Relay-switched overdrive amp with four EL34s, 1980s 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 Dumble Overdrive Special #183.

One triode drives a Fender-type tone stack straight from its plate, then a second triode. A relay sends that to the Master either straight or through two more triodes, with a Drive pot between them and an OD Volume after, into four EL34s.

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 clean path is two 12AX7 stages with the tone stack between them, hung right on CL1’s plate. The treble cap is .002 µF and 390 pF in series, about 330 pF, and Mid boost shorts the 390 pF. On Rock, Bass’s wiper feeds Treble’s bottom and .001 µF bypasses the part of Bass below the wiper. The Channel relay adds the overdrive path: 220 kΩ into OD Trim drops the level ahead of OD1, Drive sets how hard OD2 is hit, and OD Volume sets the overdrive’s level against the clean. 25 pF from each overdrive plate to its cathode rolls off the fizz. PAB lifts the bottom of the stack. The Master feeds the inverter and four EL34s, which idle at about 38 mA each with about 465 V on B+.

Download.

Parts and what they do

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

2.2k, 2.2 kΩ
Dropping resistor, 2.2 kΩ 2 W from B+4 to B+5.
23.5µ, 23.5 µF electrolytic
B+5 filter, two 47 µF 250 V in series, each with 470 kΩ across it.
150k, 150 kΩ
Load on B+5, 150 kΩ 2 W. The drawing’s note: with the FET input left out, this goes on B+5 in its place, so B+5 sits where it would with the FET input pulling current.
1M, 1 MΩ
Input resistor to ground at the Normal jack.
22k, 22 kΩ
Grid stopper into V1a. With the tube’s input capacitance it keeps radio and fizz out.
V1a, 12AX7 / ECC83 / 7025
First stage, CL1 on the drawing. Its plate drives the tone stack directly.
220k, 220 kΩ
V1a plate resistor, 220 kΩ metal film. It’s also part of the tone stack, since the stack hangs right on the plate. The drawing has 175 V on the plate.
3.3k, 3.3 kΩ
V1a cathode resistor.
4.7µ, 4.7 µF electrolytic
V1a bypass cap. Full gain down to about 30 Hz.
.002µ, 2 nF
First treble cap, .002 µF, from V1a’s plate. With the 390 pF after it the two make about 330 pF.
4.7M, 4.7 MΩ
From the two treble caps’ junction to ground. It keeps the junction at 0 V DC, so the Mid boost switch doesn’t pop.
390p, 390 pF
Second treble cap, 390 pF ceramic, to the top of Treble. The Mid boost switch shorts it.
150k, 150 kΩ
Slope resistor, from V1a’s plate to the bass and middle caps.
.1µ, 100 nF
Bass cap, to the top of Bass.
.01µ, 10 nF
Middle cap, to Middle.
Middle, 250 kΩ pot, linear taper
Middle, 250 kΩ linear as a variable resistor to ground. Turned up, its resistance rises and less of the mids go to ground.
Treble, 270 kΩ pot, audio taper
Treble, 270 kΩ audio on the drawing. Its wiper is the stack’s output, the top of Volume.
Bass, 312 kΩ pot, linear taper
Bass, 312 kΩ linear on the drawing. Its top connects to the bass cap and its wiper feeds Treble’s bottom on Rock. Turned up, the wiper moves toward the bass cap.
10k, 10 kΩ
Under Bass, to ground.
.001µ, 1 nF
On Rock, from the wiper side of Bass to its bottom, .001 µF 600 V. It lets the treble around the part of Bass below the wiper.
22M, 22 MΩ
Across the PAB relay’s first contact, from Treble’s bottom to the Rock/Jazz switch. It keeps the open contact at the same DC level, so the relay doesn’t pop.
22M, 22 MΩ
Across the PAB relay’s second contact, from Bass’s wiper to the Rock/Jazz switch, the same.
.0047µ, 4.7 nF
On Jazz, from Treble’s bottom to ground, .0047 µF 600 V.
100k, 100 kΩ
On Jazz, from Bass’s wiper to Volume, so bass reaches the volume around Treble. On Rock its switch end hangs open.
Volume, 1 MΩ pot, audio taper
Volume, 1 MΩ audio. It sets how hard CL2 and, with Overdrive on, the whole overdrive path get driven.
220p, 220 pF
Bright cap, 220 pF ceramic, from Volume’s top to its wiper on the Bright switch’s drawn throw.
100p, 100 pF
Bright cap, 100 pF ceramic, from Volume’s top to its wiper on the Bright switch’s other throw.
220k, 220 kΩ
From Volume’s wiper into V1b’s grid. With the tube’s input capacitance it trims the top end.
500p, 500 pF
Across the 220 kΩ, 500 pF ceramic. Above about 1.4 kHz it lets the treble around the resistor.
V1b, 12AX7 / ECC83 / 7025
Second stage, CL2 on the drawing. Its output feeds the Channel relay.
150k, 150 kΩ
V1b plate resistor, 150 kΩ metal film.
2.2k, 2.2 kΩ
V1b cathode resistor.
4.7µ, 4.7 µF electrolytic
V1b bypass cap.
.05µ, 50 nF
Coupling cap out of V1b, to the Channel relay.
220k, 220 kΩ
Into the top of OD Trim. With the trimmer it makes a divider, so even with the trimmer at full only about a third of CL2’s output reaches OD1.
OD Trim, 100 kΩ pot, linear taper
OD Trim, 100 kΩ linear, inside the chassis. It sets how hard OD1 is hit. Set at 0.6 of its turn, as on #124. The drawing shows no setting.
68k, 68 kΩ
Grid stopper into V2a.
V2a, 12AX7 / ECC83 / 7025
First overdrive stage, OD1 on the drawing.
220k, 220 kΩ
V2a plate resistor, 220 kΩ metal film, from B+4.
25p, 25 pF
From V2a’s plate to its cathode, 25 pF ceramic. In the top end it feeds plate signal back onto the cathode, so the stage’s gain falls off and the fizz comes down.
3.3k, 3.3 kΩ
V2a cathode resistor.
4.7µ, 4.7 µF electrolytic
V2a bypass cap, 4.7 µF.
.015µ, 15 nF
Coupling cap out of V2a, .015 µF 1200 V.
100k, 100 kΩ
In series with Drive. With the pot it makes a divider, so even at full only about 70 % of OD1’s output reaches OD2.
Drive, 272 kΩ pot, audio taper
Drive, 272 kΩ audio on the drawing, between the two overdrive stages. It sets how hard OD2 is driven.
180k, 180 kΩ
Grid stopper into V2b. With the tube’s input capacitance it trims the top end going into OD2.
V2b, 12AX7 / ECC83 / 7025
Second overdrive stage, OD2 on the drawing.
150k, 150 kΩ
V2b plate resistor, 150 kΩ metal film, from B+4.
25p, 25 pF
From V2b’s plate to its cathode, 25 pF ceramic, as on OD1.
2.2k, 2.2 kΩ
V2b cathode resistor.
4.7µ, 4.7 µF electrolytic
V2b bypass cap, 4.7 µF.
.005µ, 5 nF
Coupling cap out of V2b, .005 µF 600 V. Into the 180 kΩ and OD Volume below it, it cuts the bass under about 70 Hz.
180k, 180 kΩ
In series with OD Volume. With the pot it makes a divider, so even at full only about 60 % of OD2’s output reaches the Master.
OD Volume, 285 kΩ pot, audio taper
OD Volume, 285 kΩ audio on the drawing. It sets how much of the overdrive path reaches the Master, so it matches the overdrive’s level against the clean.
Master, 908 kΩ pot, audio taper
Master, 908 kΩ audio on the drawing. Its wiper feeds the phase inverter, through the effects send and return, so it sets the level for both paths.
.02µ, 20 nF
Coupling cap into the phase inverter. With the grid resistor it sets how low the inverter goes, around 5 Hz.
V3a, 12AX7 / ECC83 / 7025
12AX7, the driven half of the long-tail pair phase inverter. Its plate swings opposite the input and drives tube A.
V3b, 12AX7 / ECC83 / 7025
The other half of the inverter. Its grid sits at the feedback point for signal, so it’s driven through the shared cathodes, and its plate swings with the input. It drives tube B.
1M, 1 MΩ
Grid resistor for the driven half, returned to the bias point above the tail, not to ground.
1M, 1 MΩ
Grid resistor for the other half.
820, 820 Ω
Bias resistor for both halves. The cathodes sit this far above the grids’ return, a volt or so.
24k, 24 kΩ
The tail resistor. It’s large, so the pair shares a nearly fixed current. When one half conducts more, the other conducts less, which splits the phase.
.1µ, 100 nF
Ties the second grid to the feedback point for signal, so the feedback acts across the pair.
105k, 105 kΩ
Plate resistor for the driven half. It’s smaller than the other so the two outputs come out about even.
115k, 115 kΩ
Plate resistor for the other half.
4.7k, 4.7 kΩ
The feedback resistor from the speaker. With the 390 Ω below it, it sends about 8 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
390, 390 Ω
The bottom of the feedback loop, 390 Ω to ground. The fed-back speaker voltage arrives here and moves the tail.
1µ, 1 µF
The presence cap, in series with the pot across the leg. With the pot it sets where the lift starts.
Presence, 2.2 kΩ pot, linear taper
Presence, a 2.2k 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.
2.8k, 2.8 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current. Each of the two tubes has its own 5.6k. The resistors are drawn as one part, the two in parallel.
2.8k, 2.8 kΩ
Grid stopper for tube B. Each of the two tubes has its own 5.6k. The resistors are drawn as one part, the two in parallel.
V4, V6, EL34 / 6CA7
EL34, tube A of the push-pull pair, biased from the fixed bias supply. Two tubes in parallel, drawn as one.
V5, V7, EL34 / 6CA7
EL34, tube B of the pair. Two tubes in parallel, drawn as one.
0.5, 500 mΩ
Cathode resistor for tube A, 1 Ω to ground. The bias is fixed, so it’s there to set the bias by: the voltage across it is the cathode current times 1 Ω. It isn’t bypassed, so it also lowers the tube’s gain slightly. Each of the two tubes has its own 1. The resistors are drawn as one part, the two in parallel.
0.5, 500 mΩ
Cathode resistor for tube B, the same. Each of the two tubes has its own 1. The resistors are drawn as one part, the two in parallel.
270k, 270 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.
270k, 270 kΩ
Grid leak for tube B.
750, 750 Ω
Screen resistor for tube A. It limits the screen current when the tube is driven hard. Each of the two tubes has its own 1.5k. The resistors are drawn as one part, the two in parallel.
750, 750 Ω
Screen resistor for tube B. Each of the two tubes has its own 1.5k. The resistors are drawn as one part, the two in parallel.
4 H, 4 H
A choke from B+ to the screen supply, as in the amp this style is based on. It passes DC with little drop, so the screens sit close to B+. With the cap after it, it would filter ripple, but ripple isn’t modeled here. Typical of the part, not measured.
100, 100 Ω
The choke’s winding resistance, typical, not measured. It’s all the DC drop the screens see from B+.
23.5µ, 23.5 µF electrolytic
Filter cap on the screen supply.
3k, 3 kΩ
Dropping resistor from the screen supply node to the node for the inverter, as on the Dumble Overdrive Special #183 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.
23.5µ, 23.5 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.
22k, 22 kΩ
Dropping resistor from the node for the inverter to the node for the preamp, as on the Dumble Overdrive Special #183 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.
23.5µ, 23.5 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, 1.7 kΩ to 8 Ω, 20 H primary, 5 mH leakage
The output transformer, 1.7 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, 354 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 354 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, 354 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, 30 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, 30 Ω
Resistance of the other half. Typical of the part, not measured.
D1, 1N4001
One of the silicon rectifier diodes. It drops under a volt, so B+ holds up better under load than with a tube rectifier.
D2, 1N4001
The other rectifier diode, on the other half cycle.
165µ, 165 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, 56 V ac, 60 Hz
The power transformer’s bias tap, 56 V ac, as a sine source at the mains frequency. Typical of the part, not measured.
3.9k, 3.9 kΩ
Series resistor into the bias rectifier, 3.9k Ω. It limits the charging current.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
47µ, 47 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
Bias 10k, 10 kΩ pot, linear taper
Bias adjust, 10k Ω linear. Its wiper feeds both output grids through their grid leaks. Toward the diode end the grids go more negative and the tubes idle cooler. Set at 0.53 here, about 38 mA a tube, where crossover distortion on a quiet note is just gone, not measured on an amp.
22k, 22 kΩ
The bottom of the bias divider, 22k Ω to ground.
47µ, 47 µF electrolytic
Second filter cap, on the bias line at the pot’s wiper, its plus side to ground. It keeps ripple off the line the grid leaks hang from.
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. The feedback holds the output steadier against that than it would be without feedback.
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. The feedback holds the output steadier against that than it would be without feedback.
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
Four 12AX7 triodes, typical of the type, not measured.

Sources

  • jaysg, redrawn ODS #183 schematic, Rev 1.3 (2010)

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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