Two-channel amp with pitch vibrato and reverb, mid-1960s style

Pedal and tube amp circuits, solved part by part the way SPICE (the circuit simulator engineers use) does it, while you listen and watch. Turn the knobs, change any part, or tap a wire to put a probe on it.

How to use the bench

Every part is simulated. Nothing you hear is a recording.

  1. 1Pick a circuit

    Choose one from the menu, or type a name, maker or year in Find a circuit. The buttons under the menu switch between versions.

  2. 2Set the knobs and the input

    Turn the circuit’s knobs on the front panel. In the Input box, choose Guitar or Signal generator, and the note or chord.

  3. 3Press Play

    Press Play through the circuit to hear a picked note or chord. Play the string is the guitar by itself.

  4. 4Tap a part

    Tap any part on the schematic to change its value, or swap the transistor, tube or diode. The red numbers are DC voltages, and they update as you go.

  5. 5Tap a wire

    Tap a wire to put the probe there. The Probe graph and the spectrum show the signal there.

  6. 6Try a change

    Press Try it on a card under Try this to hear one change, before and after. Modify this circuit, below the schematic, adds or removes parts.

Good to know

Modeled after the Magnatone M10A.

Two channels, each a 12AX7 stage, a bass and treble stack, the volume, a second stage and a three-way tone switch. Channel 1 then runs through Magnatone’s pitch vibrato, two 12AU7 split loads with silicon carbide varistors swept by an oscillator, and the reverb driver and mixer. Both meet at a 12AU7 driver and cathodyne into two 7189As in ultra-linear.

Input

Add

Peak voltage from the pickup for each note. Strum hard and a single coil puts out a few hundred millivolts. When you play a chord, its strings share this level.

How it’s played
Tap a part to change its value, or a wire to probe it. To add or take out parts, open Modify this circuit, below. Red numbers are DC voltages to ground, not warnings. Scroll or drag sideways to see the whole circuit.
Modify this circuit Add parts, take them out, or put one in series with another, then hear what it does. Tap to open.

Pick what to do, then tap the schematic. Your changes show in purple. Any change can go on a switch beside the knobs, from the menu in its row under Your changes.

Add a part

Tap where each leg goes. Tap both ends of a part to put the new one across that part.

Change a part

With Remove a part on, tap any part of the pedal or amp to take it out, and tap it again to put it back. Tapping a purple part you added deletes it.

Turn on Add in series with a leg and tap a part. In that part’s box, below the schematic, pick which leg to lift and what goes in the gap, such as a resistor between a cap and ground or a stopper in front of a grid.

Change a wire

With Cut into a wire on, tap any wire. Pick a part from the menu and it goes in the gap, wired in. Volume pot puts a pot there with the input on one end, the output on the wiper and the other end to ground.

With Run a wire on, tap a leg, then where the wire goes. Drag any piece of a wire you ran, or of an added part’s lead, to square it up or move it.

Keep and share

Your changes stay in this browser, on this device, for your next visit. Nothing is sent to us. Anyone using this browser can see them. They’re lost if you clear your browsing data or use a private window, so save a file to keep a copy. A shared link carries the circuit after the #, a part browsers never send to a server.

Test tone for the graphs
The graphs use a steady tone. Play sounds a plucked string that dies away.

Input

pickup, unloadedat the pedal input

Output

at the amp input

Probe

Spectrum

Frequency response

What the curves mean

The ink curve is the response to a very quiet signal, from the generator to the probe. The blue curve is measured with pink noise at the level you set, like an audio analyzer. When stages clip, the blue curve pulls away from the ink one and the gray coherence line (right-hand scale) drops below 1. Then no single curve tells the whole story. Both use the signal generator, whichever Input tab is picked.

Output

Bias

every voltage and current at rest

What to listen for. Each vibrato stage makes two copies of the signal, opposite ways, at its plate and its cathode. The cathode’s copy reaches the plate side through a .047 µF cap and a varistor, a part whose resistance falls steeply as the voltage across it rises. The oscillator swings two lines opposite ways through 100 kΩ into the varistors, so their resistance rises and falls several times a second. That slides the signal’s phase back and forth, and a sliding phase is a moving pitch. The varistors are fitted to measured curves of old stock SiC parts, not measured in an M10A. The delay line’s springs are a measured tank, played from its impulse response, and the dry signal reaches the mixer’s cathode through 15 kΩ. Channel 2 skips the vibrato and reverb and meets channel 1 at two 1 MΩ mixing resistors.

Download.

Parts and what they do

Stock values. Tap a part on the schematic to change it.

R1A, 47 kΩ
The high gain jack’s 47 kΩ into the grid. With nothing in the low gain jack it sits in parallel with that jack’s 47 kΩ, so a guitar in the high gain jack sees 23.5 kΩ to the grid.
R3A, 47 kΩ
The low gain jack’s 47 kΩ into the grid. With nothing in the high gain jack, that jack’s tip is grounded, so the two 47 kΩ make a divider and the low gain jack is about 6 dB quieter.
R2A, 4.7 MΩ
The high gain jack’s 4.7 MΩ to ground, the load a guitar in that jack sees.
R1B, 47 kΩ
The high gain jack’s 47 kΩ into the grid. With nothing in the low gain jack it sits in parallel with that jack’s 47 kΩ, so a guitar in the high gain jack sees 23.5 kΩ to the grid.
R3B, 47 kΩ
The low gain jack’s 47 kΩ into the grid. With nothing in the high gain jack, that jack’s tip is grounded, so the two 47 kΩ make a divider and the low gain jack is about 6 dB quieter.
R2B, 4.7 MΩ
The high gain jack’s 4.7 MΩ to ground, the load a guitar in that jack sees.
1.5k, 1.5 kΩ
From B (+320 V) to C (+295 V), 1.5 kΩ 1 W.
30µ, 30 µF electrolytic
C’s filter cap, 30 µF 450 V.
20k, 20 kΩ
From C to D (+230 V).
30µ, 30 µF electrolytic
D’s filter cap, 30 µF 450 V.
10k, 10 kΩ
From D to E (+200 V), 10 kΩ.
1µ, 1 µF electrolytic
E’s filter cap, 1 µF 450 V.
V1a, 12AX7 / ECC83 / 7025
Channel 1’s first stage, half a 12AX7 (7025).
100k, 100 kΩ
First stage plate resistor, 100 kΩ.
470, 470 Ω
First stage cathode resistor, 470 Ω wirewound, not bypassed.
220k, 220 kΩ
220 kΩ 5 % from the supply to the cathode, as on the drawing. It sends about 0.9 mA through the 470 Ω on top of the tube’s own current, so the cathode sits slightly higher and the stage runs slightly colder.
33p, 33 pF
33 pF from the grid to ground. With the input resistors it cuts radio pickup far above the guitar’s range.
.047µ, 47 nF
Coupling cap from the first plate into the tone stack, .047 µF.
330k, 330 kΩ
The bass side’s feed, 330 kΩ from the coupling cap to the top of the bass pot.
Bass, 1 MΩ pot, audio taper
Bass, 1 MΩ. Its wiper feeds the output through 120 kΩ. Toward the top the 470 pF across the upper part of the track is shorted out and the .0047 µF across the lower part leaves the lows standing, so the bass comes up. Audio taper and its direction are typical, not on the drawing.
47k, 47 kΩ
From the bottom of the bass pot to ground, 47 kΩ. It sets how far the bass can be cut.
470p, 470 pF
470 pF from the top of the bass pot to its wiper. It lets the treble past the upper part of the track.
.0047µ, 4.7 nF
.0047 µF from the wiper to the bottom of the bass pot. It shorts the lower part of the track for the treble and mids.
120k, 120 kΩ
120 kΩ from the bass wiper to the treble wiper, where the two halves of the stack meet. It keeps the two controls from shorting each other.
470p, 470 pF
Treble cap, 470 pF from the coupling cap to the top of the treble pot.
Treble, 1 MΩ pot, audio taper
Treble, 1 MΩ, between the treble cap and the .0047 µF to ground. Its wiper is the stack’s output. Turned up, the wiper moves toward the 470 pF and the highs. Audio taper and its direction are typical, not on the drawing.
.0047µ, 4.7 nF
.0047 µF from the bottom of the treble pot to ground.
Volume, 1 MΩ pot, audio taper
Channel 1’s volume, 1 MΩ. Its wiper is the second stage’s grid.
V1b, 12AX7 / ECC83 / 7025
Channel 1’s second stage, the other half of the first 12AX7. Its grid is the volume pot’s wiper.
220k, 220 kΩ
Second stage plate resistor, 220 kΩ.
2.7k, 2.7 kΩ
Second stage cathode resistor, 2.7 kΩ, not bypassed.
.022µ, 22 nF
Coupling cap from the second stage into the tone switch’s networks, .022 µF.
470k, 470 kΩ
Mellow tap: 470 kΩ down to 68 kΩ and .01 µF to ground. The cap shorts the highs, so this tap passes lows and low mids.
68k, 68 kΩ
The mellow divider’s bottom, 68 kΩ.
.01µ, 10 nF
The mellow tap’s cap to ground, .01 µF.
470k, 470 kΩ
Bright tap: 470 kΩ over 47 kΩ, a plain divider, about a tenth of the signal at every frequency.
47k, 47 kΩ
The bright divider’s bottom, 47 kΩ.
100k, 100 kΩ
The boost tap’s low path: 100 kΩ, .047 µF to ground, then 330 kΩ on to the tap.
.047µ, 47 nF
.047 µF to ground in the boost tap’s low path. It shorts the mids, so only the lows get through this way.
330k, 330 kΩ
330 kΩ from the low path to the boost tap.
100p, 100 pF
100 pF straight to the boost tap. It carries the highs. With the low path it makes the boost tap’s bass and treble lift.
330k, 330 kΩ
The boost tap to ground, 330 kΩ.
V2a, 12AX7 / ECC83 / 7025
Channel 2’s first stage, half a 12AX7 (7025).
100k, 100 kΩ
First stage plate resistor, 100 kΩ.
470, 470 Ω
First stage cathode resistor, 470 Ω wirewound, not bypassed.
220k, 220 kΩ
220 kΩ 5 % from the supply to the cathode, as on the drawing. It sends about 0.9 mA through the 470 Ω on top of the tube’s own current, so the cathode sits slightly higher and the stage runs slightly colder.
33p, 33 pF
33 pF from the grid to ground. With the input resistors it cuts radio pickup far above the guitar’s range.
.047µ, 47 nF
Coupling cap from the first plate into the tone stack, .047 µF.
330k, 330 kΩ
The bass side’s feed, 330 kΩ from the coupling cap to the top of the bass pot.
Bass, 1 MΩ pot, audio taper
Bass, 1 MΩ. Its wiper feeds the output through 120 kΩ. Toward the top the 470 pF across the upper part of the track is shorted out and the .0047 µF across the lower part leaves the lows standing, so the bass comes up. Audio taper and its direction are typical, not on the drawing.
47k, 47 kΩ
From the bottom of the bass pot to ground, 47 kΩ. It sets how far the bass can be cut.
470p, 470 pF
470 pF from the top of the bass pot to its wiper. It lets the treble past the upper part of the track.
.0047µ, 4.7 nF
.0047 µF from the wiper to the bottom of the bass pot. It shorts the lower part of the track for the treble and mids.
120k, 120 kΩ
120 kΩ from the bass wiper to the treble wiper, where the two halves of the stack meet. It keeps the two controls from shorting each other.
470p, 470 pF
Treble cap, 470 pF from the coupling cap to the top of the treble pot.
Treble, 1 MΩ pot, audio taper
Treble, 1 MΩ, between the treble cap and the .0047 µF to ground. Its wiper is the stack’s output. Turned up, the wiper moves toward the 470 pF and the highs. Audio taper and its direction are typical, not on the drawing.
.0047µ, 4.7 nF
.0047 µF from the bottom of the treble pot to ground.
Volume, 1 MΩ pot, audio taper
Channel 2’s volume, 1 MΩ, into the second stage through .01 µF.
.01µ, 10 nF
.01 µF from the volume’s wiper to the second stage’s grid.
22M, 22 MΩ
22 MΩ grid leak. The cathode is grounded, so the stage biases itself by the grid’s own tiny current through it, about a volt.
V2b, 12AX7 / ECC83 / 7025
Channel 2’s second stage, cathode grounded, grid-leak biased.
120k, 120 kΩ
Second stage plate resistor, 120 kΩ.
.022µ, 22 nF
Coupling cap from the second stage into the tone switch’s networks, .022 µF.
470k, 470 kΩ
Mellow tap: 470 kΩ down to 68 kΩ and .01 µF to ground. The cap shorts the highs, so this tap passes lows and low mids.
68k, 68 kΩ
The mellow divider’s bottom, 68 kΩ.
.01µ, 10 nF
The mellow tap’s cap to ground, .01 µF.
470k, 470 kΩ
Bright tap: 470 kΩ over 47 kΩ, a plain divider, about a tenth of the signal at every frequency.
47k, 47 kΩ
The bright divider’s bottom, 47 kΩ.
100k, 100 kΩ
The boost tap’s low path: 100 kΩ, .047 µF to ground, then 330 kΩ on to the tap.
.047µ, 47 nF
.047 µF to ground in the boost tap’s low path. It shorts the mids, so only the lows get through this way.
330k, 330 kΩ
330 kΩ from the low path to the boost tap.
100p, 100 pF
100 pF straight to the boost tap. It carries the highs. With the low path it makes the boost tap’s bass and treble lift.
330k, 330 kΩ
The boost tap to ground, 330 kΩ.
V3a, 12AX7 / ECC83 / 7025
Channel 1’s third stage, half a 12AX7. Its plate drives the vibrato straight, with no cap.
470k, 470 kΩ
Third stage plate resistor, 470 kΩ. It also sets the vibrato tube’s grid voltage.
1.5k, 1.5 kΩ
Third stage cathode resistor, 1.5 kΩ, not bypassed.
V3b, 12AX7 / ECC83 / 7025
Channel 2’s third stage, the other half of the same 12AX7.
270k, 270 kΩ
Plate resistor, 270 kΩ.
2.7k, 2.7 kΩ
Cathode resistor, 2.7 kΩ, not bypassed.
1M, 1 MΩ
Channel 2’s mixing resistor into the power amp, 1 MΩ.
V4a, 12AU7 / ECC82
The first vibrato stage, half a 12AU7, a split load: its plate and cathode swing opposite ways.
22k, 22 kΩ
Plate resistor, 22 kΩ.
47k, 47 kΩ
Cathode resistor, 47 kΩ.
.047µ, 47 nF
.047 µF from the cathode to the first varistor.
.047µ, 47 nF
.047 µF from the cathode to the second varistor.
VR1, SiC varistor, one direction (fit), one each way
Varistor, silicon carbide, from the cathode’s cap to the plate. Its resistance falls steeply as the voltage across it rises, so the LFO current through it sets how much of the cathode’s signal reaches the plate node.
11.2k, 11.2 kΩ
Part of the varistor’s model, not a part of its own: the 11.2 kΩ in series that, with the pair beside it, follows the measured SiC curve.
VR2, SiC varistor, one direction (fit), one each way
The second varistor, the same, on the other LFO line.
11.2k, 11.2 kΩ
Part of the varistor’s model, not a part of its own: the 11.2 kΩ in series that, with the pair beside it, follows the measured SiC curve.
100k, 100 kΩ
100 kΩ from the LFO’s plate line to the first varistor.
100k, 100 kΩ
100 kΩ from the LFO’s cathode line to the second varistor.
.0022µ, 2.2 nF
.0022 µF from the first stage’s plate node to the second stage’s grid.
22M, 22 MΩ
22 MΩ from the second grid back to the first stage’s cathode, which sets its DC level.
V4b, 12AU7 / ECC82
The second vibrato stage, the other half of the 12AU7, the same split load.
22k, 22 kΩ
Plate resistor, 22 kΩ.
47k, 47 kΩ
Cathode resistor, 47 kΩ.
820p, 820 pF
820 pF from the plate to the output node.
.047µ, 47 nF
.047 µF from the cathode to the third varistor.
.047µ, 47 nF
.047 µF from the cathode to the fourth varistor.
VR3, SiC varistor, one direction (fit), one each way
The third varistor, from the cathode’s cap to the output node.
11.2k, 11.2 kΩ
Part of the varistor’s model, not a part of its own: the 11.2 kΩ in series that, with the pair beside it, follows the measured SiC curve.
VR4, SiC varistor, one direction (fit), one each way
The fourth varistor, the same, on the other LFO line.
11.2k, 11.2 kΩ
Part of the varistor’s model, not a part of its own: the 11.2 kΩ in series that, with the pair beside it, follows the measured SiC curve.
100k, 100 kΩ
100 kΩ from the LFO’s plate line.
100k, 100 kΩ
100 kΩ from the LFO’s cathode line.
.0022µ, 2.2 nF
.0022 µF from the vibrato’s output into the reverb driver.
V5a, 12AX7 / ECC83 / 7025
The oscillator, the 12AX7 half of a 12DW7, cathode grounded. The three-cap ladder from its plate back to its grid turns the phase round at a few hertz.
270k, 270 kΩ
Oscillator plate resistor, 270 kΩ from B.
.02µ, 20 nF
The ladder’s first cap, .02 µF.
220k, 220 kΩ
220 kΩ in series with the Speed pot, the slowest end’s limit.
Speed, 500 kΩ pot, linear taper
Speed, 500 kΩ as a variable resistor to ground. Turned up, less resistance, so the oscillator runs faster. Linear taper and the direction are typical, not on the drawing.
.01µ, 10 nF
The ladder’s second cap, .01 µF.
820k, 820 kΩ
820 kΩ to ground at the ladder’s middle.
.01µ, 10 nF
The ladder’s third cap, .01 µF, into the grid.
3.3M, 3.3 MΩ
The oscillator’s grid leak, 3.3 MΩ 5 %.
.068µ, 68 nF
.068 µF from the oscillator’s plate to the Intensity pot.
Intensity, 1 MΩ pot, audio taper
Vibrato intensity, 1 MΩ. Its wiper sets how much of the oscillator’s swing reaches the LFO’s split load. Audio taper and the direction are typical, not on the drawing.
47k, 47 kΩ
47 kΩ from the wiper to the footswitch node.
330k, 330 kΩ
The first of two 330 kΩ and .0047 µF sections. Together they round off the oscillator’s wave.
.0047µ, 4.7 nF
.0047 µF to ground.
330k, 330 kΩ
The second 330 kΩ.
.0047µ, 4.7 nF
.0047 µF to ground.
.047µ, 47 nF
.047 µF into the LFO split load’s grid.
8.2M, 8.2 MΩ
8.2 MΩ across the .047 µF. With the 1 MΩ to the cathode it sets the grid’s DC level.
V5b, 12AU7 / ECC82
The LFO’s split load, the 12AU7 half of the 12DW7. Its plate and cathode carry the oscillator’s swing to the varistors, one line each, opposite ways.
47k, 47 kΩ
Plate resistor, 47 kΩ from B.
47k, 47 kΩ
Cathode resistor, 47 kΩ.
1M, 1 MΩ
Grid to cathode, 1 MΩ.
.1µ, 100 nF
.1 µF from the plate line to ground. It smooths the LFO and keeps the audio off the line.
.1µ, 100 nF
.1 µF from the cathode line to ground.
V6, 12AU7 / ECC82
The reverb driver, a 12AU7 with both halves in parallel, drawn as one. Its cathode also carries the dry signal on through 15 kΩ.
1M, 1 MΩ
The driver’s grid resistor, 1 MΩ.
5k, 5 kΩ
The driver’s plate resistor, 5 kΩ 10 W from B.
270, 270 Ω
The driver’s cathode resistor, 270 Ω, not bypassed.
1µ, 1 µF
1 µF 450 V from the driver’s plate to the delay line’s input coil.
210, 210 Ω
Resistance of the delay line’s input coil, typical, not on the drawing.
Tank in, 230 mH
The delay line’s input coil, typical of a tube-driven one (about 1.5 kΩ at 1 kHz), not on the drawing. Its current drives the springs.
Tank out, 350 mH
The delay line’s output coil, from the recovery grid to ground. The springs come back in series with it. Its figures are a measured long tank’s.
181, 181 Ω
Resistance of the output coil, a measured long tank’s.
47k, 47 kΩ
47 kΩ across the delay line’s output.
V7a, 12AX7 / ECC83 / 7025
The reverb recovery stage, half a 12AX7.
270k, 270 kΩ
Recovery plate resistor, 270 kΩ.
2.7k, 2.7 kΩ
Recovery cathode resistor, 2.7 kΩ.
.047µ, 47 nF
.047 µF to the Reverb pot’s wiper and the footswitch.
Reverb, 1 MΩ pot, audio taper
Reverb depth, 1 MΩ, fed at its wiper. Its top lug is the mixer’s grid, its bottom ground. Turned up, the wiper moves toward the grid. Audio taper is typical, not on the drawing.
V7b, 12AX7 / ECC83 / 7025
The reverb mixer, the other half. The reverb comes in at its grid and the dry signal at its cathode through 15 kΩ, so the plate carries both.
270k, 270 kΩ
Mixer plate resistor, 270 kΩ.
2.7k, 2.7 kΩ
Mixer cathode resistor, 2.7 kΩ.
15k, 15 kΩ
The dry signal, 15 kΩ from the reverb driver’s cathode to the mixer’s cathode.
1M, 1 MΩ
Channel 1’s mixing resistor into the power amp, 1 MΩ.
.047µ, 47 nF
Coupling cap into the driver. With the grid resistor it sets how low the power amp goes, around 3 Hz.
V8a, 12AU7 / ECC82
12AU7 (ECC82), the driver. A cathodyne has no gain of its own, so this stage raises the preamp’s signal enough to drive the output tubes. The feedback from the speaker goes to its cathode.
1M, 1 MΩ
Grid resistor for the driver.
220k, 220 kΩ
Plate resistor for the driver.
.012µ, 12 nF
Coupling cap from the driver to the cathodyne.
V8b, 12AU7 / ECC82
The cathodyne, or split-load inverter. The same current runs through its plate resistor and its cathode resistors, which are about equal, so the plate swings one way and the cathode the other by about the same amount. The cathode drives tube A, the plate tube B. Its gain is slightly under 1 on each side.
100k, 100 kΩ
The cathodyne’s plate resistor, the output to tube B.
15k, 15 kΩ
The cathodyne’s bias resistor. The grid returns below it, so the cathode sits this far above the grid.
100k, 100 kΩ
The cathodyne’s cathode load, the same value as its plate resistor. The cathode above the bias resistor is the output to tube A.
1M, 1 MΩ
Grid resistor for the cathodyne, returned to the junction of its cathode resistors so the grid sits near the cathode’s high DC voltage.
10k, 10 kΩ
The feedback resistor from the speaker. With the 5.6k Ω below it, it sends about 36 % of the speaker voltage back to the driver’s cathode, which lowers the gain, flattens the response and tightens the bass.
5.6k, 5.6 kΩ
The driver’s cathode resistor, 5.6k Ω to ground. It sets the driver’s bias, and it’s also the bottom of the feedback loop: the feedback resistor from the speaker connects at the top of it, so the fed-back speaker voltage moves the cathode. It isn’t bypassed.
.047µ, 47 nF
Coupling cap to tube A’s grid.
.047µ, 47 nF
Coupling cap to tube B’s grid.
1k, 1 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current.
1k, 1 kΩ
Grid stopper for tube B.
V9, EL84 / 6BQ5 (Philips curves)
7189A on the drawing, a heavier-duty EL84, modeled with the EL84 preset (typical, not measured). Its screen sits straight on the ultra-linear tap, so it works between a pentode and a triode.
V10, EL84 / 6BQ5 (Philips curves)
The other 7189A, the same, driven by the cathodyne’s plate.
100, 100 Ω
One 100 Ω resistor under both output tubes’ cathodes, not bypassed, as on the drawing. The bias is fixed, so it only adds a few volts of cathode bias on top: the voltage across it is both tubes’ current times 100 Ω. The two tubes’ signal currents are opposite, so it barely lowers the gain.
270k, 270 kΩ
Grid leak for tube A, returned to the bias supply. Driven into grid current, the coupling cap charges through it and the bias goes colder for a moment. That’s blocking distortion.
270k, 270 kΩ
Grid leak for tube B.
2k, 2 kΩ
Dropping resistor from B+ to the node for the drivers and the preamp, as on the Magnatone M10A drawing. The current drawn past it drops the voltage, so each stage down the chain runs lower than the one before, and a hard note that pulls B+ down pulls these down too. The preamp runs off this node instead of its own HT supply.
40µ, 40 µF electrolytic
Filter cap at the node for the drivers and the preamp. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
OT, 10 kΩ to 16 Ω, 20 H primary, 10 mH leakage
The output transformer, 10 kΩ plate to plate into 16 Ω, center tap on B+. Tap any piece of it to change its figures. The core saturates as the flux nears its knee. Every figure is typical of the part, not measured.
PT, 300 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 300 V ac from the grounded center tap, as a sine source at the mains frequency. The value is typical of the part, not measured.
PT, 300 V ac, 60 Hz
The other half of the winding, from the grounded center tap. It runs opposite the first half, so the rectifier charges the reservoir on every half cycle.
PT R, 40 Ω
Resistance of one half of the winding, plus the primary’s share. With the rectifier’s own drop it sets how far B+ sags on a hard note. Typical of the part, not measured.
PT R, 40 Ω
Resistance of the other half. Typical of the part, not measured.
D1, 1N4001
One of the silicon rectifier diodes. It drops under a volt, so B+ holds up better under load than with a tube rectifier.
D2, 1N4001
The other rectifier diode, on the other half cycle.
40µ, 40 µF electrolytic
The reservoir cap. It feeds the output transformer’s center tap. It charges in short pulses at each peak of the mains and drains between them, so B+ carries ripple, and on a hard note it sags.
Bias tap, 35 V ac, 60 Hz
The power transformer’s bias tap, 35 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
tap R, 100 Ω
The bias tap’s own resistance, 100 Ω, typical, not measured. The drawing has no resistor between the tap and the diode.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
60µ, 60 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
22k, 22 kΩ
Second resistor of the bias supply, 22k Ω to the bias line. With the cap after it, it filters the ripple a second time.
25µ, 25 µF electrolytic
Second filter cap, on the bias line, its plus side to ground.
12k, 12 kΩ
From the bias line to ground, 12k Ω. With the 22k Ω above it, it divides the supply down to the bias. There’s no bias pot, so change this resistor to change the bias. Bigger runs the tubes colder.
Coil R, 17.4 Ω
Voice coil resistance, the floor of the speaker’s impedance. The shape of a 1x12 open-back cab with an alnico 12 in, measured (a 93 Ω peak near 74 Hz on 8 Ω), scaled to the 16 Ω tap. One real cab, not every speaker. The Speaker menu by the play buttons is separate. It plays the note through a speaker and cab impulse response, with either load.
Coil L, 512 µH
Voice coil inductance. It makes the impedance rise through the treble. The feedback holds the output steadier against that than it would be without feedback.
Loss L, 928 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 28.6 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 4.26 mH
A second step in the treble rise, with Loss R2 across it: a real voice coil’s losses keep climbing from a few hundred hertz.
Loss R2, 4.38 Ω
Limits what Loss L2 adds.
Tank R, 167 Ω
Height of the bass resonance, about 185 Ω in all at its peak. The feedback holds the output steadier against that than it would be without feedback.
Tank L, 42.2 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 110 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

About this version

M10A
From Magnatone’s M10A drawing. The 7025s are modeled as 12AX7s, the 12DW7 as a 12AX7 half and a 12AU7 half, all typical of the type, not measured. The varistors are a fit to measured old stock SiC parts.

How this tool works. Every part is an equation, and the whole circuit is solved several times per audio sample: 192,000 samples a second while you drag a knob, 384,000 once you let go. Transistors use Ebers-Moll with gain, leakage, Early effect, base resistance, junction capacitance and temperature. Their charge storage is set at the bias point, close for small signals and rough while they switch hard. Tubes use Koren’s equations fitted to each datasheet, plus grid current. Diodes are exponential junctions with series resistance. Presets are typical of each type, not measured parts. Caps are ideal, and the battery or HT is a voltage behind a resistance.

Spot an error, or want a circuit added? Email info@swellamps.com. A schematic or a clear photo of one helps. Anything you send may be used to improve these models and our desktop app.

© 2026 Swell Amplification LLC. All rights reserved. The simulator, its circuit models, drawings, presets and text are the property of Swell Amplification LLC and may not be copied, reproduced, redistributed or used to build another product without written permission. The original circuits are historical designs documented in public sources, and no claim is made to them. Everything here is provided as is, without warranty of any kind, for study and personal use. Results are simulations of typical parts, not measurements of any real unit. Don’t rely on them for the safety of any build or repair. Tube amplifiers carry lethal voltages.

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