Two-channel 6SL7 amp with shared cathode and bias tremolo, late 1950s

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 Ampeg Mercury M-12.

Instrument and accordion channels on the two halves of one 6SL7, which share a cathode resistor, each with Volume and Tone, mixed through 270 kΩ into a 6SL7 floating paraphase with feedback. Two cathode-biased 6V6s, with a dual-triode tremolo swinging their grids.

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 6SL7 stage, then Volume, with Tone’s wiper on the Volume’s top: one end of Tone sends treble around the Volume, the other sends it to ground. The two halves share a 2.2 kΩ cathode resistor with 100 µF across it. The accordion jack’s switch ties the instrument grid to the accordion grid until something goes in Acc. 1, so a guitar in an Inst. jack plays through both channels. The tremolo runs one half of a 6SL7 as the oscillator and the other half’s plate sends the swing through Intensity to the output tubes’ grid leaks. The tube models are typical of the types, not measured.

Download.

Parts and what they do

Stock values for the M-12 (6V6) version. Tap a part on the schematic to change it.

47k, 47 kΩ
Inst. 1’s resistor to the instrument grid, 47 kΩ.
47k, 47 kΩ
Inst. 2’s resistor to the instrument grid, 47 kΩ. Two guitars in the two jacks mix here.
1M, 1 MΩ
Grid leak for the instrument half, 1 MΩ.
5.6M, 5.6 MΩ
Grid leak for the accordion half, 5.6 MΩ.
V1a, 6SL7 (RCA)
Half of a 6SL7, the instrument channel’s only gain stage. It shares its cathode resistor with the other half.
V1b, 6SL7 (RCA)
The other half of the 6SL7, the accordion channel’s gain stage, on the same cathode resistor.
270k, 270 kΩ
Plate resistor for V1a, from node C.
270k, 270 kΩ
Plate resistor for V1b, from node C.
2.2k, 2.2 kΩ
The shared cathode resistor for both halves of V1, 2.2 kΩ. Both halves’ current runs through it, so each sits at about twice the bias it would get alone.
100µ, 100 µF electrolytic
Bypass for the shared cathode resistor, 100 µF. It keeps the two channels from feeding each other through the cathode.
.02µ, 20 nF
Coupling cap to the instrument Volume’s top and its Tone pot’s wiper.
Volume 1M, 1 MΩ pot, audio taper
The instrument channel’s Volume, 1 MΩ. Its wiper feeds the mixing node through 270 kΩ. The drawing marks no taper, so audio is assumed.
Tone 500k, 500 kΩ pot, audio taper
The instrument channel’s Tone, 500 kΩ. The signal comes in on the wiper. One end goes through .002 µF to the Volume’s wiper, the other through .01 µF to ground. Turned up, the wiper moves toward the .002 µF and the top end comes up. The taper isn’t marked, so audio is assumed.
.002µ, 2 nF
From the Tone pot’s bright end to the Volume’s wiper, .002 µF. It lets treble around the Volume, more with the Volume down.
.01µ, 10 nF
From the Tone pot’s other end to ground, .01 µF. With Tone down it pulls treble off the Volume’s top.
270k, 270 kΩ
Mixing resistor, 270 kΩ, from the instrument Volume’s wiper to V3’s grid, where the other channel’s 270 kΩ meets it.
.1µ, 100 nF
Coupling cap to the accordion Volume’s top and its Tone pot’s wiper.
Volume 1M, 1 MΩ pot, audio taper
The accordion channel’s Volume, 1 MΩ. Its wiper feeds the mixing node through 270 kΩ. The drawing marks no taper, so audio is assumed.
Tone 500k, 500 kΩ pot, audio taper
The accordion channel’s Tone, 500 kΩ. The signal comes in on the wiper. One end goes through .02 µF to the Volume’s wiper, the other through .02 µF to ground. Turned up, the wiper moves toward the .02 µF and the top end comes up. The taper isn’t marked, so audio is assumed.
.02µ, 20 nF
From the Tone pot’s bright end to the Volume’s wiper, .02 µF. It lets treble around the Volume, more with the Volume down.
.02µ, 20 nF
From the Tone pot’s other end to ground, .02 µF. With Tone down it pulls treble off the Volume’s top.
270k, 270 kΩ
Mixing resistor, 270 kΩ, from the accordion Volume’s wiper to V3’s grid, where the other channel’s 270 kΩ meets it.
V2a, 6SL7 (RCA)
Half of a 6SL7, the tremolo oscillator.
V2b, 6SL7 (RCA)
The other half, its grid and cathode joined to the oscillator’s. It copies the oscillator’s swing onto its own plate, so the ladder isn’t loaded by the Intensity.
270k, 270 kΩ
Plate resistor for the oscillator, from node B, the screens’ node.
270k, 270 kΩ
Plate resistor for the tremolo’s output half, from node B.
1k, 1 kΩ
The tremolo’s cathode resistor, 1 kΩ on the M-12 and 2.2 kΩ on the M-15. Its far end goes to ground through the foot switch.
25µ, 25 µF electrolytic
Bypass for the tremolo’s cathodes.
.05µ, 50 nF
First cap of the oscillator’s phase-shift ladder.
120k, 120 kΩ
First ladder leg to ground.
.05µ, 50 nF
Second ladder cap.
120k, 120 kΩ
In series with the Speed pot, the ladder’s second leg. It sets the top speed.
Speed 500k, 500 kΩ pot, audio taper
Speed, 500 kΩ, as a variable resistor in the ladder’s second leg to ground. Turned up, its resistance drops and the oscillator runs faster. The taper isn’t marked, so audio is assumed.
.05µ, 50 nF
Third ladder cap, into both grids.
270k, 270 kΩ
Grid resistor for both tremolo grids, and the ladder’s last leg. Its value isn’t printed on the drawing, so 270 kΩ is assumed, as on the other Ampegs.
.1µ, 100 nF
Couples the swing from the output half’s plate and blocks its DC.
470k, 470 kΩ
In series ahead of the Intensity pot, 470 kΩ. With the pot’s 1 MΩ it sets the most swing that reaches the grids.
Intensity 1M, 1 MΩ pot, audio taper
Intensity, 1 MΩ, to ground. Its wiper is where the output tubes’ two grid leaks meet, so it sets how much of the swing reaches both grids together. The taper isn’t marked, so audio is assumed.
V3a, 6SL7 (RCA)
6SL7GT, the driven half of the floating paraphase inverter. Its plate swings opposite the input and drives tube A. The feedback from the speaker goes to its cathode. Its grid connects straight to the preamp’s mixing resistors, which are its path to ground.
V3b, 6SL7 (RCA)
The other half. Its grid is fed from the middle of the divider, so its plate swings the other way and drives tube B.
470k, 470 kΩ
Grid resistor for the other half, to ground.
300p, 300 pF
Across the other half’s grid resistor, 300pF, as on the drawing. Above a few kilohertz it shunts the signal from the divider’s middle, so the second half gets less of it and tube B’s side gets less treble than tube A’s.
1k, 1 kΩ
Cathode resistor for both halves. It sets their bias, a couple of volts.
100µ, 100 µF electrolytic
Bypass cap across the shared cathode resistor. The cathodes hold still, so the second half gets its signal only from the divider.
47k, 47 kΩ
Plate resistor for the driven half.
47k, 47 kΩ
Plate resistor for the other half.
470k, 470 kΩ
Top of the divider, from tube A’s plate. With the resistor below it, from tube B’s, the middle sits at the average of the two sides. With the two swinging equal and opposite, the middle holds still. Whatever is left goes to the second half’s grid, which corrects it, so the inverter keeps itself even as the tube ages.
510k, 510 kΩ
Bottom of the divider, to tube B’s plate.
.02µ, 20 nF
From the middle of the divider to the second half’s grid. It keeps the plates’ DC off that grid.
8.2k, 8.2 kΩ
The feedback resistor from the speaker. With the 240 Ω below it, it sends about 3 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
240, 240 Ω
The driven half’s own cathode resistor, 240 Ω, between its cathode and the shared cathode resistor, not bypassed. It’s also the bottom of the feedback loop: the feedback resistor from the speaker connects at the top of it, so the fed-back speaker voltage moves that cathode. It lowers that half’s gain too.
.02µ, 20 nF
Coupling cap to tube A’s grid.
.02µ, 20 nF
Coupling cap to tube B’s grid.
1k, 1 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current.
1k, 1 kΩ
Grid stopper for tube B.
V4, 6V6GT
6V6GT, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V5, 6V6GT
6V6GT, tube B of the pair.
270k, 270 kΩ
Grid leak for tube A, returned with tube B’s to the tremolo’s Depth pot, so the oscillator swings both grids together. 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.
270k, 270 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.
25µ, 25 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.
22k, 22 kΩ
Dropping resistor from the screen supply node to the node for the inverter and both channels, as on the Ampeg Mercury M-12 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 inverter and both channels. 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, 20 H primary, 12 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.
V6, 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.
V6, 5Y3GT
The rectifier’s other plate, on the other half cycle.
20µ, 20 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. The feedback holds the output steadier against that than it would be without feedback.
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. The feedback holds the output steadier against that than it would be without feedback.
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.

Versions

M-12 (6V6)
The M-12 drawing: two 6SL7s in the preamp and tremolo, two 6V6s. The 6SL7 model is fitted to the RCA plate curves. The output transformer is assumed to be 8 kΩ, typical for a 6V6 pair, not on the drawing.
M-12-A (7591), modeled after the Ampeg Mercury M-12-A
The M-12-A drawing: the M-12’s preamp part for part, with 7591s on 140 Ω in the power amp. Here it runs on the M-12 power amp with 7591 tubes, 140 Ω, a 6.6 kΩ transformer and a 335 V winding, typical, not on the drawing.
M-15 (6L6), modeled after the Ampeg M-15
The M-15 drawing: 47 kΩ from each Acc. jack to the grid and 2.2 kΩ on the tremolo’s cathodes. Its power amp has 6L6s, 120 kΩ inverter plate resistors, a 5U4 and 30 µF on the rectifier. Here it runs on the M-12’s power amp with those parts, a 4 kΩ transformer and a 335 V winding, typical, not on the drawing. The 6L6s use the 6L6GC model.

Sources

  • Joe Piazza, redrawn M-12, M-12-A and M-15 schematics, 1997

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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