Two-channel bass amp with bass and treble, 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 Gibson GA-85.

Two 12AX7 channels, each with a bass control, mixed through their volume pots into a 12AX7 driver and cathodyne and a pair of 5881s. The screens run off a 6V6 cathode follower.

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 is one 12AX7 half into a passive tone network and a volume pot, and the two meet at the driver’s grid through 100 kΩ each. The bass control is a variable resistor in a path that only passes the lows: 150 kΩ into .01 µF to ground, then the pot and 200 kΩ. The highs go around it through 330 pF, on channel I through the treble pot, which sets how much of them reaches the volume. From 0 to full, bass lifts 80 Hz about 12 dB and treble lifts 2.5 kHz about 40 dB. The volume pots are fed at the wiper, with the output at the top, and channel I has 500 pF across its upper half, so it gets brighter as it’s turned down. The preamp itself stays clean. The 12AX7 driver and cathodyne after it bring the gain, and the pair of 5881s delivers about 30 W before it clips hard. The tube models are typical of the types, not measured.

Download.

Parts and what they do

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

R1, 220 kΩ
The ch i channel’s input 1 resistor into the grid. With nothing in input 2 it sits in parallel with input 2’s resistor, so a guitar in input 1 sees 110 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R3, 220 kΩ
The ch i channel’s input 2 resistor into the grid. With nothing in input 1, input 1’s tip is grounded, so the two 220 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 440 kΩ.
R2, 2.2 MΩ
Input 1’s 2.2 MΩ to ground. It sets the load a guitar in input 1 sees, and with a patch lead in input 1 it is the end of the lead’s path. On the drawing it sits on the grid. The difference is under half a dB.
R4, 220 kΩ
The ch ii channel’s input 1 resistor into the grid. With nothing in input 2 it sits in parallel with input 2’s resistor, so a guitar in input 1 sees 110 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R6, 220 kΩ
The ch ii channel’s input 2 resistor into the grid. With nothing in input 1, input 1’s tip is grounded, so the two 220 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 440 kΩ.
R5, 2.2 MΩ
Input 1’s 2.2 MΩ to ground. It sets the load a guitar in input 1 sees, and with a patch lead in input 1 it is the end of the lead’s path. On the drawing it sits on the grid. The difference is under half a dB.
V1a, 12AX7 / ECC83 / 7025
Half of the 12AX7, channel I’s only gain stage.
470k, 470 kΩ
Plate resistor, 470 kΩ. The drawing reads +150 V on the plate.
3.3k, 3.3 kΩ
Cathode resistor, +1.28 V on the drawing.
20µ, 20 µF electrolytic
Cathode bypass cap, full gain down to about 2 Hz.
.05µ, 50 nF
Coupling cap into the tone network.
150k, 150 kΩ
Into the bass path. With the .01 µF after it, it lets through only the lows, below about 100 Hz.
.01µ, 10 nF
The bass path’s cap to ground. The drawing hangs it on the bass pot’s wiper, which is looped to this end.
Bass 1.8M, 1.8 MΩ pot, reverse audio taper
Channel I bass, 1.8 MΩ as a variable resistor in the bass path. Turned up, its resistance drops and more of the lows get through. The drawing marks no taper, so reverse audio is assumed: it spreads the change across the knob.
200k, 200 kΩ
The end of the bass path, so the bass pot at full can’t short the treble side.
330p, 330 pF
Into the treble path, .00033 µF: only the highs get through.
Treble 2M, 2 MΩ pot, linear taper
Channel I treble, 2 MΩ linear. The treble path comes in at its top, its bottom goes to ground through .005 µF, and the wiper is the network’s output. Turned up, the wiper moves toward the top and more treble reaches the volume.
.005µ, 5 nF
Under the treble pot. Turned down, the wiper hangs this cap on the output and cuts treble.
500p, 500 pF
Across the volume pot’s upper half, .0005 µF. With the volume down it lets the treble past the pot, so the channel gets brighter as it gets quieter.
Vol 2M, 2 MΩ pot, linear taper
Channel I volume, 2 MΩ linear. The signal comes in on the wiper and leaves from the top end through 100 kΩ. Turned down, the wiper moves toward ground and shorts more of it away.
100k, 100 kΩ
Mixing resistor from channel I to the driver’s grid.
V1b, 12AX7 / ECC83 / 7025
The other half of the 12AX7, channel II’s gain stage.
470k, 470 kΩ
Plate resistor, +150 V on the drawing.
3.3k, 3.3 kΩ
Cathode resistor, +1.28 V on the drawing.
20µ, 20 µF electrolytic
Cathode bypass cap.
.05µ, 50 nF
Coupling cap into the tone network.
150k, 150 kΩ
Into the bass path, as on channel I.
.01µ, 10 nF
The bass path’s cap to ground.
Bass 1.8M, 1.8 MΩ pot, reverse audio taper
Channel II bass, the same as channel I’s. Turned up, more of the lows get through. The drawing marks no taper, so reverse audio is assumed.
200k, 200 kΩ
The end of the bass path.
330p, 330 pF
Straight across the bass path, .00033 µF. Channel II has no treble pot, so the highs always get through this way.
Vol 2M, 2 MΩ pot, linear taper
Channel II volume, 2 MΩ linear, wired like channel I’s but with no cap across it.
100k, 100 kΩ
Mixing resistor from channel II to the driver’s grid.
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.
470k, 470 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.
20µ, 20 µF electrolytic
Bypass cap across the driver’s cathode resistor, for full gain above a few hertz.
.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.
51k, 51 kΩ
The cathodyne’s plate resistor, the output to tube B.
1k, 1 kΩ
The cathodyne’s bias resistor. The grid returns below it, so the cathode sits this far above the grid.
51k, 51 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.
470k, 470 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.
.047µ, 47 nF
Coupling cap to tube A’s grid.
.047µ, 47 nF
Coupling cap to tube B’s grid.
V3, 5881
n5881, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V4, 5881
n5881, 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.
270, 270 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.
Screens, 350 V
The screen regulator, standing in for V5: a 6V6 wired as a triode and run as a cathode follower, its plate on B+ and its grid on 22 kΩ and 100 kΩ from B+ to ground. Its cathode holds the screens near the drawing’s 350 V and feeds the preamp’s chain. Here it’s a steady source behind 300 Ω, about the follower’s own output resistance, so it doesn’t sag with B+ the way the real one would.
4.7k, 4.7 kΩ
Dropping resistor from the screen supply node to the node for the driver and inverter, as on the Gibson GA-85 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.
20µ, 20 µF electrolytic
Filter cap at the node for the 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.
4.7k, 4.7 kΩ
Dropping resistor from the node for the driver and inverter to the node for the input stages, as on the Gibson GA-85 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.
10µ, 10 µF electrolytic
Filter cap at the node for the input stages. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
OT, 6.6 kΩ to 8 Ω, 40 H primary, 10 mH leakage
The output transformer, 6.6 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, 348 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 348 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, 348 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, 60 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, 60 Ω
Resistance of the other half. Typical of the part, not measured.
V5, 5V4G
One plate of the 5V4G 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, 5V4G
The rectifier’s other plate, on the other half cycle.
20µ, 20 µF electrolytic
The reservoir caps’ first. 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.
20µ, 20 µF electrolytic
Another reservoir cap, in parallel with the first.
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 12AX7 model is fitted to datasheet curves, 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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