Single-ended pentode practice amp, early 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 Kay 703.

One 12AU6 pentode with grid-leak bias into a 50L6 beam tube, single-ended, with a 35Z5 half-wave rectifier. Volume and a treble-cut tone control. The 703-C build is the later version with an 18GD6A and a 60FX5.

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 12AU6 is a small pentode wired with a high plate resistor, 680 kΩ, and a 2.2 MΩ screen resistor, so it runs on a fraction of a milliamp and has a lot of gain. Its cathode is grounded. The grid pulls a little current through the 3.9 MΩ leak, which biases it about a volt negative. The input jacks have no switches, so one guitar sees a 22 kΩ / 22 kΩ divider and loses 6 dB. The tone control is a cap to ground through a variable resistor. Down, it cuts treble hard. The 50L6 runs on about 120 V and idles in class A at about 50 mA. Into an 8 Ω resistor it delivers about 0.8 W at 10 % harmonics and about 2.5 W flat out.

Download.

Parts and what they do

Stock values for the 703 version. Tap a part on the schematic to change it.

22k, 22 kΩ
Input resistor from the jack (R6). The other two jacks’ 22 kΩ resistors hang from the same node and do nothing with no plug in them.
22k, 22 kΩ
From the input node to ground (R6 on the 703, 220 kΩ on the 703-C). With the input resistor it halves the guitar’s signal on the 703.
.05µ, 50 nF
Input coupling cap (C3 on the 703). It also holds the grid-leak bias: grid current charges it negative.
3.9M, 3.9 MΩ
Grid leak. The cathode is grounded, so the little grid current through this resistor sets the bias, about a volt negative.
V1, 6AU6 / EF94
12AU6 pentode, the only preamp stage. Cathode and suppressor grounded.
2.2M, 2.2 MΩ
Screen resistor, from the 110 V node. At 2.2 MΩ the screen sits low and the tube runs on a fraction of a milliamp.
.05µ, 50 nF
Screen bypass cap (C3 on the 703, .1 µF on the 703-C). It holds the screen steady so the stage keeps its full gain above about 2 Hz.
680k, 680 kΩ
Plate resistor, from the 110 V node. With a pentode, a load this large means a lot of gain.
.005µ, 5 nF
Coupling cap to the volume pot (.01 µF on the 703-C). Into 500 kΩ it passes everything above about 60 Hz.
Vol 500k, 500 kΩ pot, audio taper
Volume, 500 kΩ (R1). Its wiper is the 50L6’s grid, and the pot is that grid’s only path to ground. Audio taper is assumed.
.005µ, 5 nF
The tone control’s cap, from the plate to the tone pot.
Tone 1M, 1 MΩ pot, audio taper
Tone, 1 MΩ (R2), wired as a variable resistor from the cap to ground. Up puts more resistance in series with the cap, so less treble is bled away. Up is assumed to be brighter. Audio taper is assumed.
V2, 50L6GT
50L6GT, the output tube, single-ended: it carries the whole signal, and its idle current runs through the transformer all the time. Its cathode resistor biases it. Its grid connects straight to the preamp’s volume pot, which is its path to ground.
R11, 150 Ω
The cathode resistor. The tube’s current runs through it and lifts the cathode above the grid, and that’s the bias. Driven hard, the tube draws more current on average, so the cathode climbs and the bias goes colder.
C1A, 40 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.
R4, 3.3 kΩ
Dropping resistor from B+ to the screens. It’s large, so the screens sit about 30 V below the plates and drop further when they draw more current on a hard note.
C1B, 40 µF electrolytic
Filter cap on the screen supply.
OT, 3.5 kΩ to 8 Ω, 4.5 H primary, 40 mH leakage
The output transformer, 3.5 kΩ from the plate to B+ into 8 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, 117 V ac, 60 Hz
The transformer’s high-voltage winding, 117 V ac with one end grounded, as a sine source at the mains frequency. Turn it up and every voltage in the amp goes up, the same as with a higher wall voltage.
PT R, 10 Ω
The winding’s resistance, estimated. It and the rectifier limit the charging current into the reservoir cap.
V3, 35Z5GT
The 35Z5GT half-wave rectifier. It conducts only near the positive peak of each mains cycle, topping up the reservoir cap, and its own drop rises with the current.
C1C, 40 µF electrolytic
The reservoir cap. It feeds the output transformer’s B+ end. 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, 8.69 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 8 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, 256 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, 464 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 14.3 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 2.13 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, 2.19 Ω
Limits what Loss L2 adds.
Tank R, 83.7 Ω
Height of the bass resonance, about 92 Ω in all at its peak. With no feedback, the voltage at the speaker rises and falls with its impedance.
Tank L, 21.1 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 220 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

Versions

703
The 703 drawing. The 12AU6 uses the 6AU6 model, typical of the type. The 50L6GT and 35Z5GT are fitted to their data sheets. The output transformer is typical, not on the drawing.
703-C, modeled after the Kay 703-C
The 703-C drawing of 1963: 220 kΩ in place of the 22 kΩ to ground at the input, a .01 µF coupling cap to the volume, .1 µF on the screen, 30 µF on the output cathode. There are no 18GD6A or 60FX5 models, so the 6AU6 and 50L6GT models stand in. The 36AM3 rectifier runs as the 35Z5GT.

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.

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