Paralleled-triode tube amp, 1965 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 18 Watt normal channel.

One gain stage made of both halves of a 12AX7, sharing a plate resistor and a cathode resistor, then a two-cap tone network and a volume straight into the phase inverter. The guitar drives one grid, and the other half idles with its grid grounded.

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 empty jack’s switch grounds the idle triode’s grid, but that triode still conducts. It sits across the plate resistor, so the driven triode sees about 44 kΩ instead of 100 kΩ, for a gain of about 38. Turn on Both triodes driven and the halves work together, halving the plate resistance, for a gain of about 75. The shared 820 Ω biases each triode as 1.6 kΩ would. Tone gets brighter clockwise. At one end a 10 nF cap shorts the plate’s treble to ground. At the other a 100 kΩ hangs off the top of the volume, and the 5 nF coupling cap passes mostly treble. With only one gain stage ahead of the power amp, most of the breakup comes from the phase inverter and the output tubes. This amp’s own power amp isn’t on the bench yet, so it opens on a stand-in, the AC15’s.

Download.

Parts and what they do

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

R3, 1 MΩ
Input resistor to ground at the jack. It’s what the guitar sees.
R4, 1 MΩ
Grid resistor of the second triode.
V1b, 12AX7 / ECC83 / 7025
The driven triode.
V1a, 12AX7 / ECC83 / 7025
The other half of the same tube, in parallel. With its grid grounded it still conducts and loads the shared plate with its plate resistance.
R7, 100 kΩ
Plate resistor shared by both triodes.
R2, 820 Ω
Cathode resistor shared by both triodes. It biases each one as 1.6 kΩ would.
C11, 50 µF electrolytic
Cathode bypass cap. With it the stage has full gain down into the bass.
C5, 5 nF
Coupling cap to the volume. 5 nF into the volume and tone network is a high-pass in the low hundreds of hertz.
C6, 10 nF
Tone cap from the plate to one end of the tone pot.
Tone, 500 kΩ pot, audio taper
Tone pot with its wiper grounded. Counterclockwise, the wiper grounds C6 and cuts treble at the plate. Clockwise, it grounds R1 instead, which loads the top of the volume.
R1, 100 kΩ
From the tone pot to the top of the volume.
Volume, 500 kΩ pot, audio taper
Channel volume, the only level control before the phase inverter.
C1, 10 nF
Coupling cap into the phase inverter.
V2a, 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.
V2b, 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.
470k, 470 kΩ
Grid resistor for the driven half, returned to the bias point above the tail, not to ground.
470k, 470 kΩ
Grid resistor for the other half.
820, 820 Ω
Bias resistor for both halves. The cathodes sit this far above the grids’ return, a volt or so.
47k, 47 kΩ
The tail resistor, to ground. It’s large, so the pair shares a nearly fixed current. When one half conducts more, the other conducts less, which splits the phase.
.022µ, 22 nF
Ties the second grid to ground for signal, so that half is driven only through the shared cathodes. There’s no feedback loop in this style.
100k, 100 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.
.01µ, 10 nF
Coupling cap to tube A’s grid.
.01µ, 10 nF
Coupling cap to tube B’s grid.
8.2k, 8.2 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current.
8.2k, 8.2 kΩ
Grid stopper for tube B.
V3, EL84 / 6BQ5
EL84, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V4, EL84 / 6BQ5
EL84, tube B of the pair.
470k, 470 kΩ
Grid leak for tube A, to ground. The bias comes from the cathode resistor. Driven into grid current, the coupling cap charges through it and the bias goes colder for a moment. That’s blocking distortion.
470k, 470 kΩ
Grid leak for tube B.
100, 100 Ω
One screen resistor shared by both tubes, 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.
125, 125 Ω
The shared cathode resistor. Every output tube’s current runs through it and lifts the cathodes above the grids, and that’s the bias. Driven hard, the pair draws more current on average, so the cathodes climb and the tubes run colder, which softens the clipping.
250µ, 250 µF electrolytic
Bypass cap across the cathode resistor, so the cathodes hold still at the signal’s frequencies. It charges and discharges slowly as the average current changes on a hard note.
1.5k, 1.5 kΩ
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
16µ, 16 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 18 Watt 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, 8 kΩ to 15 Ω, 40 H primary, 10 mH leakage
The output transformer, 8 kΩ plate to plate into 15 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, 300 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 300 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, 300 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, 225 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, 225 Ω
Resistance of the other half. Typical of the part, not measured.
V5, EZ81
One plate of the EZ81 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.
V5, EZ81
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.
Coil R, 16.3 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 15 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, 480 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, 870 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 26.8 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 3.99 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, 4.11 Ω
Limits what Loss L2 adds.
Tank R, 157 Ω
Height of the bass resonance, about 173 Ω in all at its peak. With no feedback, the voltage at the speaker rises and falls with its impedance.
Tank L, 39.6 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 117 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

About this version

ECC83
One ECC83 (12AX7). The tube model is 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.

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© 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.

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