Two-channel 50 W head with a cathode follower and Fender-style stack, 1990s 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 Sovtek MIG-50.

Two channels, one with a bright mixing cap and a small cathode bypass cap, mixed into a gain stage and a cathode follower, a treble, middle and bass stack, then a long-tail pair into two output tubes.

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. A Bassman-style preamp with the two channels voiced apart: channel 2’s cathode cap is only .68 µF and its mixing resistor has 470 pF across it, so it’s thinner and brighter than channel 1. Both mix through 300 kΩ into V2a and a cathode follower that drives the tone stack.

Download.

Parts and what they do

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

R1, 68 kΩ
Channel 1 input resistor, 68 kΩ into the grid.
R2, 1 MΩ
Channel 1’s 1 MΩ from the jack’s tip to ground.
R4, 68 kΩ
Channel 2 input resistor, 68 kΩ into the grid.
R3, 1 MΩ
Channel 2’s 1 MΩ from the jack’s tip to ground.
V1a, 12AX7 (EHX, Tung-Sol reissue, Sovtek)
Channel 1 gain stage, half a 12AX7 (the drawing marks no type).
RP2, 100 kΩ
Channel 1 plate resistor, 100 kΩ.
RK1, 1 kΩ
Channel 1 cathode resistor, 1 kΩ.
CK1, 220 µF electrolytic
Channel 1 bypass cap, 220 µF 25 V, for full gain down through the bass.
C2, 22 nF
Channel 1 coupling cap, .022 µF.
Volume 1, 1 MΩ pot, audio taper
Channel 1 volume, 1 MΩ audio.
RM1, 300 kΩ
Channel 1 mixing resistor, 300 kΩ.
V1b, 12AX7 (EHX, Tung-Sol reissue, Sovtek)
Channel 2 gain stage, the other half of V1.
RP1, 100 kΩ
Channel 2 plate resistor, 100 kΩ.
RK2, 2.7 kΩ
Channel 2 cathode resistor, 2.7 kΩ, a colder bias than channel 1.
CK2, 680 nF electrolytic
Channel 2 bypass cap, only .68 µF. It bypasses the 2.7 kΩ above about 90 Hz, so this channel has full gain in the mids and treble and less in the bass.
C3, 22 nF
Channel 2 coupling cap, .022 µF.
Volume 2, 1 MΩ pot, audio taper
Channel 2 volume, 1 MΩ audio.
RM2, 300 kΩ
Channel 2 mixing resistor, 300 kΩ.
CBR, 470 pF
The bright cap, 470 pF across channel 2’s mixing resistor. It lets the treble around the 300 kΩ.
V2a, 12AX7 (EHX, Tung-Sol reissue, Sovtek)
Second stage, half a 12AX7, shared by both channels. Its plate drives the cathode follower directly.
RP3, 100 kΩ
V2a plate resistor, 100 kΩ.
RK3, 1 kΩ
V2a cathode resistor, 1 kΩ, no bypass cap.
V2b, 12AX7 (EHX, Tung-Sol reissue, Sovtek)
Cathode follower driving the tone stack, DC-coupled from V2a’s plate.
RK4, 100 kΩ
Cathode follower load resistor, 100 kΩ.
CT, 250 pF
Treble cap, 250 pF.
R7, 56 kΩ
Slope resistor, 56 kΩ.
CB, 22 nF
Bass cap, .022 µF.
CM, 22 nF
Middle cap, .022 µF.
Treble, 250 kΩ pot, audio taper
Treble, 250 kΩ audio. Its wiper is the preamp’s output, to the inverter’s coupling cap.
Bass, 1 MΩ pot, audio taper
Bass, 1 MΩ audio, a variable resistor.
Middle, 50 kΩ pot, linear taper
Middle, 50 kΩ, the middle cap on its wiper. Its taper isn’t marked, so linear is assumed.
.022µ, 22 nF
Coupling cap into the phase inverter. With the grid resistor it sets how low the inverter goes, around 5 Hz.
V3a, 12AX7 (EHX, Tung-Sol reissue, Sovtek)
12AX7, the driven half of the long-tail pair phase inverter. Its plate swings opposite the input and drives tube A.
V3b, 12AX7 (EHX, Tung-Sol reissue, Sovtek)
The other half of the inverter. Its grid sits at the feedback point for signal, so it’s driven through the shared cathodes, and its plate swings with the input. It drives tube B.
1M, 1 MΩ
Grid resistor for the driven half, returned to the bias point above the tail, not to ground.
1M, 1 MΩ
Grid resistor for the other half.
470, 470 Ω
Bias resistor for both halves. The cathodes sit this far above the grids’ return, a volt or so.
10k, 10 kΩ
The tail resistor. It’s large, so the pair shares a nearly fixed current. When one half conducts more, the other conducts less, which splits the phase.
.1µ, 100 nF
Ties the second grid to the feedback point for signal, so the feedback acts across the pair.
82k, 82 kΩ
Plate resistor for the driven half. It’s smaller than the other so the two outputs come out about even.
100k, 100 kΩ
Plate resistor for the other half.
47p, 47 pF
A small cap from plate to plate. It rolls off the far top end the inverter passes on, which keeps the feedback loop steady.
82k, 82 kΩ
The feedback resistor from the speaker. With the 4.7k Ω below it, it sends about 5 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
4.7k, 4.7 kΩ
The bottom of the feedback loop, 4.7k Ω to ground. The fed-back speaker voltage arrives here and moves the tail.
.1µ, 100 nF
The presence cap, in series with the pot across the leg. With the pot it sets where the lift starts.
Presence, 5 kΩ pot, linear taper
Presence, a 5k linear pot as a variable resistor, its resistance falling as it’s turned up, in series with the cap across the leg. Turned up, the cap shunts more of the leg’s treble, so less treble is fed back and the top end rises.
.022µ, 22 nF
Coupling cap to tube A’s grid.
.022µ, 22 nF
Coupling cap to tube B’s grid.
1.5k, 1.5 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current.
1.5k, 1.5 kΩ
Grid stopper for tube B.
V4, 5881
n5881, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V5, 5881
n5881, tube B of the pair.
220k, 220 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.
220k, 220 kΩ
Grid leak for tube B.
470, 470 Ω
Screen resistor for tube A. It limits the screen current when the tube is driven hard.
470, 470 Ω
Screen resistor for tube B.
500, 500 Ω
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
23.5µ, 23.5 µF electrolytic
Filter cap on the screen supply.
5.1k, 5.1 kΩ
Dropping resistor from the screen supply node to the node for the inverter, as on the Sovtek MIG-50 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.
23.5µ, 23.5 µF electrolytic
Filter cap at the node for the inverter. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
51k, 51 kΩ
Dropping resistor from the node for the inverter to the node for the preamp, as on the Sovtek MIG-50 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.
47µ, 47 µF electrolytic
Filter cap at the node for 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, 4.3 kΩ to 8 Ω, 20 H primary, 6 mH leakage
The output transformer, 4.3 kΩ plate to plate into 8 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, 340 V ac, 60 Hz
The power transformer’s high-voltage winding, 340 V ac end to end, into a bridge of four silicon diodes with the center tap unused. It’s drawn as two sources of 340 V ac a half cycle apart into two diodes, which charges the reservoir the same way at one diode drop (under a volt) higher. The value is typical of the part, not measured.
PT, 340 V ac, 60 Hz
The same winding on the other half cycle. It runs opposite the first source, so the bridge charges the reservoir on every half cycle.
PT R, 30 Ω
Resistance of the whole winding, plus the primary’s share. In a bridge every charging pulse runs through the whole winding. With the diodes’ drop it sets how far B+ sags on a hard note. Typical of the part, not measured.
PT R, 30 Ω
The same winding’s resistance, on the other half cycle. 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.
110µ, 110 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, 55 V ac, 60 Hz
The power transformer’s bias tap, 55 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
470, 470 Ω
Series resistor into the bias rectifier, 470 Ω. It limits the charging current.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
1500µ, 1.5 mF electrolytic
Filter cap on the bias supply, its plus side to ground.
Bias 25k, 25 kΩ pot, linear taper
Bias adjust, 25k Ω linear. Its wiper feeds both output grids through their grid leaks. Toward the diode end the grids go more negative and the tubes idle cooler.
20k, 20 kΩ
The bottom of the bias divider, 20k Ω to ground.
Coil R, 8.69 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 8 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, 256 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, 464 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 14.3 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 2.13 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, 2.19 Ω
Limits what Loss L2 adds.
Tank R, 83.7 Ω
Height of the bass resonance, about 92 Ω in all at its peak. The feedback holds the output steadier against that than it would be without feedback.
Tank L, 21.1 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 220 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

About this version

MIG-50
Bruce Collins’s redraw. The drawing marks no tube types, so 12AX7s are assumed, the Sovtek kind this amp shipped with, fitted to the New Sensor sheet (EHX, Tung-Sol reissue and Sovtek share it). The tube models are typical of each type, not measured.

Sources

  • Bruce Collins, redrawn Sovtek Mig-50 schematic, Mission Amps

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.

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