Two-channel 150 W amp with a master and four 6550s, early 1970s 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 Sunn Model T.

Brite and Normal channels on one 12AX7, mixed into a gain stage and cathode follower, a treble, middle and bass stack and a Master, then a long-tail pair into four ultra-linear 6550s.

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. The front end is close to a Fender Bassman’s: two channels sharing a cathode, mixed through 470 kΩ into a gain stage and a cathode follower that drives the tone stack. The Master sits after the stack, ahead of the inverter. The power amp is the big part: four 6550s on 500 V, ultra-linear, with 470 Ω in each plate lead and 1.5 kΩ on each screen, feedback from the 16 Ω tap and a presence control.

Download.

Parts and what they do

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

R1, 68 kΩ
Brite input resistor, 68 kΩ into the grid.
R2, 1 MΩ
Brite grid resistor, 1 MΩ to ground.
R4, 68 kΩ
Normal input resistor, 68 kΩ into the grid.
R3, 1 MΩ
Normal grid resistor, 1 MΩ to ground.
V1a, 12AX7 / ECC83 / 7025
Brite channel gain stage, half a 12AX7A. Its cathode is tied to the normal stage’s cathode.
R6, 100 kΩ
Brite stage plate resistor, 100 kΩ from C (+330 V on the drawing).
C2, 22 nF
Brite channel coupling cap, .022 µF.
Brite vol., 1 MΩ pot, linear taper
Brite volume, 1 MΩ, linear as the drawing’s note says.
R10, 470 kΩ
Brite mixing resistor, 470 kΩ. With R11 it mixes the two channels into V2a.
C4, 470 pF
The bright cap, 470 pF across the Brite mixing resistor. It lets the treble around the 470 kΩ, so that channel is brighter.
V1b, 12AX7 / ECC83 / 7025
Normal channel gain stage, the other half of the 12AX7A, sharing the cathode resistor and bypass cap.
R7, 100 kΩ
Normal stage plate resistor, 100 kΩ.
R5, 680 Ω
The shared cathode resistor, 680 Ω. It carries both triodes’ current.
C1, 250 µF electrolytic
The shared bypass cap, 250 µF 25 V.
C3, 22 nF
Normal channel coupling cap, .022 µF.
Normal vol., 1 MΩ pot, linear taper
Normal volume, 1 MΩ linear.
R11, 470 kΩ
Normal mixing resistor, 470 kΩ.
V2a, 12AX7 / ECC83 / 7025
Second stage, half a 12AX7A, shared by both channels. Its plate drives the cathode follower directly.
R12, 100 kΩ
V2a plate resistor, 100 kΩ.
R13, 820 Ω
V2a cathode resistor, 820 Ω.
CK3, 22 µF electrolytic
V2a bypass cap. The drawing marks it 22 and is hard to read, so 22 µF is assumed. With it the stage has full gain above about 10 Hz.
V2b, 12AX7 / ECC83 / 7025
Cathode follower driving the tone stack, DC-coupled from V2a’s plate.
R14, 100 kΩ
Cathode follower load resistor, 100 kΩ.
C5, 270 pF
Treble cap, 270 pF.
R17, 56 kΩ
Slope resistor, 56 kΩ. It sets how deep the mid scoop goes.
C6, 22 nF
Bass cap, .022 µF.
C7, 22 nF
Middle cap, .022 µF.
Treble, 250 kΩ pot, linear taper
Treble, R15, 250 kΩ linear. Its wiper feeds the Master.
Bass, 1 MΩ pot, linear taper
Bass, R16, 1 MΩ linear, a variable resistor.
Middle, 25 kΩ pot, linear taper
Middle, R18, 25 kΩ linear, the middle cap on its wiper and the bass pot returning through the whole track.
Master, 1 MΩ pot, linear taper
Master volume, R19, 1 MΩ linear, after the tone stack. It sets how hard the power amp is driven.
C8, 22 nF
Coupling cap into the inverter’s grid, .022 µF.
V3a, 12AX7 / ECC83 / 7025
12AX7, the driven half of the long-tail pair phase inverter. Its plate swings opposite the input and drives tube A. Its grid is fed through the preamp’s coupling cap, so there’s no second one here.
V3b, 12AX7 / ECC83 / 7025
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.
4.7k, 4.7 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.
120k, 120 kΩ
Plate resistor for the other half.
270p, 270 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.
22k, 22 kΩ
The feedback resistor from the speaker. With the 10k Ω below it, it sends about 31 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
10k, 10 kΩ
The bottom of the feedback loop, 10k Ω to ground. The fed-back speaker voltage arrives here and moves the tail.
.022µ, 22 nF
The presence cap, in series with the pot across the leg. With the pot it sets where the lift starts.
Presence, 25 kΩ pot, linear taper
Presence, a 25k 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.
.025µ, 25 nF
Coupling cap to tube A’s grid.
.025µ, 25 nF
Coupling cap to tube B’s grid.
500, 500 Ω
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current. Each of the two tubes has its own 1k. The resistors are drawn as one part, the two in parallel.
500, 500 Ω
Grid stopper for tube B. Each of the two tubes has its own 1k. The resistors are drawn as one part, the two in parallel.
V4, V6, 6550
n6550, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply. Two tubes in parallel, drawn as one.
V5, V7, 6550
n6550, tube B of the pair. Two tubes in parallel, drawn as one.
235, 235 Ω
Plate resistor for tube A, 470 Ω between the plate and the output transformer. It damps oscillation at radio frequencies. The drop across it is the plate current times 470 Ω, a few volts. Each of the two tubes has its own 470. The resistors are drawn as one part, the two in parallel.
235, 235 Ω
Plate resistor for tube B, the same. Each of the two tubes has its own 470. The resistors are drawn as one part, the two in parallel.
100k, 100 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.
100k, 100 kΩ
Grid leak for tube B.
750, 750 Ω
Screen resistor for tube A, from the ultra-linear tap on its half of the primary. The screen swings with part of the plate’s signal, so the tube sits between a pentode and a triode. Each of the two tubes has its own 1.5k. The resistors are drawn as one part, the two in parallel.
750, 750 Ω
Screen resistor for tube B, from its ultra-linear tap. Each of the two tubes has its own 1.5k. The resistors are drawn as one part, the two in parallel.
10k, 10 kΩ
Dropping resistor from B+ to the node for the inverter, as on the Sunn Model T 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.
60µ, 60 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.
10k, 10 kΩ
Dropping resistor from the node for the inverter to the node for the preamp, as on the Sunn Model T 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.
80µ, 80 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, 1.9 kΩ to 4 Ω, 20 H primary, 5 mH leakage
The output transformer, 1.9 kΩ plate to plate into 4 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, 402 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 402 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, 402 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, 20 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, 20 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.
20µ, 20 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, 54 V ac, 60 Hz
The power transformer’s bias tap, 54 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
330, 330 Ω
Series resistor into the bias rectifier, 330 Ω. It limits the charging current.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
50µ, 50 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
Bias 10k, 10 kΩ pot, linear taper
Bias adjust, 10k 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.
18k, 18 kΩ
The bottom of the bias divider, 18k Ω to ground.
50µ, 50 µF electrolytic
Second filter cap, on the bias line at the pot’s wiper, its plus side to ground. It keeps ripple off the line the grid leaks hang from.
Coil R, 4.34 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 4 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, 128 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, 232 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 7.15 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 1.06 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, 1.09 Ω
Limits what Loss L2 adds.
Tank R, 41.9 Ω
Height of the bass resonance, about 46 Ω in all at its peak. The feedback holds the output steadier against that than it would be without feedback.
Tank L, 10.6 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 440 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.
16 Ω tap, 2 V
The 16 Ω tap the feedback comes from, assumed to be an ideal tap on the same winding: 2 times the speaker voltage. The load the feedback network puts on the winding is left out. It’s tens of kilohms on a winding of a few ohms.

About this version

Model T, 1973
Drawing D-1029 revision A, September 1973. The tube models are typical of each type, not measured.

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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