Two-channel 200 W tube amp, late 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 Marshall 1967 Major (200 W).

Two channels, bright and normal, mixed into one gain stage that drives the treble, middle and bass stack straight from its plate. Feedback from the speaker goes to that stage’s cathode, with Presence there. A cathodyne and an ECC82 driver push four KT88s.

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 the 1959’s: two channels, each one triode with its own volume, mixed through 470 kΩ. From there it differs. There’s no cathode follower: the tone stack hangs straight off V2a’s plate, so the stack loses more level and the knobs interact more. The feedback from the 16 Ω tap comes back to V2a’s cathode, so the loop includes V2a and the tone stack, and Presence sits there too. After the stack, a cathodyne splits the signal and a dual triode, an ECC82, brings it up enough to drive four KT88s. On Preamp only there’s no loop at all and the feedback resistor goes to ground.

Download.

Parts and what they do

Stock values. 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.
10k, 10 kΩ
Dropping resistor, 10 kΩ 1 W, from the V2 node to V1’s node.
50µ, 50 µF electrolytic
Filter cap on V1’s node.
V1b, 12AX7 / ECC83 / 7025
Bright channel gain stage.
100k, 100 kΩ
Bright stage plate resistor.
2.7k, 2.7 kΩ
Bright stage cathode resistor, a slightly cool bias.
.68µ, 680 nF
Bright stage bypass cap. Against 2.7 kΩ it bypasses only above about 90 Hz, so the bass gets less gain.
.0022µ, 2.2 nF
Bright channel coupling cap. 2.2 nF into the 1 MΩ volume cuts below about 70 Hz. Marshall’s own sheet has .02 µF here, which passes the full bass.
Volume I, 1 MΩ pot, audio taper
Bright channel volume, 1 MΩ.
470k, 470 kΩ
Bright channel mixing resistor to V2a’s grid. With V2a’s input capacitance it rolls off some treble.
500p, 500 pF
Cap across the bright mixing resistor. It puts back the treble the 470 kΩ loses.
V1a, 12AX7 / ECC83 / 7025
Normal channel gain stage.
100k, 100 kΩ
Normal stage plate resistor.
820, 820 Ω
Normal stage cathode resistor, a warm bias.
250µ, 250 µF electrolytic
Normal stage bypass cap, full gain down through the bass.
.022µ, 22 nF
Normal channel coupling cap.
Volume II, 1 MΩ pot, audio taper
Normal channel volume, 1 MΩ.
470k, 470 kΩ
Normal channel mixing resistor.
V2a, 12AX7 / ECC83 / 7025
Third stage, shared by both channels. Its plate drives the tone stack straight, and the speaker’s feedback comes in under its cathode.
100k, 100 kΩ
V2a plate resistor. It’s also the tone stack’s source resistance, along with the tube’s plate resistance, since there’s no cathode follower.
470, 470 Ω
V2a cathode resistor, 470 Ω, from the cathode down to the feedback point.
25µ, 25 µF electrolytic
Bypass cap across the cathode resistor, to the feedback point, not to ground. It restores the stage’s gain while the feedback still reaches the cathode.
470, 470 Ω
The feedback leg, 470 Ω from the feedback point to ground. With the 100 kΩ from the speaker it sets how much comes back. It’s also part of V2a’s bias.
100k, 100 kΩ
Feedback resistor, 100 kΩ from the 16 Ω tap to the feedback point, as written over the 47 kΩ on Marshall’s own sheet (the Unicord drawing has 47 kΩ). On Preamp only, or with another power amp, its far end goes to ground.
Presence, 5 kΩ pot, linear taper
Presence, 5 kΩ linear from the feedback point to ground, its wiper to the .68 µF. Turned up, the cap shorts the feedback point to ground at high frequencies, so less treble comes back and more treble comes through.
.68µ, 680 nF
Presence cap, on the pot’s wiper to ground.
500p, 500 pF
Treble cap.
33k, 33 kΩ
Slope resistor.
.022µ, 22 nF
Bass cap.
.022µ, 22 nF
Mid cap.
Treble, 250 kΩ pot, linear taper
Treble pot, 250 kΩ. Its wiper is the preamp’s output. The drawing marks no taper, so linear is assumed, as on Marshall’s own drawings of later amps.
Bass, 1 MΩ pot, audio taper
Bass pot, 1 MΩ, a variable resistor.
Middle, 25 kΩ pot, linear taper
Mid pot, 25 kΩ, the mid cap on its wiper and its track from the bass pot to ground, the 5F6-A wiring.
.022µ, 22 nF
Coupling cap into the cathodyne’s grid.
V2b, 12AX7 / ECC83 / 7025
ECC83, the other half of V2: a cathodyne, or split-load inverter, with no driver ahead of it. 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. Its gain is slightly under 1 on each side, and the ECC82 after it brings the swing up.
100k, 100 kΩ
The cathodyne’s plate resistor, the output to tube B.
2.7k, 2.7 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. It’s the grid’s only path to ground, and the cathode follows the grid, so the preamp sees much more than 1M Ω.
.047µ, 47 nF
Coupling cap from the inverter to side A’s driver.
.047µ, 47 nF
Coupling cap from the inverter to side B’s driver.
1.5k, 1.5 kΩ
The drivers’ cathode resistor, 1.5k Ω to ground for both halves, with no bypass cap. The two cathodes share it, so the pair works somewhat like a long-tail pair: what one half gains in current the other loses, which evens out the two sides’ swings.
270k, 270 kΩ
Grid leak for side A’s driver, to ground.
V3a, 12AU7 / ECC82
12AU7, side A’s driver: a gain stage that raises the inverter’s signal about 10 times on its way to tube A’s grids. The other half drives side B, and the two share one cathode resistor.
47k, 47 kΩ
The driver’s plate resistor, 47k Ω from the node for the drivers.
.047µ, 47 nF
Coupling cap from the driver’s plate to tube A’s grids.
270k, 270 kΩ
Grid leak for side B’s driver, to ground.
V3b, 12AU7 / ECC82
The other half of the 12AU7, side B’s driver.
47k, 47 kΩ
The driver’s plate resistor, 47k Ω from the node for the drivers.
.047µ, 47 nF
Coupling cap from the driver’s plate to tube B’s grids.
2.8k, 2.8 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current. Each of the two tubes has its own 5.6k. The resistors are drawn as one part, the two in parallel.
2.8k, 2.8 kΩ
Grid stopper for tube B. Each of the two tubes has its own 5.6k. The resistors are drawn as one part, the two in parallel.
V4, V6, KT88
KT88, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply. Two tubes in parallel, drawn as one.
V5, V7, KT88
KT88, tube B of the pair. Two tubes in parallel, drawn as one.
68k, 68 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.
68k, 68 kΩ
Grid leak for tube B.
135, 135 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 270. The resistors are drawn as one part, the two in parallel.
135, 135 Ω
Screen resistor for tube B, from its ultra-linear tap. Each of the two tubes has its own 270. The resistors are drawn as one part, the two in parallel.
1.8k, 1.8 kΩ
Dropping resistor from B+ to the node for the drivers, as on the Marshall 1967 Major 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.
100µ, 100 µF electrolytic
Filter cap at the node for the drivers. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
25k, 25 kΩ
Dropping resistor from the node for the drivers to the node for the inverter and the preamp, as on the Marshall 1967 Major 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.
50µ, 50 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.
Bleeder, 112 kΩ
The bleeders, two 56 kΩ 2 W in series across the two 200 µF caps, as on the drawing. They hold the two caps’ voltages equal and drain them after the amp is switched off.
Bleeder, 100 kΩ
The bleeder, 100 kΩ 2 W from this node to ground, as on the drawing. It drains the filter caps after the amp is switched off.
OT, 2.2 kΩ to 16 Ω, 20 H primary, 5 mH leakage
The output transformer, 2.2 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, 496 V ac, 60 Hz
The power transformer’s high-voltage winding, 496 V ac end to end, into a bridge of four silicon diodes with the center tap unused. It’s drawn as two sources of 496 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, 496 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, 25 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, 25 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.
188µ, 188 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, 143 V ac, 60 Hz
The power transformer’s bias tap, 143 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
1.5k, 1.5 kΩ
Bias adjust on the drawing, a 3 kΩ variable resistor ahead of the bias diode. Here it’s fixed at 1.5 kΩ, its middle. Less resistance puts more of the winding on the grids, so the tubes idle cooler. A real Major’s chart reads −83 V on the grids, but the bench’s KT88 idles near 93 mA a tube there, as the tube’s data sheets say it would. So the winding is set colder, to about −108 V, to come out near 35 mA a tube, as the chart’s screen and wall currents say.
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.
820, 820 Ω
Second resistor of the bias supply, 820 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.
10k, 10 kΩ
From the bias line to ground, 10 kΩ, as on the drawing. With the 820 Ω and the bias adjust above it, it sets the bias.
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. With no feedback, the voltage at the speaker rises and falls with its impedance.
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. With no feedback, the voltage at the speaker rises and falls with its impedance.
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

As drawn, 1970
ECC83 (12AX7) tubes, as drawn. The tube models are typical of the type, not measured. Real tubes spread 20 % or more.

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.

Product and company names are trademarks of their owners. They are used here only to identify the circuits these models are based on; this site is not affiliated with or endorsed by any of them.

From Swell Amplification, swellamps.com.