Mic and instrument amp with bass and treble, 1939 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 EH-185.

Three 6J7 pentodes wired as triodes: a mic stage, a mixing stage for both channels, and a third stage whose output carries a bass and treble network with a choke. A 6N7 paraphase drives two cathode-biased 6L6s, each with feedback from its plate to the 6N7.

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. Three 6J7 pentodes, each wired as a triode, and none of them with much gain. The guitar goes in through 100 kΩ to the instrument volume, then 500 kΩ with .001 µF across it to V2, the mixing stage. The mic jack has its own stage first, V1, about 25 dB more for a guitar plugged in there. V2’s plate comes out through 50 kΩ and .003 µF, as the drawing’s correction has it. V3 has 10 kΩ under its cathode with no bypass, so it gains only about 4. Bass and treble work on that 10 kΩ. Each pot runs from the 6N7’s grid to a return line that reaches the 10 kΩ through .5 µF. Treble’s wiper has .01 µF to ground and bass’s wiper a choke. Down, the cap or choke shunts the 6N7’s grid and cuts its band. Up, it bypasses the 10 kΩ for that band, so V3 gains more there. The 6N7 paraphase drives two 6L6s, and 250 kΩ from each 6L6 plate back to the 6N7 plate that drives it lowers the output stage’s gain. The speaker’s field coil is the supply’s choke. The tube models are typical of the types, and the transformers and the choke are typical figures, not measured.

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 volume pot. The guitar sees it in series with the 500 kΩ pot, 600 kΩ in all.
Vol 500k, 500 kΩ pot, audio taper
Instrument volume, 500 kΩ. There’s no master, so it sets how hard the rest of the amp is pushed. The drawing marks no taper, so audio is assumed.
500k, 500 kΩ
Mixing resistor, 500 kΩ, from the instrument volume to V2’s grid. With the mic channel’s 500 kΩ it keeps one volume from shorting the other channel out.
.001µ, 1 nF
Cap across the instrument channel’s mixing resistor, .001 µF. Treble skips the 500 kΩ through it, so the instrument channel comes through brighter than the mic channel.
2M, 2 MΩ
The mic stage’s grid resistor, 2 MΩ. With nothing in the jack, the jack’s switch grounds the grid.
V1, 6J7, wired as a triode
6J7 pentode wired as a triode (screen to plate), the mic channel’s first stage. A mu of about 20, so it gains about 13 here. A guitar in the instrument jack never reaches it.
3k, 3 kΩ
Cathode resistor for V1. The drawing reads 3 V on it, 1 mA. The model reads about 3.4 V.
20µ, 20 µF electrolytic
Bypass for V1’s cathode resistor.
100k, 100 kΩ
Plate resistor for V1.
.01µ, 10 nF
Coupling cap from V1 to the mic volume.
Vol 500k, 500 kΩ pot, audio taper
Mic volume, 500 kΩ. The instruction sheet says to turn it all the way off when the mic jack isn’t in use.
500k, 500 kΩ
Mixing resistor, 500 kΩ, from the mic volume to V2’s grid. V2’s grid has no resistor of its own: the two mixing resistors and the volume pots are its path to ground.
V2, 6J7, wired as a triode
6J7 wired as a triode, the mixing stage for both channels. The drawing reads 135 V on its plate and 5.2 V on its cathode. The model reads about 155 and 5.8 V.
3k, 3 kΩ
Cathode resistor for V2.
20µ, 20 µF electrolytic
Bypass for V2’s cathode resistor.
25k, 25 kΩ
Plate resistor for V2, 25 kΩ, from the drawing’s correction. The drawing first had 100 kΩ, which can’t produce the 135 V marked there.
50k, 50 kΩ
Series resistor out of V2’s plate, 50 kΩ, from the correction. Against V3’s grid resistor beyond the cap it cuts about 0.5 dB.
.003µ, 3 nF
Coupling cap to V3, .003 µF, from the correction. V3’s grid resistor returns to a node that follows the grid, so it looks much larger than 250 kΩ, and the cap only cuts below about 60 Hz.
250k, 250 kΩ
V3’s grid resistor, 250 kΩ, returned to the node between the cathode resistor and the 10 kΩ. The drawing reads 12 V there and 15 V on the cathode, so the grid sits 3 V under the cathode.
V3, 6J7, wired as a triode
6J7 wired as a triode, the third stage. With 10 kΩ under its cathode and no bypass on it, it gains only about 4, and bass and treble work on that 10 kΩ.
3k, 3 kΩ
Cathode resistor for V3, 3 kΩ above the 10 kΩ. It sets the 3 V of bias.
20µ, 20 µF electrolytic
Bypass across V3’s 3 kΩ only. The 10 kΩ below it stays unbypassed.
10k, 10 kΩ
The 10 kΩ under V3’s cathode, not bypassed. It removes most of V3’s gain. Bass and treble bypass it for their bands through the .5 µF.
100k, 100 kΩ
Plate resistor for V3, from the first 10 kΩ node.
.1µ, 100 nF
Coupling cap from V3 to the 6N7’s grid, where the bass and treble pots also hang.
.5µ, 500 nF
.5 µF from V3’s 10 kΩ to the tone network’s return line. A cap or choke on that line bypasses the 10 kΩ for its band.
Treble 500k, 500 kΩ pot, linear taper
Treble, 500 kΩ, from the return line to the 6N7’s grid, its wiper to ground through .01 µF. Down, the cap sits on the 6N7’s grid and cuts treble, about 23 dB at 5 kHz against the middle. Up, it bypasses V3’s 10 kΩ for treble, about 8 dB more at 5 kHz. The drawing marks no taper or direction, so linear, and up is more treble.
.01µ, 10 nF
Treble cap, .01 µF from the treble pot’s wiper to ground.
Bass 500k, 500 kΩ pot, linear taper
Bass, 500 kΩ, from the return line to the 6N7’s grid, its wiper to ground through the choke. Down, the choke sits on the 6N7’s grid and cuts bass, about 10 dB at 100 Hz against the middle. Up, it bypasses V3’s 10 kΩ for bass, about 5 dB more at 100 Hz. The drawing marks no taper or direction, so linear, and up is more bass.
20 H, 20 H
The bass choke, from the bass pot’s wiper to ground. The drawing shows no value. 20 H is typical, with no winding resistance: a few hundred ohms is small beside the pot.
500k, 500 kΩ
The 6N7 grid’s 500 kΩ to ground, the grid’s DC path. The drawing has it as one of two 500 kΩ with their middle grounded.
500k, 500 kΩ
The return line’s 500 kΩ to ground, the other of the pair.
10k, 10 kΩ
Dropping resistor, 10 kΩ, from the preamp supply to V1 and V2.
16µ, 16 µF electrolytic
Filter cap, 16 µF, on V1 and V2’s supply.
V4a, 6N7
6N7, the first half of the paraphase inverter. It’s a plain gain stage: its plate swings opposite the input and drives tube A. 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.
100k, 100 kΩ
Plate resistor for the first half, the output to tube A.
1k, 1 kΩ
Cathode resistor for both halves. The tube has one cathode for its two triodes, so this carries both currents and sets both biases.
250k, 250 kΩ
Tube A’s grid resistor, down to the tap. With the resistor under the tap it feeds a small share of tube A’s grid signal to the second half’s grid. The share is about one over the second half’s gain, so the two output grids swing about the same.
12.5k, 12.5 kΩ
The tap, 12.5k Ω to ground under tube A’s grid resistor, and the second half’s grid resistor. The two sides only match while the share times the second half’s gain is 1, so they drift apart as the tube ages.
V4b, 6N7
The second half. It turns the divided signal over again, so its plate swings with the input. It drives tube B.
100k, 100 kΩ
Plate resistor for the second half, the output to tube B.
250k, 250 kΩ
Feedback, 250k Ω from tube A’s plate back to the first half’s plate, which drives tube A. The output plate swings the opposite way to the drive, so this lowers the output stage’s gain and its output impedance, which steadies the speaker. It doesn’t reach around the output transformer.
250k, 250 kΩ
The same from tube B’s plate back to the second half’s plate.
.1µ, 100 nF
Coupling cap to tube A’s grid.
.1µ, 100 nF
Coupling cap to tube B’s grid.
V5, 6L6GC
6L6GC, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V6, 6L6GC
6L6GC, tube B of the pair.
250k, 250 kΩ
Grid leak for tube B.
200, 200 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.
20µ, 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.
4 H, 4 H
The speaker’s field coil, which is the supply’s filter choke: its current makes the speaker’s magnet. On the amp it sits in the return from the HT winding’s center tap to chassis. It’s drawn here between the reservoir cap and B+, so the tubes see the same voltages. Its inductance, 4 H, is typical, not on the drawing.
750, 750 Ω
The field coil’s resistance, 750 Ω, from the drawing. All of the amp’s current runs through it. The drawing reads 106 V across it, about 141 mA. In the model it’s about 89 V, 118 mA.
16µ, 16 µF electrolytic
Filter cap on B+ after the choke, where the output transformer’s center tap and the screens hang.
10k, 10 kΩ
Dropping resistor from the screen supply node to the node for the preamp, as on the Gibson EH-185 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.
16µ, 16 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.
20k, 20 kΩ
The bleeder, 20 kΩ 20 W from B+ to ground, as on the drawing. It pulls about 14 mA all the time and drains the filter caps after the amp is switched off.
OT, 5 kΩ to 8 Ω, 40 H primary, 10 mH leakage
The output transformer, 5 kΩ plate to plate into 8 Ω, center tap on B+ after the choke. 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, 325 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 325 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, 325 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, 40 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, 40 Ω
Resistance of the other half. Typical of the part, not measured.
V7, 5U4GB
One plate of the 5U4GB 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.
V7, 5U4GB
The rectifier’s other plate, on the other half cycle.
16µ, 16 µF electrolytic
The reservoir cap. It feeds the choke, and past it the output transformer’s center tap and everything else. 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 corrected
V2’s plate as the drawing’s note corrects it. The 6J7 model is set at the type’s published triode point. The 6N7 is set at one published point. The 6L6 is the bench’s 6L6GC model. The choke, the transformers and the field coil’s inductance are typical, not on the drawing.

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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