Two-channel KT66 amp, 1963 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 Marshall JTM45.

Two channels, bright and normal, each a triode stage, sharing one cathode resistor. They mix through 270 kΩ into a second gain stage, then a cathode follower and the treble-mid-bass tone stack. No master volume: the tone stack drives the phase inverter directly.

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. With the channels jumpered, the guitar drives both first stages and the two volumes blend. The bright channel has 500 pF across its volume and its mixing resistor, so it brings up the treble most with the volume low. Both first stages share an 820 Ω cathode resistor with a big bypass cap, so each passes the full bass. V2a has no bypass cap. On Preamp only, you hear the signal as it reaches the phase inverter.

Download.

Parts and what they do

Stock values for the ECC83 throughout version. Tap a part on the schematic to change it.

R1B, 68 kΩ
The bright channel’s input 1 resistor into the grid. With nothing in input 2 it sits in parallel with input 2’s resistor, so a guitar in input 1 sees 34 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R3B, 68 kΩ
The bright channel’s input 2 resistor into the grid. With nothing in input 1, input 1’s tip is grounded, so the two 68 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 136 kΩ.
R2, 1 MΩ
Input 1’s 1 MΩ to ground. It sets the load a guitar in input 1 sees, and with a patch lead in input 1 it is the end of the lead’s path.
R1A, 68 kΩ
The normal channel’s input 1 resistor into the grid. With nothing in input 2 it sits in parallel with input 2’s resistor, so a guitar in input 1 sees 34 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R3A, 68 kΩ
The normal channel’s input 2 resistor into the grid. With nothing in input 1, input 1’s tip is grounded, so the two 68 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 136 kΩ.
R2A, 1 MΩ
Input 1’s 1 MΩ to ground. It sets the load a guitar in input 1 sees, and with a patch lead in input 1 it is the end of the lead’s path.
RD1, 10 kΩ
Dropping resistor, 1 W, from the phase inverter’s supply node to the preamp node.
CF1, 16 µF electrolytic
Filter cap on the preamp node. Every preamp stage hangs here.
V1b, 12AX7 / ECC83 / 7025
Bright channel gain stage. Its cathode is tied to V1a’s.
RP2, 100 kΩ
Bright stage plate load.
C2B, 20 nF
Bright channel coupling cap. Into the 1 MΩ volume it passes the full bass.
Volume I, 1 MΩ pot, audio taper
Bright channel volume, 1 MΩ log.
CBR, 500 pF
Bright cap across the top of volume I. Strongest with the volume low, and at full it does nothing.
RM1, 270 kΩ
Bright channel mixing resistor. With RM2 it mixes the two channels.
CM1, 500 pF
Cap across the bright mixing resistor. It lets the treble past the 270 kΩ.
V1a, 12AX7 / ECC83 / 7025
Normal channel gain stage.
RP1, 100 kΩ
Normal stage plate load.
RK1, 820 Ω
Cathode resistor shared by both V1 triodes. Both currents run through it, so each channel nudges the other’s bias.
CK1, 250 µF electrolytic
Bypass cap on the shared cathode, large enough for full gain down through the bass.
C2A, 20 nF
Normal channel coupling cap.
Volume II, 1 MΩ pot, audio taper
Normal channel volume, 1 MΩ log.
RM2, 270 kΩ
Normal channel mixing resistor.
V2a, 12AX7 / ECC83 / 7025
Second gain stage, shared by both channels. No bypass cap, so it gains less than the first stages. Its plate drives the cathode follower directly.
RP3, 100 kΩ
V2a plate load.
RK3, 820 Ω
V2a cathode resistor, not bypassed.
V2b, 12AX7 / ECC83 / 7025
Cathode follower driving the tone stack, DC-coupled from the V2a plate.
RK4, 100 kΩ
Cathode follower load resistor.
CT, 270 pF
Treble cap.
R7, 56 kΩ
Slope resistor.
CB, 20 nF
Bass cap.
CM, 20 nF
Mid cap.
Treble, 250 kΩ pot, linear taper
Treble pot, 250 kΩ linear. Its wiper is the preamp’s output.
Bass, 1 MΩ pot, audio taper
Bass pot, 1 MΩ log, a variable resistor.
Middle, 25 kΩ pot, linear taper
Mid pot, 25 kΩ linear, with the mid cap on the wiper and the bass pot returning to ground through the whole track, the 5F6-A wiring.
C7, 20 nF
Coupling cap into the phase inverter grid.
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.
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.
27k, 27 kΩ
The feedback resistor, 27 kΩ from the speaker tap. With the presence pot it sends about 16 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
Presence, 5 kΩ pot, linear taper
Presence, 5 kΩ linear, the bottom of the feedback loop. Its wiper goes to ground through the cap. Turned up, the cap shunts more of the fed-back treble to ground, so less treble is fed back and the top end rises, a few dB around 5 kHz.
.1µ, 100 nF
The presence cap. With the pot it sets where the lift starts.
.1µ, 100 nF
Coupling cap to tube A’s grid.
.1µ, 100 nF
Coupling cap to tube B’s grid.
V4, KT66
KT66, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V5, KT66
KT66, 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.
1k, 1 kΩ
One screen resistor shared by both tubes, ahead of the resistor on each screen, with no cap after it. It carries both screens’ current. The two screens’ signal currents are opposite and cancel in it, so its drop follows the even harmonics and grows as the stage is driven harder.
20 H, 20 H
A choke from B+ to the screen supply, as in the amp this style is based on. It passes DC with little drop, so the screens sit close to B+. With the cap after it, it would filter ripple, but ripple isn’t modeled here. Typical of the part, not measured.
100, 100 Ω
The choke’s winding resistance, typical, not measured. It’s all the DC drop the screens see from B+.
32µ, 32 µF electrolytic
Filter cap on the screen supply.
8.2k, 8.2 kΩ
Dropping resistor from the screen supply node to the node for the inverter and the preamp, as on the Marshall JTM45 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.
16µ, 16 µF electrolytic
Filter cap at the node for the inverter 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, 7.4 kΩ to 16 Ω, 40 H primary, 10 mH leakage
The output transformer, 7.4 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, 355 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 355 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, 355 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.
V6, GZ34
One plate of the GZ34 rectifier. It conducts near the peak of its half of the winding, and its own drop rises with the current, which adds to the sag.
V6, GZ34
The rectifier’s other plate, on the other half cycle.
32µ, 32 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.
PT, 355 V ac, 60 Hz
The end of the high-voltage winding the bias comes off, 355 V ac, the same end as the rectifier’s first plate. It’s drawn as its own source so the bias supply’s average can be found. Keep it equal to the winding beside the rectifier.
150k, 150 kΩ
Series resistor into the bias rectifier, 150k Ω. There’s no bias tap, so the bias comes straight off the high-voltage winding, and this resistor drops most of it.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
8µ, 8 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
15k, 15 kΩ
Second resistor of the bias supply, 15k Ω to the bias line. With the cap after it, it filters the ripple a second time.
8µ, 8 µF electrolytic
Second filter cap, on the bias line, its plus side to ground.
56k, 56 kΩ
From the bias line to ground, 56k Ω. With the 15k Ω and 150k 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.

Versions

ECC83 throughout
Two ECC83 (12AX7) twin triodes. The tube models are typical of the type, not measured. Real tubes spread 20 % or more.
Hand drawing: 100 pF bright cap
Marshall’s hand drawing of the JTM45 trem amps has 100 pF across the bright volume where the later drawing has 500 pF. Less treble lift with the volume low.

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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