Tweed four-input amp, late 1950s 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 Fender 5F6-A Bassman.

Two channels, bright and normal, each a triode gain stage, sharing one cathode resistor. They mix through 270 kΩ into a second gain stage, then a DC-coupled cathode follower and the treble-mid-bass tone stack later amps copied. 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. The first Marshalls copied this preamp: the same second stage, cathode follower and stack, with different voicing. The two first-stage triodes share one 820 Ω resistor and a big bypass cap, so both channels pass full bass. Jumpered, the volumes interact, since each channel loads the other through the mixers. The 56 kΩ slope resistor and 250 pF treble cap make a deeper mid scoop than the later 33 kΩ. Switch to the JTM45 build to hear a 12AX7 in the first socket. Most of the breakup happens in the power stage, which isn’t modeled here.

Download.

Parts and what they do

Stock values for the 12AY7 in V1 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 from the phase inverter’s supply node to the preamp node. All four preamp triodes pull their current through it.
CF1, 20 µF electrolytic
Preamp filter cap. Fender’s drawing shows 8 µF, but production amps used 20 µF.
V1b, 12AY7 / 6072
Bright channel gain stage. Its cathode is tied to the normal stage’s cathode.
RP2, 100 kΩ
Bright stage plate load.
C2B, 20 nF
Bright channel coupling cap.
Bright vol., 1 MΩ pot, audio taper
Bright channel volume.
CBR, 100 pF
Bright cap across the top of the bright volume, only 100 pF. It lifts the top end with the volume down and does nothing at full.
RM1, 270 kΩ
Bright channel mixer resistor. With RM2 it mixes the two channels, and through them each volume setting loads the other channel.
V1a, 12AY7 / 6072
Normal channel gain stage, sharing its cathode resistor and bypass cap with the bright stage.
RP1, 100 kΩ
Normal stage plate load.
RK1, 820 Ω
The shared cathode resistor. It carries both triodes’ current, so 820 Ω biases each about like 1.6 kΩ alone. Turning one channel’s signal up nudges the other’s bias.
CK1, 250 µF electrolytic
The shared bypass cap, 250 µF. With it both channels have full gain down through the bass.
C2A, 20 nF
Normal channel coupling cap.
Normal vol., 1 MΩ pot, audio taper
Normal channel volume.
RM2, 270 kΩ
Normal channel mixer resistor.
V2a, 12AX7 / ECC83 / 7025
Second stage, shared by both channels. Its cathode resistor is unbypassed, which holds the gain to about 40. Its plate drives the cathode follower directly.
RP3, 100 kΩ
V2a plate load.
RK3, 820 Ω
V2a cathode resistor.
V2b, 12AX7 / ECC83 / 7025
Cathode follower driving the tone stack, DC-coupled from the V2a plate.
RK4, 100 kΩ
Cathode follower load resistor.
CT, 250 pF
Treble cap.
R7, 56 kΩ
Slope resistor, 56 kΩ. Marshall later went to 33 kΩ, which makes the mid scoop shallower.
CB, 20 nF
Bass cap.
CM, 20 nF
Mid cap.
Treble, 250 kΩ pot, audio taper
Treble pot. Its wiper is the preamp’s output.
Bass, 1 MΩ pot, audio taper
Bass pot, a variable resistor.
Middle, 25 kΩ pot, linear taper
Mid pot, with the mid cap on the wiper and the bass pot returning to ground through the whole track. That’s the 5F6-A wiring, which Marshall kept. Blackface Fenders wire it as a variable resistor instead. The two match at full mid. Turned down, this wiring leaves slightly more bass.
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, 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.
4 H, 4 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.
100, 100 Ω
The choke’s winding resistance, typical. It’s all the DC drop the screens see from B+.
20µ, 20 µF electrolytic
Filter cap on the screen supply.
4.7k, 4.7 kΩ
Dropping resistor from the screen supply node to the node for the inverter and the preamp, as on the Fender 5F6-A Bassman 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.
20µ, 20 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, 4.2 kΩ to 2 Ω, 40 H primary, 10 mH leakage
The output transformer, 4.2 kΩ plate to plate into 2 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.
PT, 341 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 341 V ac from the grounded center tap, as a sine source at the mains frequency. The value is typical of the part.
PT, 341 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, 60 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.
PT R, 60 Ω
Resistance of the other half. Typical of the part.
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.
20µ, 20 µF electrolytic
The reservoir caps’ first. 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.
20µ, 20 µF electrolytic
Another reservoir cap, in parallel with the first.
Bias tap, 45 V ac, 60 Hz
The power transformer’s bias tap, 45 V ac, as a sine source at the mains frequency. Typical of the part.
tap R, 100 Ω
The bias tap’s own resistance, 100 Ω, typical. 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.
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 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, 2.17 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 2 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, 64 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, 116 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 3.58 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 533 µH
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, 548 mΩ
Limits what Loss L2 adds.
Tank R, 20.9 Ω
Height of the bass resonance, about 23 Ω in all at its peak. The feedback holds the output steadier against that than it would be without feedback.
Tank L, 5.28 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 881 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

Versions

12AY7 in V1
The factory tubes: a 12AY7 in the first socket, with about half a 12AX7’s gain, and a 12AX7 for the second stage and cathode follower.
JTM45, modeled after the Marshall JTM45
The British copy: a 12AX7 in the first socket, for about twice the first-stage gain, and 270 pF and 22 nF in the tone stack. Early examples varied. This build uses the common values.

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. 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.