Tweed two-input 6L6 amp with a cathodyne and presence, mid 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 Fender 5E6-A Bassman.

Two 12AY7 input channels mixed into another 12AY7 as gain stage and cathode follower, then a treble and bass network with its own feedback loop. A gain stage and cathodyne drive two fixed-bias 6L6Gs into four 10 in speakers, with feedback and presence from the speaker.

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 preamp is the 5E7 Bandmaster’s shape with two plain jacks and a 12AY7 in V2, so it has about half the gain ahead of the tone network, and 10 MΩ from the bass path back to V2a’s grid cuts the gain about 3 dB. In the power amp, a 12AX7 driver and cathodyne feed two 6L6Gs at about -42 V of fixed bias into four 10 in speakers, with 20 kΩ of feedback onto the driver’s cathode and a presence control there. The feedback is strong, so the amp stays tight and clean until it delivers about 37 W.

Download.

Parts and what they do

Stock values for the 5E6-A version. Tap a part on the schematic to change it.

1M, 1 MΩ
Grid resistor for channel 1, 1 MΩ from the jack’s tip to ground. The guitar sees it straight, with no 68 kΩ ahead of the grid.
1M, 1 MΩ
Grid resistor for channel 2, 1 MΩ to ground.
V1a, 12AY7 / 6072
Channel 1 input stage, half of the 12AY7. With a mu of about 45 it gains about 25, slightly over half a 12AX7 stage.
100k, 100 kΩ
Plate resistor for channel 1.
V1b, 12AY7 / 6072
Channel 2 input stage, the other half of the 12AY7. With nothing plugged in, its jack grounds its grid, and it still pulls current through the shared cathode resistor.
100k, 100 kΩ
Plate resistor for channel 2.
820, 820 Ω
Cathode resistor shared by both 12AY7 halves. It carries both currents, so each half sees about twice its value.
250µ, 250 µF electrolytic
Bypass for the shared cathode, 250 µF 6 V. Both channels get full gain down to the lowest bass notes.
.02µ, 20 nF
Coupling cap from channel 1’s plate to the top of its volume, .02 µF 600 V.
Vol 1M, 1 MΩ pot, audio taper
Channel 1 volume, 1 MΩ. Its wiper feeds the mixer. The drawing doesn’t mark the taper, so audio is assumed.
100p, 100 pF
Bright cap, 100 pF from the top of volume 1 to its wiper. It lifts the top end with the volume down and does nothing at full.
270k, 270 kΩ
Channel 1 mixing resistor. With channel 2’s it mixes the two channels into V2a, and each channel’s volume loads the other through them.
.02µ, 20 nF
Coupling cap from channel 2’s plate to the top of its volume.
Vol 1M, 1 MΩ pot, audio taper
Channel 2 volume, 1 MΩ, audio taper here. Down, it grounds channel 2’s mixing resistor, which loads channel 1.
270k, 270 kΩ
Channel 2 mixing resistor.
V2a, 12AY7 / 6072
Gain stage after the mixer, half of a 12AY7. Its cathode is bypassed, so it gains about 30 on its own. The 10 MΩ from the tone network’s bass path cancels some of that.
100k, 100 kΩ
Plate resistor for V2a.
1.5k, 1.5 kΩ
V2a cathode resistor, 1500 Ω.
25µ, 25 µF electrolytic
V2a cathode bypass, 25 µF 25 V.
V2b, 12AY7 / 6072
Cathode follower, the other half of the 12AY7, its grid straight on V2a’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.
250p, 250 pF
Treble cap, 250 pF (.00025 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 250 pF from the follower, the other goes to ground through .01 µF. Up moves the wiper toward the 250 pF, so more treble.
.01µ, 10 nF
From the far end of the treble pot to ground, .01 µF 400 V. With the pot turned down it bleeds the treble path’s top end to ground. The 5E6 has none: its treble pot’s far end is on the bass pot’s .005 µF.
.1µ, 100 nF
Coupling cap from the follower into the bass path, .1 µF 200 V.
220k, 220 kΩ
From the bass path to ground, 220 kΩ. It holds the node at 0 V for the 10 MΩ and loads the follower lightly.
10M, 10 MΩ
Feedback, 10 MΩ from the bass path back to V2a’s grid. V2a turns the signal over and the follower doesn’t, so this feeds back out of phase. It cuts V2’s gain about 3 dB.
220k, 220 kΩ
From the bass path to the bass pot’s wiper, 220 kΩ (the 5E7 and 5E4-A have 100 kΩ here).
Bass 1M, 1 MΩ pot, audio taper
Bass, 1 MΩ, its wiper on the bass path. One end goes straight to ground, the other through .005 µF. Down shorts the bass path to ground. Up leaves only the .005 µF there, which removes the treble and lets the bass through to the driver.
.005µ, 5 nF
From the bass pot’s far end to ground, .005 µF 600 V.
220k, 220 kΩ
From the bass pot’s wiper to the driver’s grid, 220 kΩ. The bass path and the treble wiper mix here.
V3a, 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. The feedback from the speaker goes to its cathode. 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 driver.
.02µ, 20 nF
Coupling cap from the driver to the cathodyne.
V3b, 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.
56k, 56 kΩ
The cathodyne’s plate resistor, the output to tube B.
1.5k, 1.5 kΩ
The cathodyne’s bias resistor. The grid returns below it, so the cathode sits this far above the grid.
56k, 56 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.
1M, 1 MΩ
Grid resistor for the cathodyne, returned to the junction of its cathode resistors so the grid sits near the cathode’s high DC voltage.
20k, 20 kΩ
The feedback resistor from the speaker. With the 1.5k Ω below it, it sends about 7 % of the speaker voltage back to the driver’s cathode, which lowers the gain, flattens the response and tightens the bass.
1.5k, 1.5 kΩ
The driver’s cathode resistor, 1.5k Ω to ground. It sets the driver’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.
.1µ, 100 nF
Coupling cap to tube A’s grid.
.1µ, 100 nF
Coupling cap to tube B’s grid.
1.5k, 1.5 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current.
1.5k, 1.5 kΩ
Grid stopper for tube B.
V4, 5881
n5881, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V5, 5881
n5881, tube B of the pair.
220k, 220 kΩ
Grid leak for tube A, returned to the bias supply. 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.
4 H, 4 H
The choke from the reservoir cap to B+, as on the amp’s drawing. The output transformer’s center tap, the screens and everything after them hang past it, so all of their current runs through it. It passes DC with little drop. With the cap after it, it would filter ripple, but ripple isn’t modeled here. Typical of the part, not measured.
50, 50 Ω
The choke’s winding resistance, typical, not measured. With the output tubes’ whole current through it, it’s most of the DC drop between the reservoir and B+.
16µ, 16 µF electrolytic
Filter cap on B+ after the choke, where the output transformer’s center tap and the screens hang.
16µ, 16 µF electrolytic
Second filter cap on the screen supply, beside the first.
10k, 10 kΩ
Dropping resistor from the screen supply node to the node for the driver and inverter, as on the Fender 5E6-A Bassman 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 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.
10k, 10 kΩ
Dropping resistor from the node for the driver and inverter to the node for the preamp, as on the Fender 5E6-A Bassman 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 2 Ω, 40 H primary, 10 mH leakage
The output transformer, 4 kΩ plate to plate into 2 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, 357 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 357 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, 357 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.
V6, 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.
Bias tap, 42 V ac, 60 Hz
The power transformer’s bias tap, 41.5 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
3.3k, 3.3 kΩ
Series resistor into the bias rectifier, 3.3k Ω. It limits the charging current.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
100µ, 100 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
56k, 56 kΩ
The bias supply’s load, 56k Ω to ground. With the 3.3k Ω above it, it sets the bias. There’s no bias pot, so change this resistor to change the bias. Bigger runs the tubes colder.
Coil R, 2.17 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 2 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, 64 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, 116 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 3.58 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 533 µH
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, 548 mΩ
Limits what Loss L2 adds.
Tank R, 20.9 Ω
Height of the bass resonance, about 23 Ω in all at its peak. The feedback holds the output steadier against that than it would be without feedback.
Tank L, 5.28 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 881 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

Versions

5E6-A
The 5E6-A drawing: a 12AY7 for both input channels and another for the gain stage and cathode follower, with 10 MΩ of feedback around them. The tube models are fitted to their data sheets, not measured.
5E6
The earlier 5E6 drawing: the treble pot’s far end on the bass pot’s .005 µF, with no .01 µF of its own, and 6800 Ω ahead of the bias rectifier in place of 3300 Ω, so the bias line sits near -35 V and the 6L6s idle hotter. A pencil note on the drawing changes it to 3300 Ω.

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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