Two-channel bass amp with a tuned midrange loop, early 1970s 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 SVT.

Two channels, each with Bright and Normal jacks and low and high switches, one with a midrange control tuned by an inductor, mixed into a 6C4 follower. The power amp is a gain stage and cathodyne, a 12BH7 gain stage and follower a side, and six 6550s.

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

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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, played here with the same guitar as every other circuit. Channel One runs a high-gain stage into a follower, then a bass switch: Bass Cut feeds the volume through two .002 µF caps, Flat goes around them, Ultra Low adds a path that passes only the lows. A second stage drives the bass and treble network, then the same midrange loop as the Ampeg V-4: the Mid-Range pot hangs a tuned inductor on either end of a feedback loop, and the select switch picks about 220 Hz, 800 Hz or 3 kHz. Channel Two has an Ultra-Low switch and its own bass and treble, no midrange. In the power amp each side gets a 12BH7 gain stage and a follower that drives the 6550 grids directly. The drawings show no voltages, so every supply figure is typical. The inductor is typical too, and 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 470 pF 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.
470p, 470 pF
Across the Bright jack’s 100 kΩ, 470 pF. Above about 3 kHz it passes the signal around the resistor.
47k, 47 kΩ
The Normal jack’s resistor to the grid, 47 kΩ. With the guitar in Bright, this jack’s switch grounds its far end and it’s the bottom of the divider.
5.6M, 5.6 MΩ
Grid leak, 5.6 MΩ.
100k, 100 kΩ
The Bright jack’s resistor to the grid, 100 kΩ with 470 pF 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.
470p, 470 pF
Across the Bright jack’s 100 kΩ, 470 pF. Above about 3 kHz it passes the signal around the resistor.
47k, 47 kΩ
The Normal jack’s resistor to the grid, 47 kΩ. With the guitar in Bright, this jack’s switch grounds its far end and it’s the bottom of the divider.
5.6M, 5.6 MΩ
Grid leak, 5.6 MΩ.
V1a, 12AX7 / ECC83 / 7025
Channel One’s first stage, the high-gain half of a 12DW7 (pins 6, 7 and 8, on the 12AX7 model). Its cathode isn’t bypassed.
3.3k, 3.3 kΩ
Cathode resistor, 3.3 kΩ, not bypassed.
220k, 220 kΩ
Plate resistor, 220 kΩ.
V1b, 12AU7 / ECC82
A cathode follower, the 12DW7’s low-gain half (pins 1, 2 and 3, on the 12AU7 model), its grid straight on V1a’s plate. It drives the bass switch’s network from a low impedance.
47k, 47 kΩ
The follower’s cathode resistor, 47 kΩ.
.1µ, 100 nF
Coupling cap, .1 µF, from the follower to the bass switch’s network.
100k, 100 kΩ
From the network’s input to ground, 100 kΩ.
.002µ, 2 nF
The first of two .002 µF caps in series to the volume. On Bass Cut they’re the only way in, so with the volume’s 1 MΩ they cut below about 160 Hz.
.002µ, 2 nF
The second .002 µF to the volume’s top.
820k, 820 kΩ
From the two caps’ middle toward ground, 820 kΩ. On Ultra Low the switch shorts it, so the 68 kΩ alone loads that middle.
68k, 68 kΩ
Under the 820 kΩ to ground, 68 kΩ.
150k, 150 kΩ
The Ultra Low path’s first 150 kΩ. With the .002 µF to ground and the second 150 kΩ it passes only the lows.
.002µ, 2 nF
From the Ultra Low path’s middle to ground, .002 µF. It removes the mids and treble from that path.
150k, 150 kΩ
The Ultra Low path’s second 150 kΩ, to the switch.
500p, 500 pF
The Ultra-High cap, 500 pF from the volume’s top. On, the switch joins its far end to the wiper, a bright cap across the pot.
Volume 1, 1 MΩ pot, linear taper
Channel One’s volume, 1 MΩ linear.
V3a, 12AU7 / ECC82
Channel One’s second stage, the low-gain half of V3, a 12DW7 (pins 1, 2 and 3, on the 12AU7 model), with a 47 kΩ plate resistor. It drives the tone network.
1.5k, 1.5 kΩ
Cathode resistor, 1.5 kΩ, not bypassed.
47k, 47 kΩ
Plate resistor, 47 kΩ.
.1µ, 100 nF
Coupling cap, .1 µF, into the tone network.
V2a, 12AX7 / ECC83 / 7025
Channel Two’s first stage, half a 12AX7 (pins 1, 2 and 3). Its cathode isn’t bypassed.
3.3k, 3.3 kΩ
Cathode resistor, 3.3 kΩ, not bypassed.
220k, 220 kΩ
Plate resistor, 220 kΩ.
.1µ, 100 nF
Coupling cap, .1 µF, to the Ultra-Low network.
.1µ, 100 nF
Top of the Ultra-Low load, .1 µF. With the switch on, it and the 68 kΩ load the plate above a few hundred hertz, so the mids drop and the lows stand out.
68k, 68 kΩ
The Ultra-Low load’s 68 kΩ.
1M, 1 MΩ
From the 68 kΩ’s far end to ground, 1 MΩ. With the switch off the load sees this and does next to nothing.
10M, 10 MΩ
From the network to the volume’s top, 10 MΩ with .001 µF across it. With the Ultra-Low switch off, only the .001 µF gets the signal through, so the lows are cut below about 160 Hz.
.001µ, 1 nF
Across the 10 MΩ, .001 µF.
500p, 500 pF
The Ultra-High cap, 500 pF from the volume’s top. On, the switch joins its far end to the wiper.
Volume 2, 1 MΩ pot, linear taper
Channel Two’s volume, 1 MΩ linear.
V2b, 12AX7 / ECC83 / 7025
Channel Two’s second stage, the other half of the 12AX7 (pins 6, 7 and 8). It drives Channel Two’s tone network. Channel Two has no midrange control.
3.3k, 3.3 kΩ
Cathode resistor, 3.3 kΩ, not bypassed.
100k, 100 kΩ
Plate resistor, 100 kΩ.
.1µ, 100 nF
Coupling cap, .1 µF, into Channel Two’s tone network.
100k, 100 kΩ
Channel Two’s mixing resistor, 100 kΩ, from its tone network to the mixing point.
220k, 220 kΩ
Into the bass pot’s top, 220 kΩ. With the 22 kΩ at the pot’s foot it sets how far the network drops the signal, about 20 dB with the knobs at noon.
Bass 1, 1 MΩ pot, linear taper
Bass, 1 MΩ, a cap across each half. Up, the lows from the 220 kΩ reach the wiper. Down, they go to the 22 kΩ at its foot. The taper isn’t marked, so linear is assumed.
.001µ, 1 nF
Across the top half of the bass pot, .001 µF. It carries the mids and treble past it.
.01µ, 10 nF
Across the bottom half of the bass pot, .01 µF.
22k, 22 kΩ
From the bass pot’s foot to ground, 22 kΩ, the bottom of the network’s divider.
47p, 47 pF
Into the treble pot’s top, 47 pF. Only the high treble gets through.
Treble 1, 1 MΩ pot, linear taper
Treble, 1 MΩ. Its wiper is the network’s output. Up, more treble from the 47 pF. Down, the 470 pF at its foot pulls treble off. The taper isn’t marked, so linear is assumed.
470p, 470 pF
From the treble pot’s foot to ground, 470 pF.
120k, 120 kΩ
From the bass wiper to the treble wiper, 120 kΩ. The lows and mids reach the output this way.
220k, 220 kΩ
Into the bass pot’s top, 220 kΩ. With the 22 kΩ at the pot’s foot it sets how far the network drops the signal, about 20 dB with the knobs at noon.
Bass 2, 1 MΩ pot, linear taper
Bass, 1 MΩ, a cap across each half. Up, the lows from the 220 kΩ reach the wiper. Down, they go to the 22 kΩ at its foot. The taper isn’t marked, so linear is assumed.
.001µ, 1 nF
Across the top half of the bass pot, .001 µF. It carries the mids and treble past it.
.01µ, 10 nF
Across the bottom half of the bass pot, .01 µF.
22k, 22 kΩ
From the bass pot’s foot to ground, 22 kΩ, the bottom of the network’s divider.
47p, 47 pF
Into the treble pot’s top, 47 pF. Only the high treble gets through.
Treble 2, 1 MΩ pot, linear taper
Treble, 1 MΩ. Its wiper is the network’s output. Up, more treble from the 47 pF. Down, the 470 pF at its foot pulls treble off. The taper isn’t marked, so linear is assumed.
470p, 470 pF
From the treble pot’s foot to ground, 470 pF.
120k, 120 kΩ
From the bass wiper to the treble wiper, 120 kΩ. The lows and mids reach the output this way.
.01µ, 10 nF
Coupling cap, .01 µF, from Channel One’s tone network to the midrange loop.
V3b, 12AX7 / ECC83 / 7025
The midrange loop’s first stage, the high-gain half of V3 (pins 6, 7 and 8, on the 12AX7 model). Its cathode is where the loop works: the follower’s output comes back through 56 kΩ, and the mid network hangs on it.
1M, 1 MΩ
Grid leak, 1 MΩ, to the foot of the 560 Ω.
560, 560 Ω
Under the cathode, 560 Ω. The grid returns below it, so this sets the bias.
7.5k, 7.5 kΩ
To ground, 7.5 kΩ. The follower’s current through the 56 kΩ also flows here and lifts the whole cathode about 20 V.
220k, 220 kΩ
Plate resistor, 220 kΩ.
.01µ, 10 nF
Coupling cap, .01 µF.
1M, 1 MΩ
Grid leak, 1 MΩ.
V4b, 12AX7 / ECC83 / 7025
The loop’s second stage, the high-gain half of V4, a 12DW7 (pins 6, 7 and 8). Its plate drives the follower directly.
3.3k, 3.3 kΩ
Cathode resistor, 3.3 kΩ, not bypassed.
470k, 470 kΩ
Plate resistor, 470 kΩ.
V4a, 12AU7 / ECC82
A cathode follower, V4’s low-gain half (pins 1, 2 and 3, on the 12AU7 model). It drives the mid network, the mixing point and, through 56 kΩ, V3b’s cathode. From V3b’s grid to here the signal is turned over twice, so that path is negative feedback.
47k, 47 kΩ
The follower’s load, 47 kΩ, above the 6.8 kΩ. The signal goes on from their middle.
6.8k, 6.8 kΩ
Bottom of the follower’s load, 6.8 kΩ.
56k, 56 kΩ
The loop’s feedback, 56 kΩ from the follower back to V3b’s cathode. It also carries DC, which is why that cathode sits about 20 V up.
470, 470 Ω
From V3b’s cathode to the Mid-Range pot’s boost end, 470 Ω.
.68µ, 680 nF
.68 µF to the pot’s boost end. With the inductor it sets the band’s center.
620, 620 Ω
From the follower’s divider to the pot’s cut end, 620 Ω.
.68µ, 680 nF
.68 µF to the pot’s cut end.
Mid-Range, 50 kΩ pot, linear taper
Mid-Range, 50 kΩ linear. Its wiper goes to ground through the inductor and the select switch’s cap, a path that’s low only near one frequency. Up, that path pulls V3b’s cathode down there, so less is fed back and that band comes up. Down, it pulls on the follower’s output instead and the band drops.
L 4–3, 89.4 mH
The inductor from its top (terminal 4) to tap 3. Typical, not on the drawing: set so the three select positions center near 220 Hz, 800 Hz and 3 kHz.
L 3–2, 72.1 mH
The inductor from tap 3 to tap 2. Typical, not measured.
L 2–1, 96.1 mH
The inductor from tap 2 to its foot (terminal 1). Typical, not measured. The whole winding is about 0.77 H.
K, coupling factor 0.990
Coupling between the inductor’s top two sections. Typical.
K, coupling factor 0.990
Coupling between the middle and bottom sections.
K, coupling factor 0.990
Coupling between the top and bottom sections.
220k, 220 kΩ
From tap 3 to ground, 220 kΩ. It keeps the .033 µF from sitting charged when it’s switched in.
220k, 220 kΩ
From tap 2 to ground, 220 kΩ, the same for the .15 µF.
.033µ, 33 nF
.033 µF from tap 3 to the select switch. On position 3 it’s grounded, so the top of the inductor and this cap tune the band near 3 kHz.
.15µ, 150 nF
.15 µF from tap 2 to the select switch. On position 2 the inductor down to tap 2 and this cap tune the band near 800 Hz.
.1µ, 100 nF
Coupling cap, .1 µF, from the follower’s divider toward the mixing point.
100k, 100 kΩ
Channel One’s mixing resistor, 100 kΩ. With Channel Two’s it mixes the channels, and each channel loads the other through the pair.
.02µ, 20 nF
Coupling cap, .02 µF, from the mixing point to the output follower.
V5, 12AU7 / ECC82
The output cathode follower, a 6C4 (half a 12AU7, on the 12AU7 model). It drives the cable to the power amp chassis.
1M, 1 MΩ
Grid return, 1 MΩ, to the middle of the cathode resistors, so the grid sits 1 kΩ’s drop below the cathode.
1k, 1 kΩ
Under the cathode, 1 kΩ. Its drop is the follower’s bias.
47k, 47 kΩ
The follower’s load, 47 kΩ.
.1µ, 100 nF
Coupling cap, .1 µF, out to the connector’s pin 1.
100k, 100 kΩ
From the preamp’s output to ground, 100 kΩ.
1k, 1 kΩ
On the power amp chassis: 1 kΩ from pin 1 to the Ext. Amp jacks (not modeled).
1k, 1 kΩ
On the power amp chassis: 1 kΩ on to the power amp’s first grid.
Diodes, 1N914 / 1N4148, one each way
On the power amp chassis: two silicon diodes back to back from the power amp’s first grid to ground. They hold its input to about 0.6 V either way, so a hot signal clips here. The drawing doesn’t name them, so the 1N914 model is assumed.
V6a, 12AX7 / ECC83 / 7025
12AU7 (ECC82), 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.
470k, 470 kΩ
Grid resistor for the driver.
220k, 220 kΩ
Plate resistor for the driver.
2.2k, 2.2 kΩ
Cathode resistor for the driver. It sets the driver’s bias, a volt or so. It sits on the feedback leg, so the fed-back signal moves the driver’s cathode.
.1µ, 100 nF
Coupling cap from the driver to the cathodyne.
V6b, 12AU7 / ECC82
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.
15k, 15 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.
14k, 14 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.
47k, 47 kΩ
The feedback resistor from the speaker. With the 220 Ω below it, it sends about 0 % of the speaker voltage back to the driver’s cathode, which lowers the gain, flattens the response and tightens the bass.
120p, 120 pF
A small cap across the feedback resistor. It feeds back more of the top end, which keeps the loop steady out where the output transformer turns the phase round.
220, 220 Ω
The bottom of the feedback loop, 220 Ω to ground under the driver’s cathode resistor. It isn’t bypassed, so the fed-back speaker voltage moves the cathode. The driver’s current runs through it too, so it’s kept small.
.1µ, 100 nF
Coupling cap from the inverter to side A’s driver.
.1µ, 100 nF
Coupling cap from the inverter to side B’s driver.
1k, 1 kΩ
Dropping resistor from B+ to the drivers’ plate supply. The drawing marks this line H, with 1 k and 40 µF near it. Where its 1 k starts can’t be traced on the redraw, so here it starts at B+.
40µ, 40 µF electrolytic
Filter cap on the drivers’ plate supply. It keeps B+ ripple off the drivers, whose plates connect to the output grids through the followers.
470k, 470 kΩ
Grid leak for side A’s driver, to ground.
V13a, 12BH7 (GE, JJ)
12BH7, side A’s driver: a gain stage that raises the inverter’s signal about 9 times on its way to the output grids.
1.8k, 1.8 kΩ
The driver’s cathode resistor, not bypassed.
47k, 47 kΩ
The driver’s plate resistor, 47k Ω from the drivers’ plate supply.
.047µ, 47 nF
Coupling cap from the driver’s plate to the middle of the two 150k Ω.
150k, 150 kΩ
From the coupling cap to the follower’s grid.
150k, 150 kΩ
From the coupling cap to the bias pot’s wiper. Through it and the resistor above, the pot sets the follower’s grid, so the follower sets the output grids’ bias.
V13b, 12BH7 (GE, JJ)
12BH7, side A’s cathode follower, its plate on the screen supply. Its cathode drives the output grids directly through their stoppers, so they get a low-impedance drive that can push them toward 0 V without the blocking a coupling cap causes.
470k, 470 kΩ
Grid leak for side B’s driver, to ground.
V14a, 12BH7 (GE, JJ)
12BH7, side B’s driver: a gain stage that raises the inverter’s signal about 9 times on its way to the output grids.
1.8k, 1.8 kΩ
The driver’s cathode resistor, not bypassed.
47k, 47 kΩ
The driver’s plate resistor, 47k Ω from the drivers’ plate supply.
.047µ, 47 nF
Coupling cap from the driver’s plate to the middle of the two 150k Ω.
150k, 150 kΩ
From the coupling cap to the follower’s grid.
150k, 150 kΩ
From the coupling cap to the bias pot’s wiper. Through it and the resistor above, the pot sets the follower’s grid, so the follower sets the output grids’ bias.
V14b, 12BH7 (GE, JJ)
12BH7, side B’s cathode follower, its plate on the screen supply. Its cathode drives the output grids directly through their stoppers, so they get a low-impedance drive that can push them toward 0 V without the blocking a coupling cap causes.
15.7k, 15.7 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current. Each of the three tubes has its own 47k. The resistors are drawn as one part, the three in parallel.
15.7k, 15.7 kΩ
Grid stopper for tube B. Each of the three tubes has its own 47k. The resistors are drawn as one part, the three in parallel.
V7, V9, V11, 6550
n6550, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply. Three tubes in parallel, drawn as one.
V8, V10, V12, 6550
n6550, tube B of the pair. Three tubes in parallel, drawn as one.
3.33, 3.33 Ω
Plate resistor for tube A, 10 Ω between the plate and the output transformer. It damps oscillation at radio frequencies. The drop across it is the plate current times 10 Ω, a few volts. Each of the three tubes has its own 10. The resistors are drawn as one part, the three in parallel.
3.33, 3.33 Ω
Plate resistor for tube B, the same. Each of the three tubes has its own 10. The resistors are drawn as one part, the three in parallel.
47k, 47 kΩ
The follower’s cathode resistor, to the bias supply ahead of the pots. The follower’s cathode is the output grids’ bias.
47k, 47 kΩ
Side B’s follower’s cathode resistor, the same.
7.33, 7.33 Ω
Screen resistor for tube A. It limits the screen current when the tube is driven hard. Each of the three tubes has its own 22. The resistors are drawn as one part, the three in parallel.
7.33, 7.33 Ω
Screen resistor for tube B. Each of the three tubes has its own 22. The resistors are drawn as one part, the three in parallel.
8.2k, 8.2 kΩ
Dropping resistor from B+ to the screens. It’s large, so the screens sit about 30 V below the plates and drop further when they draw more current on a hard note.
30µ, 30 µF electrolytic
Filter cap on the screen supply.
40µ, 40 µF electrolytic
Second filter cap on the screen supply, beside the first.
15k, 15 kΩ
Dropping resistor from the screen supply node to the node for the connector’s pin 5, as on the Ampeg SVT 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.
8.2k, 8.2 kΩ
Dropping resistor from the node for the connector’s pin 5 to the node for the preamp, as on the Ampeg SVT 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 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 4 Ω, 20 H primary, 5 mH leakage
The output transformer, 1.7 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, 488 V ac, 60 Hz
The power transformer’s high-voltage winding, 488 V ac end to end, into a bridge of four silicon diodes. It’s drawn as two sources of 488 V ac a half cycle apart into two diodes, which charges the reservoir the same way at one diode drop (under a volt) higher. The winding’s marked value, on the drawing.
PT, 488 V ac, 60 Hz
The same winding on the other half cycle. It runs opposite the first source, so the bridge charges the reservoir on every half cycle.
PT R, 15 Ω
Resistance of the whole winding, plus the primary’s share. In a bridge every charging pulse runs through the whole winding. With the diodes’ drop it sets how far B+ sags on a hard note. Typical of the part, not measured.
PT R, 15 Ω
The same winding’s resistance, on the other half cycle. 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.
100µ, 100 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, 96 V ac, 60 Hz
The power transformer’s bias tap, 96 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.
100µ, 100 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
19.5k, 19.5 kΩ
Second resistor of the bias supply, 19.5k Ω. With the cap after it, it filters the ripple a second time.
Bias A, 15 kΩ pot, linear taper
Bias adjust for tube A, 15k Ω linear, from the second cap down to the resistor below it. Its wiper feeds tube A’s grid through its grid leak. Toward the cap end the grid goes more negative and the tube idles cooler.
Bias B, 15 kΩ pot, linear taper
Bias adjust for tube B, the same, beside the first. With one for each tube, the two can be set to idle alike.
22k, 22 kΩ
The bottom of the bias divider, 22k Ω to ground, under both pots. It sets the least bias the pots can set.
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.

About this version

As drawn
Joe Piazza’s 1972 preamp drawing on his 6550 power amp drawing. Each 12DW7 half runs on the model of the tube it’s half of (12AX7 or 12AU7), the 6C4 on the 12AU7’s. The inductor’s values aren’t on the drawing and are typical. The tube models are typical of each type, not measured.

Sources

  • Joe Piazza, redrawn SVT preamp and power amp schematics, 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.

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.

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