Single-ended practice amp, 1959 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 Harmony H303 and Reyes Amplifonics 303.

A whole amp from the mains to the speaker. One 12SQ7 triode drives a 50L6GT beam tube, single-ended through an output transformer to an 8 Ω speaker, with a 35Z5GT half-wave rectifier. It delivers about 1 W at 10 % distortion and slightly under 2 W flat out. The only control is the volume.

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 stage has no cathode resistor. Its grid pulls a little current through the 2.7 MΩ leak, which sets the bias about 0.85 V below ground. The 50L6GT idles in class A at about 36 mA with about 139 V on its plate. One rectifier tube charges a 40 µF cap once per mains cycle, so B+ carries about 13 V of 60 Hz ripple. R7 and 60 µF clean it up for the screen and the first stage, but the output tube’s plate is fed straight from the rippling supply, and some ripple reaches the speaker as hum. Turned up, the output stage clips. Its grid pulls current, which charges C2 and pushes the grid negative, so the tube’s average current drops and B+ rises a few volts instead of sagging. Probe B+ and the speaker, and try the ideal-supply switch to hear what the rectifier adds. The amp plays into a measured 1x12. With no feedback, and the amp at about 50 Ω seen from the speaker, the speaker voltage follows the speaker’s impedance: up about 3 dB just above the bass resonance and about 8 dB from 3 to 5 kHz. Switch off “Speaker as a reactive load” for a plain 8 Ω resistor, and compare the two in the frequency response.

Download.

Parts and what they do

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

R1, 33 kΩ
Input resistor from J1. With the second input’s 33 kΩ it mixes the two jacks. Alone it’s a grid stopper, making a low-pass with the tube’s input capacitance far above the audio band.
C1, 5 nF
Input coupling cap. With the 2.7 MΩ grid leak it passes everything above about 12 Hz. It also holds the grid-leak bias: grid current charges it negative.
R3, 2.7 MΩ
Grid leak. The 12SQ7’s cathode is grounded, so the grid current through this resistor sets the bias, about −0.85 V. A larger value biases colder: with the second drawing’s 6.8 MΩ the bias is about −0.95 V and the plate about 6 V higher.
V1, 12SQ7
12SQ7 triode (the tube’s two detector diodes are unused). It has a high mu, about 100, and runs at a fraction of a milliamp.
R4, 560 kΩ
Plate load, from the filtered screen supply. At about 0.1 mA the plate sits near 40 V, low enough that big signals run into grid current and cutoff soon.
C2, 50 nF
Coupling cap to the volume control. With the 500 kΩ it makes a high-pass near 6 Hz.
Volume, 500 kΩ pot, audio taper
Volume (R5), and the output tube’s grid resistor through its wiper. Audio taper is assumed. The owner’s model has it linear.
V3, 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.
R6, 180 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.
C3, 20 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.
R7, 22 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.
C5, 40 µF electrolytic
Filter cap on the screen supply.
C6, 20 µF electrolytic
Second filter cap on the screen supply, beside the first.
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, 125 V ac, 60 Hz
The transformer’s high-voltage winding, 125 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.
V2, 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.
C4, 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.

About this version

As drawn
The tube presets are fitted to their data sheets: Tung-Sol 12SQ7, RCA 50L6GT and GE 35Z5GT. The output transformer’s coupling and losses are assumed, from the owner’s model, not measured. Its 4.5 H primary is an LCR meter reading on a similar old 50L6 transformer, small-signal with no standing current, not the one in this amp.

Sources

  • Miguel Reyes, sent in the schematic and his LTspice model of this amp and asked for it on the bench

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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