Small 6V6 amp with instrument and mic channels, 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 Magnatone 110.

A small two-channel amp: an instrument and a microphone channel, each half a 12AX7 into a volume pot fed at its wiper, meeting at one tone control. A 12AX7 driver and cathodyne feed two 6V6s in cathode bias, with no feedback, on a 5Y3.

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. Each channel’s .05 µF goes to its volume pot’s wiper, and the two pots’ top lugs are the shared line. So the pots mix the channels as well as set their levels, and turning one up loads the other. The tone control is a 1 MΩ variable resistor and .01 µF from that line to ground. The power amp has no feedback loop, and the drawing shows no voltages or supply resistors, so those are typical.

Download.

Parts and what they do

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

100k, 100 kΩ
The instrument jack’s 100 kΩ into the grid.
100k, 100 kΩ
The second instrument jack’s 100 kΩ. With nothing in that jack its switch grounds the tip, so this ties the grid to ground: the grid’s only path there, and with the 100 kΩ in front a 6 dB divider.
V1a, 12AX7 / ECC83 / 7025
The instrument channel’s gain stage, half a 12AX7.
220k, 220 kΩ
Plate resistor, 220 kΩ.
3.3k, 3.3 kΩ
Cathode resistor, 3.3 kΩ, not bypassed.
.05µ, 50 nF
.05 µF from the plate to the volume pot’s wiper.
Instrument, 1 MΩ pot, audio taper
Instrument volume, 1 MΩ, fed at its wiper: the signal comes in at the wiper and leaves from the top lug, which both channels share. Turned down, the wiper sits near ground and the track between it and the top keeps the other channel from being shorted. Audio taper and its direction are typical, not on the drawing.
100k, 100 kΩ
The mic jack’s 100 kΩ into the grid.
100k, 100 kΩ
The second mic jack’s 100 kΩ, its tip grounded with nothing plugged in.
V1b, 12AX7 / ECC83 / 7025
The microphone channel’s gain stage, the other half of the 12AX7.
220k, 220 kΩ
Plate resistor, 220 kΩ.
3.3k, 3.3 kΩ
Cathode resistor, 3.3 kΩ, not bypassed.
.05µ, 50 nF
.05 µF from the plate to the mic volume’s wiper.
Mic, 1 MΩ pot, audio taper
Microphone volume, 1 MΩ, wired the same way. Even at 0 its track loads the shared line with 1 MΩ.
Tone, 1 MΩ pot, audio taper
Tone, 1 MΩ as a variable resistor in series with .01 µF from the shared line to ground. Turned down, less resistance, so the cap shorts more treble. Audio taper and its direction are typical, not on the drawing.
.01µ, 10 nF
The tone cap, .01 µF.
V2a, 12AX7 / ECC83 / 7025
12AX7, the driver. A cathodyne has no gain of its own, so this stage raises the preamp’s signal about 30 times, enough to drive the output tubes. Its grid connects straight to the preamp’s output with no coupling cap, so its path to ground is through the preamp’s volume and tone network.
220k, 220 kΩ
Plate resistor for the driver.
3.3k, 3.3 kΩ
Cathode resistor for the driver. It sets the driver’s bias, a volt or so.
.01µ, 10 nF
Coupling cap from the driver to the cathodyne.
V2b, 12AX7 / ECC83 / 7025
The cathodyne, or split-load inverter. 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. The cathode drives tube A, the plate tube B. Its gain is slightly under 1 on each side.
220k, 220 kΩ
The cathodyne’s plate resistor, the output to tube B.
220k, 220 kΩ
The cathodyne’s cathode load, the same value as its plate resistor, and the output to tube A. There’s no separate bias resistor.
10M, 10 MΩ
Grid resistor for the cathodyne, 10M Ω straight to its cathode, as on the drawing. With no bias resistor, the grid’s own small current through it sets the bias, a volt or so below the cathode.
.1µ, 100 nF
Coupling cap to tube A’s grid.
.1µ, 100 nF
Coupling cap to tube B’s grid.
V3, 6V6GT
6V6GT, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V4, 6V6GT
6V6GT, 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.
250, 250 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.
40µ, 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.
1k, 1 kΩ
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
20µ, 20 µF electrolytic
Filter cap on the screen supply.
10k, 10 kΩ
Dropping resistor from the screen supply node to the node for the preamp, driver and inverter, as on the Magnatone 110 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 preamp, driver and inverter. 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 8 Ω, 10 H primary, 10 mH leakage
The output transformer, 8 kΩ plate to plate 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, 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, 70 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, 70 Ω
Resistance of the other half. Typical of the part, not measured.
V5, 5Y3GT
One plate of the 5Y3GT 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, 5Y3GT
The rectifier’s other plate, on the other half cycle.
40µ, 40 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, 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

110
From Magnatone’s drawing for the 110 and 112. The 12AX7s are typical of the type, not measured.

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.