Brownface two-channel bass amp head with presence, 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 Fender 6G6-A Bassman.

Two channels on 7025s. The bass channel is a gain stage, a cathode follower, a bass network with no treble cap of its own, two more stages and a treble cut. The normal channel is the brownface two-stage layout with a tapped treble pot. Both mix through 470 kΩ into a 7025 long-tail pair and two 5881s.

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 bass channel’s first network passes everything through 100 pF and a 1 MΩ path, with the bass pot setting how much low end goes around them, and its Treble knob after the last stage only cuts. The normal channel is the brownface guitar circuit of the same years. The power amp has 56 kΩ of feedback with a Presence pot. The 6G6 and 6G6-B builds are the earlier and later drawings.

Download.

Parts and what they do

Stock values for the 6G6-A version. Tap a part on the schematic to change it.

R1B, 68 kΩ
The bass channel’s input 1 resistor into the grid. With nothing in input 2 it sits in parallel with input 2’s resistor, so a guitar in input 1 sees 34 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R3B, 68 kΩ
The bass channel’s input 2 resistor into the grid. With nothing in input 1, input 1’s tip is grounded, so the two 68 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 136 kΩ.
R2B, 1 MΩ
Input 1’s 1 MΩ to ground. It sets the load a guitar in input 1 sees, and with a patch lead in input 1 it is the end of the lead’s path.
R1A, 68 kΩ
The normal channel’s input 1 resistor into the grid. With nothing in input 2 it sits in parallel with input 2’s resistor, so a guitar in input 1 sees 34 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R3A, 68 kΩ
The normal channel’s input 2 resistor into the grid. With nothing in input 1, input 1’s tip is grounded, so the two 68 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 136 kΩ.
R2A, 1 MΩ
Input 1’s 1 MΩ to ground. It sets the load a guitar in input 1 sees, and with a patch lead in input 1 it is the end of the lead’s path.
27k, 27 kΩ
Dropping resistor, 27 kΩ 1 W, from the inverter’s node (+450 V) to box A (+390 V), the normal channel’s supply.
20µ, 20 µF electrolytic
Filter cap on box A, 20 µF 600 V.
56k, 56 kΩ
Dropping resistor, 56 kΩ 1 W, from box B (+470 V) to the bass channel’s node (+280 V). All four bass channel triodes pull their current through it.
20µ, 20 µF electrolytic
Filter cap on the bass channel’s node, 20 µF 600 V.
V1a, 12AX7 / ECC83 / 7025
Bass channel first stage, half a 7025, on a 220 kΩ plate resistor. Its plate drives the cathode follower directly.
2700, 2.7 kΩ
V1a cathode resistor, 2700 Ω.
25µ, 25 µF electrolytic
V1a bypass cap, 25 µF 25 V.
220k, 220 kΩ
V1a plate resistor, 220 kΩ. It also sets the follower’s grid voltage, so the follower’s cathode sits about where V1a’s plate does.
V1b, 12AX7 / ECC83 / 7025
Cathode follower, the other half of V1, DC-coupled from V1a’s plate. It drives the bass network from a low impedance.
100k, 100 kΩ
Cathode follower load, 100 kΩ.
CT, 100 pF
The network’s top cap, 100 pF on the 6G6-A and 6G6 (250 pF on the 6G6-B). It passes the treble to the volume’s top around the 1 MΩ path.
R1, 27 kΩ
From the 100 pF to the volume’s top, 27 kΩ on the 6G6-A (47 kΩ on the 6G6-B). The 6G6 has none.
R2, 27 kΩ
From the volume’s top to the bass pot’s top, 27 kΩ on the 6G6-A (56 kΩ on the 6G6, 10 kΩ on the 6G6-B). With the bass pot it sets how much the lows come up.
1M, 1 MΩ
From the follower’s cathode, 1 MΩ to the two .25 µF caps. Most of the mids and lows reach the volume through here and the caps.
.25µ, 250 nF
.25 µF from the 1 MΩ to the bass pot’s top.
.25µ, 250 nF
.25 µF from the 1 MΩ to the bass pot’s foot. With the pot turned up, the lows go through it around the pot.
Bass, 25 kΩ pot, linear taper
Bass channel bass, 25 kΩ linear on the layout, a variable resistor between the two .25 µF caps’ far ends.
820, 820 Ω
The network’s foot, 820 Ω to ground.
Volume, 250 kΩ pot, linear taper
Bass channel volume, 250 kΩ, marked linear on the layout.
V2a, 12AX7 / ECC83 / 7025
Bass channel second stage, half a 7025.
1500, 1.5 kΩ
V2a cathode resistor, 1500 Ω.
25µ, 25 µF electrolytic
V2a bypass cap, 25 µF.
100k, 100 kΩ
V2a plate resistor, 100 kΩ.
CC, 50 nF
V2a coupling cap, .05 µF (.1 µF on the 6G6-B).
1M, 1 MΩ
V2b grid resistor, 1 MΩ.
V2b, 12AX7 / ECC83 / 7025
Bass channel third stage, the other half of V2. Its cathode isn’t bypassed.
1500, 1.5 kΩ
V2b cathode resistor, 1500 Ω, not bypassed.
100k, 100 kΩ
V2b plate resistor, 100 kΩ.
.1µ, 100 nF
Coupling cap from V2b to the treble pot, .1 µF.
Treble, 100 kΩ pot, linear taper
Bass channel treble, 100 kΩ linear. Its foot has .02 µF to ground, so turned down the wiper slides toward a treble cut. At the top it passes everything.
.02µ, 20 nF
Under the treble pot, .02 µF to ground.
470k, 470 kΩ
Bass channel mixing resistor, 470 kΩ.
V3a, 12AX7 / ECC83 / 7025
Normal channel first stage, half a 7025.
1500, 1.5 kΩ
V3a cathode resistor, 1500 Ω.
25µ, 25 µF electrolytic
V3a bypass cap, 25 µF.
100k, 100 kΩ
V3a plate resistor, 100 kΩ from box A.
250p, 250 pF
Treble cap, 250 pF, straight off the plate.
Treble, 250 kΩ pot, linear taper
Normal treble, 250 kΩ tapped 70 kΩ from its bottom lug. The lows reach the tap and the highs the top, and the wiper mixes the two. Below the tap the wiper slides toward the .05 µF. No taper is marked, so it’s assumed to be linear.
.05µ, 50 nF
From the treble pot’s foot lug to ground, .05 µF.
100k, 100 kΩ
Slope resistor, 100 kΩ.
.1µ, 100 nF
Bass cap, .1 µF, to the treble pot’s tap and the bass pot’s top.
CM, 50 nF
Middle cap, .05 µF (.02 µF on the 6G6).
Bass, 250 kΩ pot, audio taper
Normal bass, 250 kΩ audio, a variable resistor.
RM, 6.8 kΩ
The network’s foot, 6800 Ω, as on the 6G6 and 6G6-B.
Volume, 1 MΩ pot, audio taper
Normal volume, 1 MΩ audio.
V3b, 12AX7 / ECC83 / 7025
Normal channel second stage, the other half of V3, on a 220 kΩ plate resistor.
2700, 2.7 kΩ
V3b cathode resistor, 2700 Ω.
CK, 25 µF electrolytic
V3b bypass cap, 25 µF (read as .1 µF on the 6G6, which bypasses only the top end).
220k, 220 kΩ
V3b plate resistor, 220 kΩ from box A.
.05µ, 50 nF
V3b coupling cap, .05 µF.
470k, 470 kΩ
Normal channel mixing resistor, 470 kΩ. The inverter’s 1 MΩ grid resistor is the far side of the mix, so each channel loses about a third of its signal here.
V4a, 12AX7 / ECC83 / 7025
12AX7, the driven half of the long-tail pair phase inverter. Its plate swings opposite the input and drives tube A. Its grid is fed through the preamp’s coupling cap, so there’s no second one here.
V4b, 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.
6.8k, 6.8 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.
82k, 82 kΩ
Plate resistor for the driven half. It’s smaller than the other so the two outputs come out about even.
100k, 100 kΩ
Plate resistor for the other half.
100p, 100 pF
A small cap from plate to plate. It rolls off the far top end the inverter passes on, which keeps the feedback loop steady.
56k, 56 kΩ
The feedback resistor from the speaker. With the 4.7k 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.
4.7k, 4.7 kΩ
The bottom of the feedback loop, 4.7k Ω to ground. The fed-back speaker voltage arrives here and moves the tail.
.1µ, 100 nF
The presence cap, in series with the pot across the leg. With the pot it sets where the lift starts.
Presence, 25 kΩ pot, linear taper
Presence, a 25k 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.
V5, 5881
n5881, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V6, 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.
470, 470 Ω
Screen resistor for tube A. It limits the screen current when the tube is driven hard.
470, 470 Ω
Screen resistor for tube B.
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+.
20µ, 20 µF electrolytic
Filter cap on the screen supply.
4.7k, 4.7 kΩ
Dropping resistor from the screen supply node to the node for the inverter and the preamp, as on the Fender 6G6-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.
20µ, 20 µF electrolytic
Filter cap at the node for the inverter and 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 4 Ω, 40 H primary, 10 mH leakage
The output transformer, 4 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, 350 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 350 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, 350 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, 50 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, 50 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.
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, 50 V ac, 60 Hz
The power transformer’s bias tap, 50 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.
8µ, 8 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
10k, 10 kΩ
Second resistor of the bias supply, 10k Ω to the bias line. With the cap after it, it filters the ripple a second time.
8µ, 8 µF electrolytic
Second filter cap, on the bias line, its plus side to ground.
56k, 56 kΩ
From the bias line to ground, 56k Ω. With the 10k 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. The feedback holds the output steadier against that than it would be without feedback.
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. The feedback holds the output steadier against that than it would be without feedback.
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.

Versions

6G6-A
The 6G6-A drawing: 27 kΩ on each side of the bass channel’s volume tap, V3b’s cathode bypassed by 25 µF, and a silicon rectifier in the power amp. The tube models are typical of each type, not measured.
6G6, modeled after the Fender 6G6 Bassman
The earlier 6G6 drawing: 56 kΩ from the volume’s top to the bass pot, 1 MΩ of feedback around V2b, a .02 µF middle cap and 6.8 kΩ in the normal channel, and V3b’s cathode with a cap read as .1 µF. Its power amp opens as the 6G6-A’s with 100 kΩ feedback and 47 pF plate to plate. The real 6G6 has a GZ34 rectifier, which sags more.
6G6-B, modeled after the Fender 6G6-B Bassman
The later 6G6-B drawing: 47 kΩ and 10 kΩ in the bass network, the treble pot moved between V2a and V2b, no coupling caps ahead of the 470 kΩ mixing resistors, and 500 pF into the inverter. Its power amp opens as the 6G6-A’s with .1 µF coupling caps. The real 6G6-B’s bias supply is 1 kΩ, one 25 µF and 27 kΩ to ground, with no second filter step.

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.

Spot an error, or want a circuit added? Email info@swellamps.com. A schematic or a clear photo of one helps. Anything you send may be used to improve these models and our desktop app.

© 2026 Swell Amplification LLC. All rights reserved. The simulator, its circuit models, drawings, presets and text are the property of Swell Amplification LLC and may not be copied, reproduced, redistributed or used to build another product without written permission. The original circuits are historical designs documented in public sources, and no claim is made to them. Everything here is provided as is, without warranty of any kind, for study and personal use. Results are simulations of typical parts, not measurements of any real unit. Don’t rely on them for the safety of any build or repair. Tube amplifiers carry lethal voltages.

Product and company names are trademarks of their owners. They are used here only to identify the circuits these models are based on; this site is not affiliated with or endorsed by any of them.

From Swell Amplification, swellamps.com.