Mic and instrument amp with a 6N7 paraphase, early 1940s

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-150 (6SQ7 version).

A mic channel with its own 6SQ7 stage and an instrument channel, mixed in two more 6SQ7s that share one plate resistor. A one-knob tone control feeds a 6N7 paraphase and two cathode-biased 6L6s. The speaker’s field coil is the supply’s choke.

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 guitar goes in through 100 kΩ into the instrument volume, then one 6SQ7 stage. The mic jack has one more 6SQ7 stage ahead of its volume, so a guitar in it gets about 30 dB more gain. The two channels’ 6SQ7s share a cathode resistor and a plate resistor, so they add into one signal. Into the 6N7, Tone is a 1 MΩ pot in series with .1 µF, beside 750 pF. Turned down, the .1 µF passes the whole range. Turned up, the pot’s resistance leaves mostly the 750 pF, which with the 100 kΩ grid resistor cuts the bass. The 6N7 is a paraphase inverter into two 6L6s with no feedback. The speaker’s field coil is the filter choke, so all the amp’s current runs through it. The tube models are typical of the types, and the transformers 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. Its wiper is V3’s grid, and the pot is that grid’s only path to ground. The drawing marks no taper, so audio is assumed.
2M, 2 MΩ
The mic stage’s grid resistor, 2 MΩ. A mic or a guitar sees it as its load. With nothing in the jack, the jack’s switch grounds the grid.
V1, 6SQ7
6SQ7, the mic channel’s first stage (its two diodes are unused). A mu of about 100, so it gains about 35 here. A guitar in the instrument jack never reaches it.
3k, 3 kΩ
Cathode resistor for V1. The 6SQ7 runs on about a third of a milliamp, so this sets about 1 V of bias.
20µ, 20 µF electrolytic
Bypass for V1’s cathode resistor, full gain down to a few hertz.
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. Its wiper is V2’s grid. The instruction sheet says to turn it all the way off when the mic jack isn’t in use.
V2, 6SQ7
6SQ7, the mic channel’s half of the mixer. It shares its plate resistor and cathode resistor with V3, so the two channels add into one signal at the plates.
V3, 6SQ7
6SQ7, the instrument channel’s stage, and the other half of the mixer. Its plate shares one resistor with V2’s. Each tube’s plate resistance loads the other’s output, so each gains less than it would alone.
1k, 1 kΩ
One cathode resistor for both mixer tubes. It carries both currents, about 0.7 mA, so the bias is only about 0.7 V, where the 6SQ7 already pulls a little grid current. That loads whatever drives each grid.
20µ, 20 µF electrolytic
Bypass for the mixer’s cathode resistor.
100k, 100 kΩ
The mixer’s one plate resistor, shared by V2 and V3.
750p, 750 pF
The small coupling cap, 750 pF, straight to the 6N7’s grid. With the 100 kΩ grid resistor it passes only treble, from about 2 kHz up. With Tone up, this is most of what gets through.
.1µ, 100 nF
The big coupling cap, .1 µF, to the tone pot. Through the pot it passes the whole range.
Tone 1M, 1 MΩ pot, audio taper
Tone, 1 MΩ as a variable resistor in series with the .1 µF. Turned down, its resistance drops and the .1 µF passes the bass. Turned up, its resistance rises against the 100 kΩ grid resistor, so mostly the 750 pF is left and the bass drops away. The drawing marks no taper or direction, so audio is assumed, and up is brighter, as on every tone control on the bench.
100k, 100 kΩ
The 6N7’s grid resistor, 100 kΩ. It’s low for a grid, so it sets where the 750 pF cuts the bass.
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.
.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.
4 H, 4 H
The speaker’s field coil, which is the supply’s filter choke: its current makes the speaker’s magnet. Everything past it runs on its current, the output tubes, the bleeder and the preamp. 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, about 115 mA here, so it drops about 86 V between the reservoir cap and B+.
10µ, 10 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 inverter, as on the Gibson EH-150 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.
10µ, 10 µF electrolytic
Filter cap at the node for the inverter. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
100k, 100 kΩ
Dropping resistor from the node for the inverter to the node for the preamp, as on the Gibson EH-150 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 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.
10µ, 10 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 drawn
The 6SQ7 is the 12SQ7’s fit (the same triode). The 6N7 is set at one published point. The 6L6 is the bench’s 6L6GC model. 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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