Pentode channel with a passive bass and treble network, 1958 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 Selmer Stadium (TV19/T).

An EF86 pentode stage and its volume, a low-gain triode, a passive bass and treble network and a second triode. Then a driver and cathodyne into two EL84s with feedback.

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 EF86 does nearly all the gain, about 150 times, as on the AC15’s normal channel. After the volume, the 680 kΩ mixing resistor into channel 2’s 270 kΩ drops the signal about 11 dB. V4a, half an ECC82 with only 22 kΩ on its plate, makes about 7 times back and drives the bass and treble network from a low impedance. The network loses about 21 dB with its knobs at the middle, and V4b makes another 7 times back. In the power amp, a 12AX7 driver and a cathodyne feed two EL84s on one cathode resistor, with 5 kΩ of feedback from the 15 Ω tap, about 15 dB of it. Channel 2 and the tremolo aren’t modeled.

Download.

Parts and what they do

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

R4, 220 kΩ
From the jack’s tip to ground, 220 kΩ. With R1 it divides the guitar down a little and sets the load the pickup sees, about 150 kΩ.
R1, 470 kΩ
Into the EF86’s grid, 470 kΩ. With the tube’s input capacitance it trims the top end slightly.
V1, EF86 / 6267
EF86 pentode, channel 1’s first stage. With a 220 kΩ plate resistor and 1 MΩ to its screen it gains well over 100.
R11, 220 kΩ
The EF86’s plate resistor, 220 kΩ.
R8, 1 MΩ
The EF86’s screen resistor, 1 MΩ. The screen sags when the stage is driven hard, which softens its clipping.
C1, 100 nF
Screen bypass. Its value isn’t in the drawing’s table, so a typical .1 µF is assumed.
R12, 2 kΩ
The EF86’s cathode resistor, 2 kΩ.
C3, 50 µF electrolytic
Cathode bypass, 50 µF 25 V.
C5, 100 nF
Coupling cap to the volume, .1 µF.
Volume, 1 MΩ pot, audio taper
Channel 1’s volume, 1 MΩ.
R19, 680 kΩ
Mixing resistor from channel 1, 680 kΩ.
R21, 270 kΩ
Channel 2’s mixing resistor, 270 kΩ. Channel 2 isn’t modeled. With its volume at 0 this goes to ground, so with R19 it makes a divider that drops channel 1 by about 11 dB.
V4a, 12AU7 / ECC82
Half an ECC82, a low-gain stage ahead of the tone network: 22 kΩ plate, 1 kΩ cathode unbypassed. Gain about 7, and a low output impedance to drive the network.
R25, 22 kΩ
V4a’s plate resistor, 22 kΩ.
R26, 1 kΩ
V4a’s cathode resistor, 1 kΩ, not bypassed.
C13, 100 nF
Coupling cap into the tone network, .1 µF.
C19, 75 pF
Treble cap, 75 pF, to the top of the treble pot.
Treble, 1 MΩ pot, linear taper
Treble, 1 MΩ. It gets the signal through 75 pF with .01 µF to ground at its far end, and its wiper goes straight to V4b’s grid. Up moves the wiper toward the 75 pF, for more treble.
C20, 10 nF
From the treble pot’s far end to ground, .01 µF.
R29, 220 kΩ
Top of the bass section, 220 kΩ.
Bass, 1 MΩ pot, linear taper
Bass, 1 MΩ. Its wiper is the network’s output and V4b’s grid. Up moves the wiper toward the 220 kΩ end, for more bass.
C14, 1 nF
Across the bass pot’s upper half, .001 µF.
C15, 10 nF
Across the bass pot’s lower half, .01 µF.
R30, 22 kΩ
Bottom of the bass section, 22 kΩ to ground.
V4b, 12AU7 / ECC82
The other half of the ECC82, a recovery stage after the tone network: 22 kΩ plate, 1 kΩ cathode unbypassed. Gain about 7.
R35, 22 kΩ
Dropping resistor to V4b’s supply, 22 kΩ.
C21, 8 µF electrolytic
Filter cap on V4b’s supply, 8 µF.
R33, 22 kΩ
V4b’s plate resistor, 22 kΩ.
R34, 1 kΩ
V4b’s cathode resistor, 1 kΩ, not bypassed.
C22, 100 nF
Coupling cap to the power amp, .1 µF, through the plug between the two chassis.
V1a, 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 is fed through the preamp’s coupling cap, so there’s no second one here.
1M, 1 MΩ
Grid resistor for the driver.
100k, 100 kΩ
Plate resistor for the driver.
1.5k, 1.5 kΩ
Cathode resistor for the driver. It sets the driver’s bias, a volt or so. It sits on the feedback leg, so the fed-back signal moves the driver’s cathode.
25µ, 25 µF electrolytic
Bypass cap across the driver’s cathode resistor, for full gain above a few hertz. It goes to the top of the feedback leg, not to ground, so the feedback still reaches the cathode.
.01µ, 10 nF
Coupling cap from the driver to the cathodyne.
V1b, 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.
68k, 68 kΩ
The cathodyne’s plate resistor, the output to tube B.
2k, 2 kΩ
The cathodyne’s bias resistor. The grid returns below it, so the cathode sits this far above the grid.
68k, 68 kΩ
The cathodyne’s cathode load, the same value as its plate resistor. The cathode above the bias resistor is the output to tube A.
220k, 220 kΩ
Grid resistor for the cathodyne, returned to the junction of its cathode resistors so the grid sits near the cathode’s high DC voltage.
5k, 5 kΩ
The feedback resistor from the speaker. With the 100 Ω below it, it sends about 2 % of the speaker voltage back to the driver’s cathode, which lowers the gain, flattens the response and tightens the bass.
100, 100 Ω
The bottom of the feedback loop, 100 Ω to ground under the driver’s cathode resistor. It isn’t bypassed, so the fed-back speaker voltage moves the cathode. The driver’s current runs through it too, so it’s kept small.
.02µ, 20 nF
Coupling cap to tube A’s grid.
.02µ, 20 nF
Coupling cap to tube B’s grid.
V2, EL84 / 6BQ5
EL84, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V3, EL84 / 6BQ5
EL84, tube B of the pair.
220k, 220 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.
220k, 220 kΩ
Grid leak for tube B.
100, 100 Ω
Screen resistor for tube A. It limits the screen current when the tube is driven hard.
100, 100 Ω
Screen resistor for tube B.
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.
50µ, 50 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 node for the driver and cathodyne, as on the Selmer Stadium 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.
32µ, 32 µF electrolytic
Filter cap at the node for the driver and cathodyne. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
15k, 15 kΩ
Dropping resistor from the node for the driver and cathodyne to the node for the preamp, as on the Selmer Stadium 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.
32µ, 32 µF electrolytic
Filter cap at the node for 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, 600 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, 600 Ω
Resistance of the other half. Typical of the part, not measured.
V4, 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.
V4, 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. The feedback holds the output steadier against that than it would be without feedback.
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. The feedback holds the output steadier against that than it would be without feedback.
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

As drawn, 1958
An EF86 and an ECC82 (12AU7), as the drawing’s table lists them. The tube models 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.

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