Two-channel 6SL7 bass amp with Ultra switches, 1968 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 Ampeg B-15-N Portaflex (1968).

Two channels, each a 6SL7 with a volume between its halves, two switches that reshape the lows and highs, and a bass and treble network, mixed into a 6SL7 that is both the last gain stage and the phase inverter.

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. This is a bass amp, but the note played into it is the same guitar as everywhere else on the bench, so the low notes here are a guitar’s. Each channel is a 6SL7 with both cathodes unbypassed, so the gain stays low and the channel stays clean. Ultra Low loads the first half’s plate above about 200 Hz, which leaves the lows standing out. Ultra Hi is a 500 pF bright cap across the volume. The bass and treble network drops about 20 dB with both knobs at noon and moves each band up or down from there. The two channels meet through 270 kΩ each at V3’s first grid, in the power amp. 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 Bright jack’s resistor to the grid, 100 kΩ with the .005 µF across it. With the guitar here, the empty Normal jack grounds its own 47 kΩ, so the two make a divider: the lows come in about 10 dB down, and the cap passes the top end around the 100 kΩ.
.005µ, 5 nF
Across the Bright jack’s 100 kΩ. Above about 300 Hz it starts to pass the signal around the resistor, so the top end gets through the divider with less loss than the lows.
47k, 47 kΩ
The Normal jack’s resistor to the grid, 47 kΩ. With the guitar here, the signal goes straight through it, full range. With the guitar in Bright, this jack’s switch grounds the far end and it’s the bottom of the divider.
5.6M, 5.6 MΩ
Grid leak, 5.6 MΩ, as drawn. It ties the grid to ground for DC.
V1a, 6SL7 (RCA)
The first half of the channel one’s 6SL7, pins 4, 5 and 6. Its cathode resistor isn’t bypassed, so the gain is about 30 and the stage stays clean on a bass or guitar.
5.6k, 5.6 kΩ
Cathode resistor, 5.6 kΩ, not bypassed. It sets a cold bias and halves the gain compared with a bypassed stage.
470k, 470 kΩ
Plate resistor, 470 kΩ.
.1µ, 100 nF
Coupling cap from the plate to the Ultra Low network and the volume.
.01µ, 10 nF
Into the top of the volume pot. With the 1 MΩ pot it rolls off below about 16 Hz.
.02µ, 20 nF
Top of the Ultra Low network. With the switch on, it and the 39 kΩ load the plate above about 200 Hz, so the mids and treble drop and the lows stand out.
39k, 39 kΩ
The Ultra Low network’s resistor. The plate’s own output resistance is a couple of hundred kΩ, so with the switch on this pulls the mids and treble down about 15 dB.
1M, 1 MΩ
Under the 39 kΩ to ground. With the Ultra Low switch off the network sees this 1 MΩ and does next to nothing.
Volume 1, 1 MΩ pot, audio taper
The channel one’s volume, 1 MΩ. It sits between the two halves of the 6SL7, so the second half and the tone network see what it lets through. The drawing marks no taper, so audio is assumed.
V1b, 6SL7 (RCA)
The second half of the 6SL7, pins 1, 2 and 3. It makes up the level lost in the tone network. Its cathode isn’t bypassed either.
2.2k, 2.2 kΩ
Cathode resistor, 2.2 kΩ, not bypassed.
220k, 220 kΩ
Plate resistor, 220 kΩ.
.1µ, 100 nF
Coupling cap into the tone network.
220k, 220 kΩ
Feeds the Bass pot’s top. With the 22 kΩ under the pot it sets how far the network drops the signal, about 20 dB with the knobs centered.
Bass 1, 1 MΩ pot, linear taper
Bass, 1 MΩ, with a cap across each half. Its wiper feeds the output through 120 kΩ. Toward the top the lows from the 220 kΩ get through, toward the bottom they’re pulled to the 22 kΩ. The caps let the mids and treble past whatever it’s set to. The taper isn’t marked, so linear is assumed.
.001µ, 1 nF
From the Bass pot’s top to its wiper. It passes the mids and treble around the upper half of the pot.
.01µ, 10 nF
From the Bass pot’s wiper to its bottom. It passes the mids and treble around the lower half.
22k, 22 kΩ
From the Bass pot’s bottom to ground, the bottom of the network’s divider.
270k, 270 kΩ
From the Bass pot’s wiper to ground. It drops the level slightly more and keeps the Bass knob from moving the mids as much.
470p, 470 pF
Into the Treble pot’s top, .00047 µF. Only the treble gets through it.
Treble 1, 1 MΩ pot, linear taper
Treble, 1 MΩ. Its top gets the treble through the 470 pF, its bottom goes to ground through .0047 µF, and its wiper is the network’s output. Up, more treble from the top. Down, the .0047 µF pulls treble off the output. The taper isn’t marked, so linear is assumed.
.0047µ, 4.7 nF
Under the Treble pot to ground. With the knob down it pulls the treble off the output.
120k, 120 kΩ
From the Bass wiper to the Treble wiper. The lows and mids reach the output this way.
270k, 270 kΩ
Mixing resistor, 270 kΩ, from the channel one’s tone network to V3’s grid, where the other channel’s 270 kΩ meets it. Each channel loads the other through the pair, about 6 dB.
100k, 100 kΩ
The Bright jack’s resistor to the grid, 100 kΩ with the .005 µF across it. With the guitar here, the empty Normal jack grounds its own 47 kΩ, so the two make a divider: the lows come in about 10 dB down, and the cap passes the top end around the 100 kΩ.
.005µ, 5 nF
Across the Bright jack’s 100 kΩ. Above about 300 Hz it starts to pass the signal around the resistor, so the top end gets through the divider with less loss than the lows.
47k, 47 kΩ
The Normal jack’s resistor to the grid, 47 kΩ. With the guitar here, the signal goes straight through it, full range. With the guitar in Bright, this jack’s switch grounds the far end and it’s the bottom of the divider.
5.6M, 5.6 MΩ
Grid leak, 5.6 MΩ, as drawn. It ties the grid to ground for DC.
V2a, 6SL7 (RCA)
The first half of the channel two’s 6SL7, pins 4, 5 and 6. Its cathode resistor isn’t bypassed, so the gain is about 30 and the stage stays clean on a bass or guitar.
5.6k, 5.6 kΩ
Cathode resistor, 5.6 kΩ, not bypassed. It sets a cold bias and halves the gain compared with a bypassed stage.
470k, 470 kΩ
Plate resistor, 470 kΩ.
.1µ, 100 nF
Coupling cap from the plate to the Ultra Low network and the volume.
.01µ, 10 nF
Into the top of the volume pot. With the 1 MΩ pot it rolls off below about 16 Hz.
.02µ, 20 nF
Top of the Ultra Low network. With the switch on, it and the 39 kΩ load the plate above about 200 Hz, so the mids and treble drop and the lows stand out.
39k, 39 kΩ
The Ultra Low network’s resistor. The plate’s own output resistance is a couple of hundred kΩ, so with the switch on this pulls the mids and treble down about 15 dB.
1M, 1 MΩ
Under the 39 kΩ to ground. With the Ultra Low switch off the network sees this 1 MΩ and does next to nothing.
Volume 2, 1 MΩ pot, audio taper
The channel two’s volume, 1 MΩ. It sits between the two halves of the 6SL7, so the second half and the tone network see what it lets through. The drawing marks no taper, so audio is assumed.
V2b, 6SL7 (RCA)
The second half of the 6SL7, pins 1, 2 and 3. It makes up the level lost in the tone network. Its cathode isn’t bypassed either.
2.2k, 2.2 kΩ
Cathode resistor, 2.2 kΩ, not bypassed.
220k, 220 kΩ
Plate resistor, 220 kΩ.
.1µ, 100 nF
Coupling cap into the tone network.
220k, 220 kΩ
Feeds the Bass pot’s top. With the 22 kΩ under the pot it sets how far the network drops the signal, about 20 dB with the knobs centered.
Bass 2, 1 MΩ pot, linear taper
Bass, 1 MΩ, with a cap across each half. Its wiper feeds the output through 120 kΩ. Toward the top the lows from the 220 kΩ get through, toward the bottom they’re pulled to the 22 kΩ. The caps let the mids and treble past whatever it’s set to. The taper isn’t marked, so linear is assumed.
.001µ, 1 nF
From the Bass pot’s top to its wiper. It passes the mids and treble around the upper half of the pot.
.01µ, 10 nF
From the Bass pot’s wiper to its bottom. It passes the mids and treble around the lower half.
22k, 22 kΩ
From the Bass pot’s bottom to ground, the bottom of the network’s divider.
270k, 270 kΩ
From the Bass pot’s wiper to ground. It drops the level slightly more and keeps the Bass knob from moving the mids as much.
470p, 470 pF
Into the Treble pot’s top, .00047 µF. Only the treble gets through it.
Treble 2, 1 MΩ pot, linear taper
Treble, 1 MΩ. Its top gets the treble through the 470 pF, its bottom goes to ground through .0047 µF, and its wiper is the network’s output. Up, more treble from the top. Down, the .0047 µF pulls treble off the output. The taper isn’t marked, so linear is assumed.
.0047µ, 4.7 nF
Under the Treble pot to ground. With the knob down it pulls the treble off the output.
120k, 120 kΩ
From the Bass wiper to the Treble wiper. The lows and mids reach the output this way.
270k, 270 kΩ
Mixing resistor, 270 kΩ, from the channel two’s tone network to V3’s grid, where the other channel’s 270 kΩ meets it. Each channel loads the other through the pair, about 6 dB.
V3a, 6SL7 (RCA)
6SL7GT, the driven half of the floating paraphase inverter. Its plate swings opposite the input and drives tube A. The feedback from the speaker goes to its cathode. Its grid connects straight to the preamp’s mixing resistors, which are its path to ground.
V3b, 6SL7 (RCA)
The other half. Its grid is fed from the middle of the divider, so its plate swings the other way and drives tube B.
470k, 470 kΩ
Grid resistor for the other half, to ground.
1k, 1 kΩ
Cathode resistor for both halves. It sets their bias, a couple of volts.
25µ, 25 µF electrolytic
Bypass cap across the shared cathode resistor. The cathodes hold still, so the second half gets its signal only from the divider.
120k, 120 kΩ
Plate resistor for the driven half.
120k, 120 kΩ
Plate resistor for the other half.
470k, 470 kΩ
Top of the divider, from tube A’s plate. With the resistor below it, from tube B’s, the middle sits at the average of the two sides. With the two swinging equal and opposite, the middle holds still. Whatever is left goes to the second half’s grid, which corrects it, so the inverter keeps itself even as the tube ages.
510k, 510 kΩ
Bottom of the divider, to tube B’s plate.
.022µ, 22 nF
From the middle of the divider to the second half’s grid. It keeps the plates’ DC off that grid.
10k, 10 kΩ
The feedback resistor from the speaker. With the 220 Ω below it, it sends about 2 % of the speaker voltage back to the inverter, which lowers the gain, flattens the response and tightens the bass.
220, 220 Ω
The driven half’s own cathode resistor, 220 Ω, between its cathode and the shared cathode resistor, not bypassed. It’s also the bottom of the feedback loop: the feedback resistor from the speaker connects at the top of it, so the fed-back speaker voltage moves that cathode. It lowers that half’s gain too.
.022µ, 22 nF
Coupling cap to tube A’s grid.
.022µ, 22 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.
V5, 6L6GC
6L6GC, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V4, 6L6GC
6L6GC, tube B of the pair.
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.
1k, 1 kΩ
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
40µ, 40 µF electrolytic
Filter cap on the screen supply.
22k, 22 kΩ
Dropping resistor from the screen supply node to the node for the inverter, as on the Ampeg B-15-N Portaflex 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 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 screen supply node to the node for both channels, as on the Ampeg B-15-N Portaflex 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 both channels. 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 8 Ω, 40 H primary, 10 mH leakage
The output transformer, 4 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, 385 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 385 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, 385 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, 60 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, 60 Ω
Resistance of the other half. Typical of the part, not measured.
V6, 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.
V6, GZ34
The rectifier’s other plate, on the other half cycle.
30µ, 30 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.
PT, 385 V ac, 60 Hz
The end of the high-voltage winding the bias comes off, 385 V ac, the same end as the rectifier’s first plate. It’s drawn as its own source so the bias supply’s average can be found. Keep it equal to the winding beside the rectifier.
100k, 100 kΩ
Series resistor into the bias rectifier, 100k Ω. There’s no bias tap, so the bias comes straight off the high-voltage winding, and this resistor drops most of it.
56k, 56 kΩ
From the diode’s winding side to ground, 56k Ω. With the 100k Ω above it, it divides the winding’s swing down before the diode, so it sets the bias along with the load after 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.
47k, 47 kΩ
The bias supply’s load, 47k Ω to ground. With the 100k Ω above it, it sets the bias. There’s no bias pot, so change this resistor to change the bias. Bigger runs the tubes colder.
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.
16 Ω tap, 1 V
The 16 Ω tap the feedback comes from, assumed to be an ideal tap on the same winding: 1.414 times the speaker voltage. The load the feedback network puts on the winding is left out. It’s tens of kilohms on a winding of a few ohms.

About this version

As drawn
Two 6SL7s, one a channel. The 6SL7 model is fitted to the RCA plate curves. The drawing shows no preamp voltages, so the preamp runs on the voltage from the power amp’s supply chain.

Sources

  • Joe Piazza, redrawn B-15-N schematic, 1996

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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