is where an inductive load takes it at full drive. Pick the table for the board: B2 bare board (0.6 mm to 150 V, 1.25 mm to 300 V, 2.5 mm to 500 V, then 0.005 mm per volt), B4 under a permanent coating, A6 conformally coated, or Off. A mounting hole counts as chassis, 0 V. Routing, pours and the check all use it; the net table's To GND column shows each net's clearance to 0 V copper.

Heaters and shields. The schematic draws a valve's anode, grids and cathode; the board also wires its heater. Every indirectly heated valve gets its heater pins on heater nets, one pair per heater voltage (HTR6.3_A, HTR6.3_B), fed from a heater-supply connector XH1 that carries the sum of the heater currents, so the current check sizes the heater tracks. 12AX7-type valves run at 6.3 V (pins 4 + 5 against 9) or, if chosen, 12.6 V (4 to 5); a heater raised to a DC potential (the tube's Heater DC potential) gets its own supply at that voltage; a heater joined to its cathode (GZ34) gets a winding of its own. Directly heated valves are left alone: their filament is the cathode. Internal shield pins (6N23P pin 9) go to GND. Both can be turned off.

Mounting holes (H, or Board setup): Ø 3.2 mm for M3 by default. In the inspector, Plating makes one plated: a copper ring on both sides (Ring Ø, 6 mm by default), bare of solder mask, drilled with the plated holes. Give a plated hole a Net (GND for a chassis screw that grounds the board) and a pour of that net joins it; the check reports one its net does not reach. A hole that is not plated keeps every net's chassis clearance.

The design-rule check runs live: clearances, shorts, edge, annular rings and drills, widths, track width for its simulated DC current per IPC-2221, overlapping parts, mounting holes under parts (the screw head, washer and standoff need Ø 7 mm free round an M3 hole), keep-outs, silkscreen over pads, dangling tracks and vias, unplaced parts, pinouts to verify. Click an entry to go to it. Errors are red; warnings amber; the status bar turns green only when nothing is left to route.

Net names say what a net is: a sheet connector's name; B+ (or B+1, B+2 … from the highest voltage down) for supply rails, +12V for low-voltage ones; a valve electrode as VL1.1-A, VL2-G1, VL1.2-K; OUT for the output transformer's secondary, IN for a generator's output; otherwise the parts it joins (R8_C3). Boards saved with the older Net-(R8-2) names move to the new ones by themselves.

Star grounding (Ground scheme: Star point). A star terminal XG1 (a plated M3 or M4 hole for a solder lug) joins every ground return, and nothing else may. Each part with a ground pad belongs to a return: Supply (sources, rectifiers, reservoir and filter capacitors, bleeders), Output (power valves, the output transformer's secondary, the speaker), Input (the input jack), or the small-signal stage it serves (VL1: its cathode resistor, bypass and grid leak). A part's inspector shows its return and can move it. The ratsnest joins each return's pads and runs the return to XG1 on its own; ground copper that joins two returns anywhere else is reported, since the reservoir's charging pulses would then flow through the signal ground. A ground pour joins everything, so it is for the Free scheme.

Teardrops (on by default): where a track meets a round pad or a via, copper widens from the track to most of the pad's width over about a pad's length, so the joint survives drilling slop, soldering heat and a heavy part's leverage. They go to the Gerbers, PDFs and SVG; one is left out where it would break a clearance, and pours keep clear of them. A track as wide as its pad needs none.

Valve layout checks (warnings, each with its distance in the rules):

• Grid stopper at the socket: a resistor that is the only part on a grid net, further than 10 mm from its grid pin. The track before the stopper is an antenna.
• Grid tracks near anodes and heaters: a grid track running alongside an anode track (oscillation) or a heater track (hum) for more than 10 mm, closer than 2 mm, on either side of the board.
• Electrolytics near hot parts: closer than 10 mm to a power valve, a rectifier or a resistor dissipating 1 W or more in the simulation.
• Mounting hole near each socket: a board-mounted valve socket with no mounting hole within 40 mm; pulling a valve flexes the board.

11.4 3D and exports

• 3D (3): the board with every part, valves in their glass, transformers; drag to turn; Iso/Top/Bottom/Front/Side views; Save PNG.
• STEP (AP214) 3D model for chassis design in FreeCAD, KiCad or any MCAD.
• Gerber + drill (ZIP) for JLCPCB, PCBWay, Aisler… with a README of the board's specification. RS-274X with X2 attributes (each aperture marked pad, via pad, conductor, washer pad or profile) and Excellon drill files with each tool marked component, via or mechanical drill. The silkscreen is cut to the board outline and kept 0.1 mm off every mask opening, so the board maker clips nothing.
• Print set (PDF): assembly drawing, copper at 1:1, drill map; a 50 mm bar checks the printer's scale.
• Toner transfer (PDF): mirrored copper for home etching.
• SVG at 1:1 and the board as JSON.

12. Files and exports

• Open / Save / Save as (JSON). In Chrome and Edge, Save writes back to the opened file after asking once; in Safari and others it downloads a file. The circuit is also autosaved in the browser.
• A circuit file holds the schematic, every sheet, the board layout and any imported SPICE tube models: one file is the whole project.
• Save as PDF: every sheet as a vector page. Export sheet…: the current sheet, each sheet as its own file, or all in one PDF; PNG at 150/300/600 dpi, black on white or in the editor's colours.
• Export SPICE netlist: an LTspice netlist; tubes as behavioural sources with the same equations the simulator uses, transistors as Q and M elements with their .model lines.
• Bill of materials (B, or View ▾): equal parts on one line with designator ranges; tube sockets; ratings; the simulated worst case (dissipation, capacitor peak voltage, LED current); a part number you type. Click a line to select its parts. Export TXT, CSV, XLSX or PDF.
• Checks (View ▾, or the tab next to the bill of materials): the inspector's Checks list in the panel under the sheet, wider: wiring problems (no ground, unconnected pins, shorted parts) and every part beyond its rating. The tab shows how many there are.

13. Keyboard shortcuts

Shortcuts go by the key's position, so they work with a Russian or any other layout.

14. Design cookbook

Worked procedures, with the formulas the simulator lets you check.

14.1 A triode voltage amplifier

1. Pick Va and Ia from the curve tracer: a 12AX7 at 150–200 V and 0.8–1.2 mA; a 6N2P similar; a 12AU7 / 6N8S at 8–10 mA.
2. Plate resistor: Ra = (B+ − Va) / Ia. 100–220 kΩ for a 12AX7.
3. Cathode resistor: Rk = −Vg / Ia (read Vg at the operating point on the curves),; try the nearest E24 values and read Ia in the tube's inspector.
4. Gain, bypassed: A = µ · Ra / (Ra + rp). Unbypassed: A = µ · Ra / (Ra + rp + (µ + 1) · Rk). Compare with the inspector's Plate/grid gain.
5. Cathode bypass corner: f ≈ 1 / (2π · Ck · (Rk ‖ (Ra + rp)/(µ + 1))).
6. Coupling capacitor corner: f = 1 / (2π · Cc · (Rout + Rnext)). Check both on the Bode plot.
7. Top corner (Miller): see Bode plotter.

14.2 A single-ended output stage

1. Choose the load: roughly Zp ≈ 2 · Va / Ia for a pentode, 2–3 × rp for a triode.
2. Bias for 60–70 % of Pa max: adjust the cathode resistor (or the bias voltage) and read Pa in the tube's inspector against Pa max.
3. Check the output transformer's DC current rating (the check does it) and its primary inductance for the bass: f−3dB ≈ (rp ‖ Zp) / (2π · Lp).
4. Drive it to just below clipping: watch the scope for flat tops and the spectrum's THD. Read Output (speakers) in the circuit summary.
5. Check Pg2 at full drive (pentodes): it is highest there.

14.3 A push-pull stage

• Use the push-pull OPT; drive the grids 180° apart (a phase splitter, or two generators at 0° and 180°).
• Ultralinear: screens to the UL taps (about 40 % for KT88/EL34, 20–25 % elsewhere).
• Fixed bias: one bias pot per tube on a negative line; check with the pot at both ends that it covers tubes drawing ±30 % (a fresh, unmatched set).
• The spectrum should show H2 cancelled and H3 dominant; a large H2 means unbalance.

14.4 A power supply

• Mains → power transformer → rectifier (GZ34, 5U4G, or 1N4007s) → reservoir C → choke or resistor → filter C.
• Capacitor-input DC ≈ 1.2–1.4 × Vrms at load. Ripple ≈ Idc / (2 · f · C) peak-to-peak for full-wave.
• Check the rectifier's current and PIV, the capacitors' peak voltages (BOM) and the resistor dissipation.
• Watch the power-on transient on the scope for the turn-on surge and voltage overshoot.

14.5 Global feedback

• Take a resistor from the speaker to the cathode of the first stage (unbypassed part of Rk).
• Check stability on the Bode plot: no peak above the 1 kHz gain, and phase not reaching −180° where the loop gain is still above 1. A square wave on the scope should show little ringing.
• Feedback reduces gain, THD and output impedance by about the same factor (1 + loop gain).

15. How the simulator works

• Solver. Modified nodal analysis. Newton–Raphson with gmin and source stepping for the DC operating point; transient with Gear's second-order method (BDF2), which keeps resonances at their true Q without trapezoidal ringing.
• Steady state by shooting. The steady state is the state that one period maps onto itself; Newton–GMRES finds it, so slow coupling, cathode and filter capacitors settle in a few dozen periods instead of hundreds. If it does not converge in time, plain settling continues, extrapolating the slowly decaying response (minimal polynomial extrapolation).
• Sparse solves. A sparse LU (Gilbert–Peierls, as KLU) whose numeric part runs as WebAssembly; work grows about linearly with the circuit.
• Triodes: Koren's published equation unmodified:

E1 = (Va / kp) · ln(1 + exp(kp · (1/µ + Vg / √(kvb + Va²))))
Ia = 2 · E1^x / kg1

• Pentodes: Koren's space-charge term, divided between plate and screen with a separate knee and slope; what the plate loses in the knee goes to the screen:

Is  = 2 · E1^x / kg1      with E1 = (Vg2 / kp) · ln(1 + exp(kp · (1/µ + Vg / Vg2)))
Ia  = Is · tanh(Va / vk) · (1 + Va / λ)
Ig2 = (Vg + Vg2/µ)^x / kg2 + ks · Is · (1 − tanh(Va / vk))

• Grid conduction is included; below 2 V on the screen the currents fade smoothly to zero.
• Interelectrode capacitances: Cgk, Cga, Cak (triode); Cin, Cg1a, Cout (pentode), from the datasheets, a Soviet tube taking its Western analog's, else typical values.
• AC analysis: every device linearised at the operating point (its Jacobian by central differences of the same equations); the complex system G + jωC solved at 40 frequencies per decade.
• Rectifier tubes: vacuum diodes (Child–Langmuir), perveance from datasheet drops.
• Transistors: SPICE Gummel–Poon (IS, BF, BR, VAF, IKF) with junction capacitances, and base and emitter resistance (RB, RE) for the 2N3055 / MJ2955, TIP41C / TIP42C and MJL21194 / MJL21193, so their VBE rises with the current as a real part's does; the inspector's VBE and VCE are read at the pins. MOSFETs: square law (level 1) with a smoothed threshold, body diode and gate capacitances. Diodes/zeners/LEDs: exponential junctions.
• Transformers: coupled inductors with leakage. Loudspeakers: Re + Le in series with a parallel R‖L‖C motional impedance at fs.
• It runs in a Web Worker, so the editor stays responsive.

16. Accuracy and limits

• The engine is tested against closed-form results (dividers, RC/RL, resonance), an independent RK4 integration of a choke-input supply (0.02 %), Kirchhoff's law at every node, and the frequency response against RC corners and the Miller estimate.
• Every amplifier tube is fitted to and tested against its datasheet operating points; fitted currents are within a few percent.
• 76 reference circuits (RCA, Philips, GE datasheet circuits; textbook stages) are measured through the instruments as a user would.

17. Troubleshooting and FAQ

The instruments say "Waiting for the Circuit CAD…"

They need the Circuit CAD window open in the same browser. Open them from its Windows ▾ menu.

An instrument says "no scope in circuit"

Place an Oscilloscope part (palette → Instruments) and wire its channels.

The Bode plotter shows nothing

It needs a signal generator in the circuit as well as a scope part.

A voltage sticker is missing

There was no free room for it. Zoom in, or hover the wire to read it in the status bar.

The gain is higher than my hand calculation

Check rp and µ at your operating point in the curve tracer (markers give gm, rp, µ at a point): they change with current. See the 12AX7 note in Accuracy.

My real amp draws a different current

Normal tube spread: real tubes of one type differ by ±30 % in anode current at the same bias. Adjust the cathode resistor (or the bias voltage) in the circuit until the simulation draws what you measured.

Where are my circuits kept?

The open circuit is autosaved in this browser. Save files (Ctrl+S) to keep them safely or move them between computers.

Can I run it offline?

Yes: any static web server, e.g. python3 -m http.server in the folder. The simulator needs http://, not file://.

Tube Amp CAD · · MIT licence.