Three-channel amp with 6550s, early 1960s 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 GA-400 Super 400.

Three channels, each a 12AX7 with a jack on each half and its own bass and treble network, mixed into a 12AY7 mixer and splitter, 6SK7 pentode drivers and a pair of 6550s. The compressor isn’t modeled.

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 has two jacks, each on its own 12AX7 half, and the two halves meet in the channel’s tone network through 100 kΩ each. Channel I splits the signal into a bass path (100 kΩ, the bass pot and .02 µF to ground) and a treble path (.0005 µF around it), with a second treble pot fed from the plates through .005 µF. At full bass the two paths cancel around 500 Hz. Channels II and III have a treble cut (a pot in series with a cap to ground) and a bass pot with a small cap across it, so the treble always gets through. Channel I’s volume sits straight on the mixing bus, II and III through 220 kΩ. The mixer, half a 12AY7, has no cathode bypass, so it gains only a few times. The other half is the splitter, in the power amp. The tube models are typical of the types, not measured. The compressor isn’t modeled: on the real amp it turns the power amp’s 6SK7 drivers down on loud notes.

Download.

Parts and what they do

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

1M, 1 MΩ
Channel I input 1’s 1 MΩ to ground. The jack goes straight to the grid of its own 12AX7 half, so this is the grid’s path to ground.
1M, 1 MΩ
Channel I input 2’s 1 MΩ to ground. The jack goes straight to the grid of its own 12AX7 half, so this is the grid’s path to ground.
1M, 1 MΩ
Channel II input 1’s 1 MΩ to ground. The jack goes straight to the grid of its own 12AX7 half, so this is the grid’s path to ground.
1M, 1 MΩ
Channel II input 2’s 1 MΩ to ground. The jack goes straight to the grid of its own 12AX7 half, so this is the grid’s path to ground.
1M, 1 MΩ
Channel III input 1’s 1 MΩ to ground. The jack goes straight to the grid of its own 12AX7 half, so this is the grid’s path to ground.
1M, 1 MΩ
Channel III input 2’s 1 MΩ to ground. The jack goes straight to the grid of its own 12AX7 half, so this is the grid’s path to ground.
V1a, 12AX7 / ECC83 / 7025
Half of channel i’s 12AX7, the stage for input 1.
V1b, 12AX7 / ECC83 / 7025
The other half, the stage for input 2. The two share one cathode resistor, so a guitar in one input moves the other half’s cathode too.
220k, 220 kΩ
Plate resistor for input 1’s half. The drawing reads +135 V on the plates.
220k, 220 kΩ
Plate resistor for input 2’s half.
820, 820 Ω
The shared cathode resistor, +1.15 V on the drawing.
20µ, 20 µF electrolytic
Cathode bypass cap.
100k, 100 kΩ
From input 1’s plate into the tone network. The two plates meet here through 100 kΩ each.
100k, 100 kΩ
From input 2’s plate into the tone network.
V2a, 12AX7 / ECC83 / 7025
Half of channel ii’s 12AX7, the stage for input 1.
V2b, 12AX7 / ECC83 / 7025
The other half, the stage for input 2. The two share one cathode resistor, so a guitar in one input moves the other half’s cathode too.
220k, 220 kΩ
Plate resistor for input 1’s half. The drawing reads +110 V on the plates.
220k, 220 kΩ
Plate resistor for input 2’s half.
820, 820 Ω
The shared cathode resistor, +0.85 V on the drawing.
20µ, 20 µF electrolytic
Cathode bypass cap.
100k, 100 kΩ
From input 1’s plate into the tone network. The two plates meet here through 100 kΩ each.
100k, 100 kΩ
From input 2’s plate into the tone network.
V3a, 12AX7 / ECC83 / 7025
Half of channel iii’s 12AX7, the stage for input 1.
V3b, 12AX7 / ECC83 / 7025
The other half, the stage for input 2. The two share one cathode resistor, so a guitar in one input moves the other half’s cathode too.
220k, 220 kΩ
Plate resistor for input 1’s half. The drawing reads +105 V on the plates.
220k, 220 kΩ
Plate resistor for input 2’s half.
820, 820 Ω
The shared cathode resistor, +0.9 V on the drawing.
20µ, 20 µF electrolytic
Cathode bypass cap.
100k, 100 kΩ
From input 1’s plate into the tone network. The two plates meet here through 100 kΩ each.
100k, 100 kΩ
From input 2’s plate into the tone network.
.005µ, 5 nF
From input 1’s plate to the first tone pot’s wiper. Through these two .005 µF the plates also meet, but only for the treble.
.005µ, 5 nF
From input 2’s plate to the same point.
Treble 2M, 2 MΩ pot, linear taper
Channel I’s treble, 2 MΩ linear. Its top is on the 100 kΩ junction, its wiper on the .005 µF caps from the plates, its bottom on .02 µF to ground. Turned up, the wiper moves toward the top and more of the plates’ treble reaches the network.
.02µ, 20 nF
Under the treble pot, to ground.
500p, 500 pF
From the 100 kΩ junction straight to the output point, .0005 µF: the treble’s way around the bass path.
100k, 100 kΩ
Into the bass path.
Bass 2M, 2 MΩ pot, linear taper
Channel I’s bass, 2 MΩ linear as a variable resistor in the bass path, ahead of the .02 µF to ground. Turned up, its resistance drops and more of the lows get through.
.02µ, 20 nF
The bass path’s cap to ground. With the resistance ahead of it, only the lows pass.
100k, 100 kΩ
From the bass path to the output point.
.05µ, 50 nF
Coupling cap to the volume’s wiper.
.02µ, 20 nF
Coupling cap from the 100 kΩ junction to the tone pots’ wipers.
Treble 1M, 1 MΩ pot, audio taper
Channel II’s treble, 1 MΩ audio as a variable resistor in series with .005µF to ground. Turned up, its resistance rises, the cap cuts less treble and the channel gets brighter.
.005µ, 5 nF
Under the treble pot, to ground.
Bass 2M, 2 MΩ pot, linear taper
Channel II’s bass, 2 MΩ linear as a variable resistor between the tone pots’ wipers and the volume, with 500pF across it. The treble always gets across through the cap. Turned up, the resistance drops and the lows get across too.
500p, 500 pF
Across the bass pot: the treble’s way around it.
220k, 220 kΩ
Mixing resistor from channel II’s volume to the mixer’s grid.
.05µ, 50 nF
Coupling cap from the 100 kΩ junction to the tone pots’ wipers.
Treble 1M, 1 MΩ pot, audio taper
Channel III’s treble, 1 MΩ audio as a variable resistor in series with .01µF to ground. Turned up, its resistance rises, the cap cuts less treble and the channel gets brighter. A .00025 µF across the pot lets some treble to the cap at every setting.
.01µ, 10 nF
Under the treble pot, to ground.
250p, 250 pF
Across the treble pot, .00025 µF.
Bass 2M, 2 MΩ pot, linear taper
Channel III’s bass, 2 MΩ linear as a variable resistor between the tone pots’ wipers and the volume, with .001µF across it. The treble always gets across through the cap. Turned up, the resistance drops and the lows get across too.
.001µ, 1 nF
Across the bass pot: the treble’s way around it.
220k, 220 kΩ
Mixing resistor from channel III’s volume to the mixer’s grid.
Vol 1M, 1 MΩ pot, linear taper
Channel I’s volume, 1 MΩ linear. The signal comes in on the wiper and leaves from the top, which is the mixing bus itself. Turned down, the wiper shorts it to ground.
Vol 1M, 1 MΩ pot, linear taper
Channel II’s volume, 1 MΩ linear. The signal comes in on the wiper and leaves from the top, through 220 kΩ to the mixing bus. Turned down, the wiper shorts it to ground.
Vol 1M, 1 MΩ pot, linear taper
Channel III’s volume, 1 MΩ linear. The signal comes in on the wiper and leaves from the top, through 220 kΩ to the mixing bus. Turned down, the wiper shorts it to ground.
V4a, 12AY7 / 6072
Half of the 12AY7, the mixer: the three channels meet at its grid. The other half is the splitter, in the power amp.
3.3k, 3.3 kΩ
The mixer’s cathode resistor, not bypassed, +2.2 V on the drawing. Without a bypass cap it lowers the stage’s gain to a few times.
100k, 100 kΩ
From the mixer’s plate to the point the coupling cap connects to. With the 47 kΩ above it, it makes up the plate load, 147 kΩ.
47k, 47 kΩ
The rest of the mixer’s plate load, from the preamp’s node. On the drawing the signal comes off between the two, so the splitter gets about a third of the plate’s swing.
.01µ, 10 nF
Coupling cap to the splitter’s grid.
V5, 12AY7 / 6072
ay7, a cathodyne, or split-load inverter, with no driver ahead of it. 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, so the preamp has to bring the whole swing the output tubes need.
22k, 22 kΩ
The cathodyne’s plate resistor, the output to tube B.
470, 470 Ω
The cathodyne’s bias resistor. The grid returns below it, so the cathode sits this far above the grid.
22k, 22 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. It’s the grid’s only path to ground, and the cathode follows the grid, so the preamp sees much more than 1M Ω.
.05µ, 50 nF
Coupling cap from the inverter to side A’s driver.
.05µ, 50 nF
Coupling cap from the inverter to side B’s driver.
150k, 150 kΩ
The drivers’ screen resistor, 150k Ω from the node for the drivers and the splitter to both screens, with no cap on them. It holds the screens low, so the drivers run on little current.
220, 220 Ω
The drivers’ cathode resistor, 220 Ω to ground for both halves, with no bypass cap. The two cathodes share it, so the pair works somewhat like a long-tail pair: what one half gains in current the other loses, which evens out the two sides’ swings.
220k, 220 kΩ
Grid leak for side A’s driver, to ground.
V8, 6SK7
6SK7 pentode, side A’s driver: a gain stage that raises the inverter’s signal about 5 times on its way to tube A’s grids. Side B has its own, and the two share one cathode resistor and one screen resistor. The 6SK7 is a remote-cutoff pentode, the tube the compressor turns down. The compressor isn’t modeled here.
47k, 47 kΩ
The driver’s plate resistor, 47k Ω from the node for the drivers and the splitter.
.05µ, 50 nF
Coupling cap from the driver’s plate to tube A’s grids.
220k, 220 kΩ
Grid leak for side B’s driver, to ground.
V9, 6SK7
The other 6SK7, side B’s driver.
47k, 47 kΩ
The driver’s plate resistor, 47k Ω from the node for the drivers and the splitter.
.05µ, 50 nF
Coupling cap from the driver’s plate to tube B’s grids.
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.
V6, 6550
n6550, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V7, 6550
n6550, tube B of the pair.
47k, 47 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.
47k, 47 kΩ
Grid leak for tube B.
Screens, 300 V
The screen regulator, standing in for V11: a 6V6 wired as a triode and run as a cathode follower, its plate on B+ and its grid on two 100 kΩ from B+ to ground. Its cathode holds both 6550 screens near the drawing’s 300 V and feeds the drivers and the preamp through 2.7 kΩ. Here it’s a steady source behind 300 Ω, about the follower’s own output resistance, so it doesn’t sag with B+ the way the real one would.
2.7k, 2.7 kΩ
Dropping resistor from the screen supply node to the node for the drivers and the splitter, as on the Gibson GA-400 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.
20µ, 20 µF electrolytic
Filter cap at the node for the drivers and the splitter. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
4.7k, 4.7 kΩ
Dropping resistor from the node for the drivers and the splitter to the node for the preamp, as on the Gibson GA-400 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.
OT, 5 kΩ to 4 Ω, 40 H primary, 10 mH leakage
The output transformer, 5 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, 470 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 470 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, 470 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, 300 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, 300 Ω
Resistance of the other half. Typical of the part, not measured.
V8, GZ34
One plate of the GZ34 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, GZ34
The rectifier’s other plate, on the other half cycle.
20µ, 20 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.
20µ, 20 µF electrolytic
Another reservoir cap, in parallel with the first.
Bias tap, 36 V ac, 60 Hz
The power transformer’s bias tap, 36 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
tap R, 100 Ω
The bias tap’s own resistance, 100 Ω, typical, not measured. The drawing has no resistor between the tap and the diode.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
10µ, 10 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
470, 470 Ω
Second resistor of the bias supply, 470 Ω to the bias line. With the cap after it, it filters the ripple a second time.
50µ, 50 µF electrolytic
Second filter cap, on the bias line, its plus side to ground.
3.3k, 3.3 kΩ
From the bias line to ground, 3.3k Ω. With the 470 Ω above it, it divides the supply down to the bias. There’s no bias pot, so change this resistor to change the bias. Bigger runs the tubes colder.
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. With no feedback, the voltage at the speaker rises and falls with its impedance.
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. With no feedback, the voltage at the speaker rises and falls with its impedance.
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

As drawn
The 12AX7 and 12AY7 models are fitted to datasheet curves, 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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