Tweed four-input 6L6 amp with a self-balancing paraphase and presence, 1955 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 Fender 5D8 Twin.

The first tweed Twin with treble, bass and presence. Two channels of a 12AY7 each, mixed into a 12AY7 gain stage and cathode follower, then the tone network. A 12AX7 self-balancing paraphase drives two cathode-biased 6L6Gs into two 12 in speakers, with feedback and presence.

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’s 12AY7 has one half on each jack, their plates mixed through 100 kΩ each. With the guitar in jack 1, jack 2’s switch grounds the other grid. With the guitar in jack 2, jack 1’s switch feeds the channel’s own output back into the other grid, which pulls the level far down. A 12AY7 gain stage and cathode follower drive the treble and bass network. In the power amp, the 12AX7’s first half drives tube A, and the second half has its grid at the point where both output grid resistors meet, so the two sides stay close to even. 27 kΩ from the speaker comes back to the first half’s cathode, with the presence control across it. The transformers are typical figures, not measured.

Download.

Parts and what they do

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

1M, 1 MΩ
Bright 1’s grid resistor, 1 MΩ from the jack’s tip to ground. The tip is the grid, with no 68 kΩ ahead of it.
1M, 1 MΩ
Bright 2’s grid resistor, 1 MΩ to ground.
V1a, 12AY7 / 6072
The Bright channel’s jack 1 triode, half the 12AY7. With the guitar in jack 2, jack 1’s switch ties this grid to the channel’s output, so this half feeds the output back turned over and the channel comes out far quieter.
V1b, 12AY7 / 6072
The Bright channel’s jack 2 triode, the other half. With the guitar in jack 1, jack 2’s switch grounds this grid, and the half loads the channel’s output.
100k, 100 kΩ
Plate resistor for the jack 1 triode, 100 kΩ.
100k, 100 kΩ
Plate resistor for the jack 2 triode, 100 kΩ.
100k, 100 kΩ
From the jack 1 triode’s plate to the channel’s output, 100 kΩ. With its partner it mixes the two plates.
100k, 100 kΩ
From the jack 2 triode’s plate to the channel’s output, 100 kΩ. When only the other triode is driven, this half and its plate resistor make the bottom of a divider, about 6 dB down.
.02µ, 20 nF
Coupling cap from the Bright channel’s output to the top of its volume, .02 µF 600 V. Jack 1’s switch is wired to this side of it.
Vol 1M, 1 MΩ pot, audio taper
Bright volume, 1 MΩ. Its wiper feeds the mixer. The drawing doesn’t mark the taper, so audio is assumed.
270k, 270 kΩ
Bright mixing resistor into V3a, 270 kΩ. Each channel’s volume loads the other through the two of them.
1M, 1 MΩ
Normal 1’s grid resistor, 1 MΩ from the jack’s tip to ground. The tip is the grid, with no 68 kΩ ahead of it.
1M, 1 MΩ
Normal 2’s grid resistor, 1 MΩ to ground.
V2a, 12AY7 / 6072
The Normal channel’s jack 1 triode, half the 12AY7. With the guitar in jack 2, jack 1’s switch ties this grid to the channel’s output, so this half feeds the output back turned over and the channel comes out far quieter.
V2b, 12AY7 / 6072
The Normal channel’s jack 2 triode, the other half. With the guitar in jack 1, jack 2’s switch grounds this grid, and the half loads the channel’s output.
100k, 100 kΩ
Plate resistor for the jack 1 triode, 100 kΩ.
100k, 100 kΩ
Plate resistor for the jack 2 triode, 100 kΩ.
100k, 100 kΩ
From the jack 1 triode’s plate to the channel’s output, 100 kΩ. With its partner it mixes the two plates.
100k, 100 kΩ
From the jack 2 triode’s plate to the channel’s output, 100 kΩ. When only the other triode is driven, this half and its plate resistor make the bottom of a divider, about 6 dB down.
.02µ, 20 nF
Coupling cap from the Normal channel’s output to the top of its volume, .02 µF 600 V. Jack 1’s switch is wired to this side of it.
Vol 1M, 1 MΩ pot, audio taper
Normal volume, 1 MΩ. Its wiper feeds the mixer. The drawing doesn’t mark the taper, so audio is assumed. Down, it grounds the Normal mixing resistor, which loads the Bright channel.
270k, 270 kΩ
Normal mixing resistor into V3a, 270 kΩ. Each channel’s volume loads the other through the two of them.
470, 470 Ω
Cathode resistor shared by all four input triodes, 470 Ω. It carries all four currents, so each half sees about four times its value.
250µ, 250 µF electrolytic
Bypass for the shared cathode, 250 µF 6 V. Every input stage gets full gain down to the lowest notes.
100p, 100 pF
Bright cap, 100 pF from the top of the Bright volume to its wiper. It lifts the top end with the volume down and does nothing at full.
V3a, 12AY7 / 6072
Gain stage after the mixer, half of a 12AY7. Its cathode is bypassed, so it gains about 30.
100k, 100 kΩ
Plate resistor for V3a.
1.5k, 1.5 kΩ
V3a cathode resistor, 1500 Ω.
25µ, 25 µF electrolytic
V3a cathode bypass, 25 µF 25 V.
V3b, 12AY7 / 6072
Cathode follower, the other half of the 12AY7, its grid straight on V3a’s plate. Its gain is slightly under 1, and its low output impedance drives the tone network.
100k, 100 kΩ
Cathode follower load, 100 kΩ.
220p, 220 pF
Treble cap, 220 pF (.00022 µF on the drawing), into the bright end of the treble pot.
Treble 1M, 1 MΩ pot, audio taper
Treble, 1 MΩ. Its wiper is the driver’s grid. One end has the 220 pF from the follower, the other goes to ground through .003 µF. Clockwise moves the wiper toward the 220 pF, so more treble.
.003µ, 3 nF
From the far end of the treble pot to ground, .003 µF 600 V. Turned down, the wiper sits near it and the highs bleed off.
.1µ, 100 nF
Coupling cap from the follower into the bass path, .1 µF 200 V.
270k, 270 kΩ
From the bass path to ground, 270 kΩ. It holds the node at 0 V and loads the follower lightly.
270k, 270 kΩ
From the bass path to the top of the bass pot, 270 kΩ.
Bass 2M, 2 MΩ pot, audio taper
Bass, 2 MΩ, with a cap across each half of its track and its wiper on the way to the driver. Clockwise the wiper moves to the top and the lows come straight through, with the .0047 µF and 56 kΩ under them. Counterclockwise the wiper shorts the .0047 µF to the 56 kΩ, and only the highs get through the 100 pF.
100p, 100 pF
From the bass pot’s top to its wiper, 100 pF (.0001 µF). It carries the highs around the upper track.
.0047µ, 4.7 nF
From the bass pot’s bottom to its wiper, .0047 µF. With the 56 kΩ it sets how much of the lows drain to ground.
56k, 56 kΩ
From the bass pot’s bottom to ground, 56 kΩ.
270k, 270 kΩ
From the bass pot’s wiper to the driver’s grid, 270 kΩ. The bass path and the treble wiper mix here.
V4a, 12AX7 / ECC83 / 7025
12AX7, the first half of the paraphase inverter. It’s a plain gain stage: its plate swings opposite the input and drives tube A. Its cathode is the feedback point. 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.
220k, 220 kΩ
Tube A’s grid resistor, 220k Ω down to the tap. Tube B’s grid resistor comes up to the same tap, and the tap is the second half’s grid. The two output grids swing opposite ways, so the tap gets only what’s left over when they don’t match. The second half turns that over and drives tube B with it, so the inverter keeps the two sides close to even as the tubes age.
56k, 56 kΩ
The tap, 56k Ω to ground, and the second half’s grid resistor. Both output tubes’ grid resistors return through it, so it’s part of each one’s grid leak.
V4b, 12AX7 / ECC83 / 7025
The second half. Its grid is the tap between the two output grids. It turns what’s there over and drives tube B, so tube B’s grid swings opposite tube A’s.
100k, 100 kΩ
Plate resistor for the second half, the output to tube B.
1.5k, 1.5 kΩ
Cathode resistor for the second half, straight to ground.
100p, 100 pF
A small cap from plate to plate. It rolls off the far top end the inverter passes on.
27k, 27 kΩ
The feedback resistor from the speaker. With the 1.5k Ω below it, it sends about 5 % of the speaker voltage back to the first half’s cathode, which lowers the gain, flattens the response and tightens the bass.
1.5k, 1.5 kΩ
The first half’s cathode resistor, 1.5k Ω to ground, not bypassed. It sets that half’s bias, and 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 the cathode. The presence cap and pot across it bypass part of it in the treble.
.1µ, 100 nF
The presence cap, in series with the pot across the leg. With the pot it sets where the lift starts.
Presence, 5 kΩ pot, linear taper
Presence, a 5k linear pot as a variable resistor, its resistance falling as it’s turned up, in series with the cap across the leg. Turned up, the cap shunts more of the leg’s treble, so less treble is fed back and the top end rises.
.05µ, 50 nF
Coupling cap to tube A’s grid.
.05µ, 50 nF
Coupling cap to tube B’s grid.
V5, 5881
n5881, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V6, 5881
n5881, tube B of the pair.
270k, 270 kΩ
Tube B’s grid resistor, 270k Ω up to the tap, the inverter’s second grid, which sits near ground. With tube A’s resistor and the one under the tap, it sets how much of tube B’s swing reaches the second half’s grid.
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.
10k, 10 kΩ
Dropping resistor from B+ to the screens. It’s large, so the screens sit about 30 V below the plates and drop further when they draw more current on a hard note.
16µ, 16 µF electrolytic
Filter cap on the screen supply.
10k, 10 kΩ
Dropping resistor from the screen supply node to the node for the inverter, as on the Fender 5D8 Twin 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.
16µ, 16 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.
10k, 10 kΩ
Dropping resistor from the node for the inverter to the node for the preamp, as on the Fender 5D8 Twin 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.
8µ, 8 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, 4 kΩ to 4 Ω, 40 H primary, 10 mH leakage
The output transformer, 4 kΩ plate to plate into 4 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, 330 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 330 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, 330 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, 50 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, 50 Ω
Resistance of the other half. Typical of the part, not measured.
V7, 5U4G
One plate of the 5U4G 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.
V8, 5U4G
The rectifier’s other plate, on the other half cycle.
16µ, 16 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.
16µ, 16 µF electrolytic
Another reservoir cap, in parallel with the first.
Coil R, 4.34 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 4 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, 128 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, 232 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 7.15 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 1.06 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, 1.09 Ω
Limits what Loss L2 adds.
Tank R, 41.9 Ω
Height of the bass resonance, about 46 Ω in all at its peak. The feedback holds the output steadier against that than it would be without feedback.
Tank L, 10.6 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 440 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

About this version

5D8
The 5D8 drawing: a 12AY7 for each input channel and another for the gain stage and cathode follower. The tube models are fitted to their data sheets, 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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