Two-stage amp with a pentode driver and KT88s, late 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 Sunn Sorado (and 200S).

A 12AX7 with Lo Boost and Hi Boost switches, a treble and bass stack, then a 6AN8 pentode wired straight into a cathodyne and a pair of ultra-linear KT88s.

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. A 12AX7 preamp with a treble and bass stack, then a lot of gain from the 6AN8’s pentode, which feeds its own triode, a cathodyne, with no coupling cap between them. The two switches change what gets to the second stage: Lo Boost lets the lows through to the Volume pot, Hi Boost deepens the mid scoop. Two KT88s run ultra-linear on about 480 V, with the speaker’s feedback going in under the pentode’s cathode.

Download.

Parts and what they do

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

R1, 68 kΩ
Input resistor, 68 kΩ into V1a’s grid. Each of the two jacks has its own, with nothing to switch the other out.
R2, 1 MΩ
Grid resistor, 1 MΩ to ground.
V1a, 12AX7 / ECC83 / 7025
First stage, half a 12AX7, fully bypassed for full gain.
RP1, 100 kΩ
V1a plate resistor, 100 kΩ from C (+320 V on the drawing).
RK1, 1.5 kΩ
V1a cathode resistor, 1.5 kΩ.
CK1, 250 µF electrolytic
V1a bypass cap, 250 µF 15 V. Full gain down through the bass.
CLB, 7.5 nF
Lo Boost cap, .0075 µF. With the switch on it feeds V1a’s plate to the Volume pot’s top, full range.
R10M, 10 MΩ
10 MΩ from V1a’s plate to the Volume pot’s top, across the cap and switch. With Lo Boost off it’s the only path to the pot’s top. It also puts a few volts of DC on the pot, as drawn.
CBR, 2 nF
.002 µF from V1a’s plate straight to the Volume wiper. With Lo Boost off it’s the main path: with the pot’s lower part it passes the mids and treble and cuts the lows.
Volume, 500 kΩ pot, audio taper
Volume, 500 kΩ. Its taper isn’t marked on the drawing, so audio is assumed. Its wiper is V1b’s grid.
V1b, 12AX7 / ECC83 / 7025
Second stage, the other half of the 12AX7, its cathode unbypassed. Its plate drives the tone stack directly.
RP2, 100 kΩ
V1b plate resistor, 100 kΩ.
RK2, 1.5 kΩ
V1b cathode resistor, 1.5 kΩ, no bypass cap. It holds the stage’s gain to about 33.
CT, 470 pF
Treble cap, 470 pF, to the Treble pot’s top.
RSL, 56 kΩ
Slope resistor, 56 kΩ.
CB, 22 nF
Bass cap, .022 µF, from the slope node into the Bass pot’s wiper.
Treble, 250 kΩ pot, linear taper
Treble, 250 kΩ. Its wiper drives the pentode’s grid. The taper isn’t marked, so linear is assumed. Clockwise moves the wiper toward the 470 pF, brighter.
Bass, 1 MΩ pot, audio taper
Bass, 1 MΩ, the bass cap on its wiper. Clockwise moves the wiper toward the Treble pot’s bottom, so the lows reach the Treble pot through less resistance and come up. The taper isn’t marked, so audio is assumed.
RBB, 22 kΩ
22 kΩ from the Bass pot’s bottom to ground.
R47, 47 kΩ
47 kΩ from the Treble pot’s bottom, with the .0075 µF below it to ground. It shunts some of the treble at the Treble pot’s bottom.
C75, 7.5 nF
.0075 µF under the 47 kΩ to ground.
CM, 22 nF
Mid cap, .022 µF, from the slope node to the 22 kΩ. It sets how deep the mid scoop goes.
RHB, 22 kΩ
22 kΩ under the mid cap. The Hi Boost switch shorts it.
RGP, 1 MΩ
Grid resistor for the pentode, 1 MΩ to ground.
V2a, 6AN8 pentode (RCA sheet)
The 6AN8’s pentode, the driver. Its model is typical: the 6AU6’s shape, set at the one operating point on the drawing, not fitted to a 6AN8 sheet. Its plate is wired straight to the cathodyne’s grid.
RP3, 270 kΩ
Pentode plate resistor, 270 kΩ. Its plate sits at about 75 V, and that sets the cathodyne’s grid.
C12, 12 pF
12 pF from the pentode’s plate to C. It rolls off the far top end.
RS3, 1.5 MΩ
Screen resistor, 1.5 MΩ. It holds the screen near 35 V, so the pentode runs on little current with a lot of gain.
CS3, 100 nF
Screen cap, .1 µF from the screen to the cathode, so the screen follows the cathode for signal.
RK3, 680 Ω
Pentode cathode resistor, 680 Ω, not bypassed, down to the feedback point.
V2b, 6AN8 (triode section)
6AN8 triode, a cathodyne, or split-load inverter, its grid wired straight to the plate of the stage before it, with no coupling cap. That plate’s voltage sets the grid’s, and the cathode sits a few volts above it. The same current runs through its plate resistor and its cathode resistor, which are 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. On the drawing it’s the triode half of the 6AN8 whose pentode is the stage before. Its model is fitted to the RCA 6AN8-A sheet.
47k, 47 kΩ
The cathodyne’s plate resistor, the output to tube B.
47k, 47 kΩ
The cathodyne’s cathode resistor, the same value as its plate resistor, straight to ground, and the output to tube A. There’s no separate bias resistor.
1k, 1 kΩ
The feedback resistor from the speaker. With the 47 Ω below it, it sends about 4 % of the speaker voltage back to the driver’s cathode, which lowers the gain, flattens the response and tightens the bass.
750p, 750 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.
47, 47 Ω
The bottom of the feedback loop, 47 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.
.25µ, 250 nF
Coupling cap to tube A’s grid.
.25µ, 250 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.
V3, KT88
kt88, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V4, KT88
kt88, tube B of the pair.
100k, 100 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.
100k, 100 kΩ
Grid leak for tube B.
15k, 15 kΩ
Dropping resistor from B+ to the node for the cathodyne, as on the Sunn Sorado / 200S 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.
20µ, 20 µF electrolytic
Filter cap at the node for the cathodyne. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
33k, 33 kΩ
Dropping resistor from the node for the cathodyne to the node for the preamp, as on the Sunn Sorado / 200S 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 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 8 Ω, 25 H primary, 8 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, 378 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 378 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, 378 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, 40 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, 40 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.
Bias tap, 61 V ac, 60 Hz
The power transformer’s bias tap, 61 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.
50µ, 50 µF electrolytic
Filter cap on the bias supply, its plus side to ground.
1k, 1 kΩ
Second resistor of the bias supply, 1k Ω. With the cap after it, it filters the ripple a second time.
50µ, 50 µF electrolytic
Second filter cap, its plus side to ground. The bias pot hangs from here.
Bias 10k, 10 kΩ pot, linear taper
Bias adjust, 10k Ω linear, from the second cap down to the resistor below it. Its wiper feeds both output grids through their grid leaks. Toward the cap end the grid goes more negative and the tube idles cooler.
18k, 18 kΩ
The bottom of the bias divider, 18k Ω to ground. It sets the least bias the pot can set.
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

Sorado / 200S, 1968
The May 1968 drawing, the same for the Sorado and the 200S. The 6AN8’s pentode model is set at the drawing’s operating point, a typical stand-in, and its triode is fitted to the RCA 6AN8-A sheet. The tube models are typical of each type, not measured.

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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