Two-channel 6973 amp with cathode tremolo, mid 1960s

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 Supro S6424.

Two 12AX7 channels, each with Volume and a treble-cut Tone, mixed through 270 kΩ into a paraphase inverter and two cathode-biased 6973s. The tremolo’s cathode follower moves channel 2’s cathode.

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 12AX7 stage. The High jacks come in through .005 µF, which cuts the bass of a guitar, and the Low jacks through 100 kΩ. Channel 1 couples straight to its Volume. Channel 2 goes through two .005 µF caps with 270 kΩ between, so it’s thinner still. Each Volume has .0005 µF from its top to its wiper, which keeps the treble up as the Volume comes down. Each Tone pot runs from the Volume’s wiper to .0033 µF to ground, and its wiper feeds the mix through 270 kΩ. The tremolo’s oscillator drives a cathode follower whose cathode is channel 2’s cathode, so the LFO moves that stage’s bias and its gain. Channel 1 has no tremolo. The tube models are typical of the types, not measured.

Download.

Parts and what they do

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

.005µ, 5 nF
The High jack’s cap into channel 1. Into the 82 kΩ to ground behind it, it cuts the bass below about 400 Hz.
100k, 100 kΩ
The Low jack’s resistor. With nothing in J2 its switch grounds this end, so it’s 100 kΩ to ground beside the 470 kΩ.
470k, 470 kΩ
Channel 1’s grid resistor to ground.
2.2k, 2.2 kΩ
Grid stopper for V1b.
V1b, 12AX7 / ECC83 / 7025
Half of a 12AX7, channel 1’s only gain stage, its cathode not bypassed.
2.2k, 2.2 kΩ
Cathode resistor for V1b, with no bypass, so the gain is about half what it would be bypassed.
270k, 270 kΩ
Plate resistor for V1b.
.05µ, 50 nF
Filter cap on V1b’s own supply, small for a filter: with the 100 kΩ above it, it starts to work around 30 Hz.
100k, 100 kΩ
Dropping resistor from the 290 V node to V1b’s plate resistor.
.005µ, 5 nF
Coupling cap to Volume 1. Into 500 kΩ it passes everything above about 60 Hz.
Vol 1 500k, 500 kΩ pot, audio taper
Channel 1’s Volume, 500 kΩ. The drawing marks no taper, so audio is assumed.
.0005µ, 500 pF
Bright cap from the Volume’s top to its wiper. It lets treble around the Volume, more as the Volume comes down.
Tone 1 500k, 500 kΩ pot, audio taper
Channel 1’s Tone, 500 kΩ, from the Volume’s wiper to the .0033 µF to ground. Its wiper feeds the mix. Turned up, the wiper moves toward the Volume end and the treble comes up. The taper and the way it turns aren’t marked, so audio and brighter clockwise are assumed.
.0033µ, 3.3 nF
Tone 1’s cut cap to ground.
270k, 270 kΩ
Channel 1’s mixing resistor into V2’s grid.
.005µ, 5 nF
The High jack’s cap into channel 2, as channel 1’s.
100k, 100 kΩ
The Low jack’s resistor for channel 2. With nothing in J4 its switch grounds this end.
470k, 470 kΩ
Channel 2’s grid resistor to ground.
2.2k, 2.2 kΩ
Grid stopper for V1a.
V1a, 12AX7 / ECC83 / 7025
The other half of the 12AX7, channel 2’s gain stage. The tremolo’s cathode follower shares its cathode.
1500, 1.5 kΩ
Cathode resistor for V1a. The cathode follower’s current runs through it too, so the follower sets this cathode’s voltage and the stage’s bias.
35µ, 35 µF electrolytic
Bypass for V1a’s cathode. At the tremolo’s few hertz it only partly holds the cathode still, so the LFO gets through.
270k, 270 kΩ
Plate resistor for V1a.
.05µ, 50 nF
Filter cap on V1a’s own supply.
100k, 100 kΩ
Dropping resistor from the 290 V node to V1a’s plate resistor.
.005µ, 5 nF
First coupling cap after V1a.
270k, 270 kΩ
To ground between the two .005 µF. With them it cuts more of channel 2’s bass than channel 1 loses.
.005µ, 5 nF
Second coupling cap, to Volume 2.
Vol 2 500k, 500 kΩ pot, audio taper
Channel 2’s Volume, 500 kΩ, audio assumed.
.0005µ, 500 pF
Bright cap across Volume 2’s top half.
Tone 2 500k, 500 kΩ pot, audio taper
Channel 2’s Tone, wired as channel 1’s. Brighter clockwise.
.0033µ, 3.3 nF
Tone 2’s cut cap to ground.
270k, 270 kΩ
Channel 2’s mixing resistor into V2’s grid.
.0005µ, 500 pF
From the mixing point to ground. With the two 270 kΩ it trims the top end above about 2 kHz. The value is faint on the drawing, so .0005 µF is assumed.
V3b, 12AX7 / ECC83 / 7025
Half of a 12AX7, the tremolo’s oscillator.
270k, 270 kΩ
Plate resistor for the oscillator.
1000, 1 kΩ
Cathode resistor for the oscillator, with no bypass.
.03µ, 30 nF
First cap of the oscillator’s ladder.
100k, 100 kΩ
In series with Speed, the ladder’s first leg to ground. It sets the top speed.
Speed 500k, 500 kΩ pot, audio taper
Speed, 500 kΩ as a variable resistor in the first leg. Turned up, its resistance drops and the oscillator runs faster. Audio taper assumed.
.02µ, 20 nF
Second ladder cap.
1M, 1 MΩ
Second ladder leg to ground.
.01µ, 10 nF
Third ladder cap.
1.5M, 1.5 MΩ
Third ladder leg, the oscillator’s grid leak.
2.2k, 2.2 kΩ
Grid stopper for the oscillator.
.005µ, 5 nF
Couples the LFO from the oscillator’s plate. At a few hertz it passes very little, so the swing that reaches channel 2 is small.
1M, 1 MΩ
With the .02 µF below it, a filter that rounds the LFO’s wave.
.02µ, 20 nF
To ground after the 1 MΩ. It rounds the LFO and cuts a fast speed more.
2.2M, 2.2 MΩ
Feeds the LFO to the Intensity pot. With the pot below it, a divider.
.05µ, 50 nF
On the Intensity pot, to ground. It smooths the LFO further.
Int 500k, 500 kΩ pot, audio taper
Intensity, 500 kΩ as a variable resistor to ground. Turned up, its resistance rises and more of the LFO reaches the follower’s grid. Audio taper assumed.
2.2k, 2.2 kΩ
Grid stopper for the cathode follower.
V3a, 12AX7 / ECC83 / 7025
The other half of V3, a cathode follower. Its cathode is channel 2’s cathode, so as its grid swings with the LFO it pushes more or less current through the 1500 Ω and moves that stage’s bias.
V2a, 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 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.
270k, 270 kΩ
Plate resistor for the first half, the output to tube A.
2.2k, 2.2 kΩ
Cathode resistor for the first half. It sets its bias, a volt or so.
270k, 270 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.
12k, 12 kΩ
The tap, 12k 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.
V2b, 12AX7 / ECC83 / 7025
The second half. It turns the divided signal over again, so its plate swings with the input. It drives tube B.
270k, 270 kΩ
Plate resistor for the second half, the output to tube B.
3.9k, 3.9 kΩ
Cathode resistor for the second half, straight to ground.
.01µ, 10 nF
Coupling cap to tube A’s grid.
.01µ, 10 nF
Coupling cap to tube B’s grid.
V4, 6973
6973, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V5, 6973
6973, tube B of the pair.
470k, 470 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.
35µ, 35 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.
10µ, 10 µF electrolytic
Filter cap on the screen supply.
15k, 15 kΩ
Dropping resistor from the screen supply node to the node for the inverter, the preamp and the tremolo, as on the Supro S6424 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 inverter, the preamp and the tremolo. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
OT, 6.6 kΩ to 8 Ω, 20 H primary, 12 mH leakage
The output transformer, 6.6 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, 337 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 337 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, 337 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, 100 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, 100 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. 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 12AX7 model is fitted to its data sheet (Brimar). Values from Supro’s 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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