Three-input 6GW8 amp with bias 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 Guild Thunder 1 T1RVT.

A 12AX7 stage into a 3 MΩ Bass and Treble network, a cathode follower, a Norm/Bright switch and Volume, another 12AX7 stage, then a 6GW8 triode driving a 6GW8 cathodyne and two 6GW8 pentodes whose bias the tremolo swings.

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. One 12AX7 stage with 3.3 MΩ back from the far side of its coupling cap to the input node, then a Bass and Treble network on 3 MΩ pots whose wipers are tied. A cathode follower drives the Norm/Bright switch and the Volume, and a second 12AX7 stage brings the level back up for the 6GW8s. The tremolo works on the output tubes’ bias, from the power amp. The reverb isn’t modeled. 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.

100k, 100 kΩ
The Mic jack’s resistor into the input node.
330k, 330 kΩ
The Norm jack’s resistor, 330 kΩ, with 115 pF across it.
115p, 115 pF
Across the Norm jack’s 330 kΩ. It lets the top end past the resistor.
330k, 330 kΩ
The Bright jack’s resistor, 330 kΩ, with 1000 pF across it.
1000p, 1 nF
Across the Bright jack’s 330 kΩ, ten times the Norm jack’s cap, so more of the treble gets past.
3.3M, 3.3 MΩ
From the input node to the far side of V1a’s coupling cap, 3.3 MΩ. It feeds some of the plate’s signal back, turned the other way, and it’s the input node’s DC path to ground through the tone network.
1k, 1 kΩ
Grid stopper for V1a.
V1a, 12AX7 / ECC83 / 7025
Half of a 12AX7, the first gain stage.
1k, 1 kΩ
Cathode resistor for V1a.
32µ, 32 µF electrolytic
Bypass for V1a’s cathode.
150k, 150 kΩ
Plate resistor for V1a, from node 4 (+280 V on the drawing).
.033µ, 33 nF
Coupling cap into the tone network.
1000p, 1 nF
Treble cap, 1000 pF to the top of the Treble pot.
Treble 3M, 3 MΩ pot, audio taper
Treble, 3 MΩ, from the 1000 pF to the 5000 pF to ground. Its wiper is the network’s output, tied to the Bass wiper. Turned up, the wiper moves toward the 1000 pF. Taper and direction aren’t marked, so audio and brighter clockwise are assumed.
5000p, 5 nF
From the Treble pot’s bottom to ground.
330k, 330 kΩ
From the tone network’s input to the top of the Bass pot.
Bass 3M, 3 MΩ pot, audio taper
Bass, 3 MΩ, with 1000 pF from its top to its wiper and .01 µF from its wiper to its bottom, and 33 kΩ from the bottom to ground. Its wiper is the output. Turned up, the wiper moves toward the top and the lows come up. Taper and direction aren’t marked, so audio and more bass clockwise are assumed.
1000p, 1 nF
From the Bass pot’s top to its wiper.
.01µ, 10 nF
From the Bass wiper to the pot’s bottom.
33k, 33 kΩ
From the Bass pot’s bottom to ground.
.033µ, 33 nF
Coupling cap from the tone network to V2a.
V2a, 12AX7 / ECC83 / 7025
Half of a 12AX7, a cathode follower: its plate straight on node 3, its output off the cathode. It drives the Norm and Bright network and the Volume without loading the tone network.
100k, 100 kΩ
V2a’s grid resistor, returned to the tap between its two cathode resistors, so the grid sits slightly below the cathode.
390, 390 Ω
V2a’s bias resistor.
8.2k, 8.2 kΩ
V2a’s cathode load to ground.
.033µ, 33 nF
Coupling cap off the follower’s cathode to the Norm and Bright network.
150k, 150 kΩ
The Norm position’s resistor, 150 kΩ.
330k, 330 kΩ
The Bright position’s resistor, 330 kΩ, with .001 µF across it, so the treble loses less than the rest.
.001µ, 1 nF
Across the Bright position’s 330 kΩ.
100k, 100 kΩ
From the Norm/Bright switch to the top of the Volume.
Volume 1M, 1 MΩ pot, audio taper
Volume, 1 MΩ. There’s no master, so it sets how hard the stages after it and the 6GW8s are pushed. Audio taper assumed.
.033µ, 33 nF
Coupling cap to V2b.
1M, 1 MΩ
Grid resistor for V2b.
V2b, 12AX7 / ECC83 / 7025
The other half of V2, a gain stage with its cathode not bypassed.
860, 860 Ω
Cathode resistor for V2b, no bypass.
100k, 100 kΩ
Plate resistor for V2b, from node 3.
.033µ, 33 nF
Coupling cap to V3a, the 6GW8’s triode, the driver ahead of the inverter.
V3a, 6GW8 (triode section)
6GW8, the driver. A cathodyne has no gain of its own, so this stage raises the preamp’s signal enough to drive the output tubes. Its grid is fed through the preamp’s coupling cap, so there’s no second one here.
1M, 1 MΩ
Grid resistor for the driver.
150k, 150 kΩ
Plate resistor for the driver.
680, 680 Ω
Cathode resistor for the driver. It sets the driver’s bias, a volt or so.
32µ, 32 µF electrolytic
Bypass cap across the driver’s cathode resistor, for full gain above a few hertz.
.0094µ, 9.4 nF
Coupling cap from the driver to the cathodyne.
V4a, 6GW8 (triode section)
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.
75k, 75 kΩ
The cathodyne’s plate resistor, the output to tube B.
1k, 1 kΩ
The cathodyne’s bias resistor. The grid returns below it, so the cathode sits this far above the grid.
75k, 75 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.
.15µ, 150 nF
Coupling cap to tube A’s grid.
.15µ, 150 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.
V3b, 6GW8
6GW8, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V4b, 6GW8
6GW8, tube B of the pair.
390k, 390 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.
390k, 390 kΩ
Grid leak for tube B.
4.7k, 4.7 kΩ
Dropping resistor from B+ to the node for the driver, the inverter and the tremolo, as on the Guild Thunder 1 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.
40µ, 40 µF electrolytic
Filter cap at the node for the driver, the inverter 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.
12k, 12 kΩ
Dropping resistor from the node for the driver, the inverter and the tremolo to the node for V2, as on the Guild Thunder 1 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.
40µ, 40 µF electrolytic
Filter cap at the node for V2. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
8.2k, 8.2 kΩ
Dropping resistor from the node for the driver, the inverter and the tremolo to the node for V1a, as on the Guild Thunder 1 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.
40µ, 40 µF electrolytic
Filter cap at the node for V1a. 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, 15 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, 150 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, 150 Ω
Resistance of the other half. Typical of the part, not measured.
V7, 6CA4
One plate of the 6CA4 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, 6CA4
The rectifier’s other plate, on the other half cycle.
40µ, 40 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.
Bias tap, 12 V ac, 60 Hz
The power transformer’s bias tap, 12.1 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
tap R, 50 Ω
The bias tap’s own resistance, 50 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.
32µ, 32 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
33k, 33 kΩ
Second resistor of the bias supply, 33k Ω to the Strength pot’s bias end. With the cap after it, it filters the ripple a second time.
68k, 68 kΩ
From the Strength pot’s bias end to ground, 68k Ω. With the 33k 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.
V1b, 12AX7 / ECC83 / 7025
The tremolo’s oscillator, half a 12AX7. The three caps after its plate turn the signal around at one frequency, and the oscillator runs at that frequency, a few times a second. In the amp it’s the other half of V1, the first stage’s tube.
220k, 220 kΩ
Oscillator plate resistor, from the inverter’s supply node.
3.3k, 3.3 kΩ
Oscillator cathode resistor.
32µ, 32 µF electrolytic
Oscillator bypass cap. At a few hertz it isn’t a full short, and it adds some of the phase shift that keeps the loop oscillating.
.022µ, 22 nF
First ladder cap.
Speed, 1.5 MΩ pot, linear taper
Speed, 1.5M Ω linear, used as a variable resistor. Less resistance runs the oscillator faster.
330k, 330 kΩ
Sets the top speed. It’s all that’s left with Speed on 10.
.01µ, 10 nF
Second ladder cap.
1M, 1 MΩ
Middle resistor of the ladder, returned to the cathode. The footswitch grounds this point.
.01µ, 10 nF
Third ladder cap.
1M, 1 MΩ
Oscillator grid leak.
820k, 820 kΩ
Couples the LFO from the oscillator’s plate. It keeps the bias network from loading the oscillator much.
.15µ, 150 nF
Blocks the oscillator’s DC, so only its swing reaches the Intensity pot.
Strength, 250 kΩ pot, linear taper
Strength, 250k Ω linear. One end gets the LFO, the other sits on the bias supply, and the wiper is the bias line for both output tubes. At 0 the wiper is on the bias supply and the bias holds still. Turned up, more of the LFO rides on the bias, so both grids move together and the output stage’s gain rises and falls. Which way it turns isn’t marked on the drawing. Up is assumed to be more.
.22µ, 220 nF
On the bias line, to ground. With the Strength pot it rounds off the LFO slightly.
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 Guild’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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