Two-channel amp with a tuned midrange loop, 1970 style

Parts and what they do

Stock values for the V-4 head version. Tap a part on the schematic to change it.

R1, 1 kΩ

Grid stopper, 1 kΩ from the jack to the grid. With the 50 pF it keeps radio out. Ampeg’s drawing marks it as left out of some units.

R2, 5.6 MΩ

Grid leak, 5.6 MΩ from the jack to ground.

C1, 50 pF

50 pF from the jack to the first stage’s cathode, for radio frequencies. At audio it does next to nothing. Left out of some units.

V1a, 12AX7 / ECC83 / 7025

The channel one’s first stage, half a 12AX7 (pins 6, 7 and 8 of V1). The sensitivity switch sets how much of its cathode resistance is bypassed, so its gain runs from about 30 to about 75.

R6, 390 kΩ

Plate resistor, 390 kΩ. The drawing reads 195 V on the plate.

R5, 6.8 kΩ

Cathode resistor, 6.8 kΩ. It alone sets the bias, about 2 V, since everything else on the cathode goes through a cap.

R4, 33 kΩ

From the cathode to the 10 µF, 33 kΩ. With the switch on Low it’s the only bypass, so the cathode sees 6.8 kΩ beside 33 kΩ for signal, about 5.6 kΩ.

C2, 10 µF electrolytic

The bypass cap, 10 µF 25 V. The switch sets how it reaches the cathode: through 33 kΩ, through 33 kΩ and 6.8 kΩ side by side, or straight.

R3, 6.8 kΩ

From the cathode to the sensitivity switch, 6.8 kΩ. On Mid the switch joins its foot to the 10 µF, so it bypasses beside the 33 kΩ, about 3.1 kΩ left for signal.

C3, 10 nF

Coupling cap, .01 µF, from the plate to the volume pot.

Volume 1, 1 MΩ pot, linear taper

The channel one’s volume, 1 MΩ linear on the drawing. Its wiper feeds the second stage.

C102, 120 pF

The Ultra Hi bright cap, 120 pF from the volume’s wiper. Up, the switch joins its other end to the volume’s top, so treble skips past the pot. It does the most with the volume low and nothing at full.

C101, 1 nF

The Ultra Hi cut cap, .001 µF from the volume’s wiper. Down, the switch grounds its other end, so treble from the wiper goes to ground: the volume’s own resistance and this cap roll off the top.

V1b, 12AX7 / ECC83 / 7025

The channel one’s second stage, the other half of V1 (pins 1, 2 and 3). Its plate is tied to the other channel’s second stage, so the two channels mix on one 68 kΩ plate resistor.

R14, 3.3 kΩ

Cathode resistor, 3.3 kΩ, not bypassed. The drawing reads 2.1 V.

R7, 1 kΩ

Grid stopper, 1 kΩ from the jack to the grid. With the 50 pF it keeps radio out. Ampeg’s drawing marks it as left out of some units.

R8, 5.6 MΩ

Grid leak, 5.6 MΩ from the jack to ground.

C4, 50 pF

50 pF from the jack to the first stage’s cathode, for radio frequencies. At audio it does next to nothing. Left out of some units.

V2a, 12AX7 / ECC83 / 7025

The channel two’s first stage, half a 12AX7 (pins 6, 7 and 8 of V2). The sensitivity switch sets how much of its cathode resistance is bypassed, so its gain runs from about 30 to about 75.

R12, 390 kΩ

Plate resistor, 390 kΩ. The drawing reads 195 V on the plate.

R11, 6.8 kΩ

Cathode resistor, 6.8 kΩ. It alone sets the bias, about 2 V, since everything else on the cathode goes through a cap.

R10, 33 kΩ

From the cathode to the 10 µF, 33 kΩ. With the switch on Low it’s the only bypass, so the cathode sees 6.8 kΩ beside 33 kΩ for signal, about 5.6 kΩ.

C5, 10 µF electrolytic

The bypass cap, 10 µF 25 V. The switch sets how it reaches the cathode: through 33 kΩ, through 33 kΩ and 6.8 kΩ side by side, or straight.

R9, 6.8 kΩ

From the cathode to the sensitivity switch, 6.8 kΩ. On Mid the switch joins its foot to the 10 µF, so it bypasses beside the 33 kΩ, about 3.1 kΩ left for signal.

C6, 10 nF

Coupling cap, .01 µF, from the plate to the volume pot.

Volume 2, 1 MΩ pot, linear taper

The channel two’s volume, 1 MΩ linear on the drawing. Its wiper feeds the second stage.

C103, 120 pF

The Ultra Hi bright cap, 120 pF from the volume’s wiper. Up, the switch joins its other end to the volume’s top, so treble skips past the pot. It does the most with the volume low and nothing at full.

C104, 1 nF

The Ultra Hi cut cap, .001 µF from the volume’s wiper. Down, the switch grounds its other end, so treble from the wiper goes to ground: the volume’s own resistance and this cap roll off the top.

V2b, 12AX7 / ECC83 / 7025

The channel two’s second stage, the other half of V2 (pins 1, 2 and 3). Its plate is tied to the other channel’s second stage, so the two channels mix on one 68 kΩ plate resistor.

R15, 3.3 kΩ

Cathode resistor, 3.3 kΩ, not bypassed. The drawing reads 2.1 V.

R13, 68 kΩ

The second stages’ shared plate resistor, 68 kΩ. Both channels’ second stages pull their current through it, so the two channels mix here. The drawing reads 230 V.

C7, 10 nF

Coupling cap, .01 µF, into the tone network.

220k, 220 kΩ

Into the bass pot’s top. The network is a printed part with no values on the drawing, so this and the network’s other values are the SVT’s, the same shape. Typical, not measured.

Bass, 1 MΩ pot, audio taper

Bass, 1 MΩ audio on the drawing, with 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 caps carry the mids and treble past it at any setting.

.001µ, 1 nF

Across the top half of the bass pot, .001 µF. Typical, the SVT’s value.

.01µ, 10 nF

Across the bottom half of the bass pot, .01 µF. Typical, the SVT’s value.

22k, 22 kΩ

From the bass pot’s foot to ground, 22 kΩ, the bottom of the network’s divider. Typical, the SVT’s value.

47p, 47 pF

Into the treble pot’s top, 47 pF. Only the high treble gets through. Typical, the SVT’s value.

Treble, 1 MΩ pot, audio taper

Treble, 1 MΩ audio on the drawing. Its wiper is the network’s output. Up, more treble from the 47 pF. Down, the 470 pF at its foot pulls treble off.

470p, 470 pF

From the treble pot’s foot to ground, 470 pF. Typical, the SVT’s value.

R101, 120 kΩ

From the bass wiper to the treble wiper, 120 kΩ, as drawn. The lows and mids reach the output this way.

C201, 10 nF

Coupling cap, .01 µF, from the tone network to V201. The reverb driver’s grid hangs on the network’s output too.

V201a, 12AX7 / ECC83 / 7025

First section of V201, a 6K11 (three high-gain triodes in one bottle, modeled as a 12AX7). Its cathode is where the midrange loop works: the follower’s output comes back through 56 kΩ, and the mid network hangs on it too.

R201, 1 MΩ

Grid leak, 1 MΩ, to the foot of the 560 Ω, so the grid sits only that far below the cathode.

R202, 560 Ω

Under the cathode, 560 Ω. The grid returns below it, so this sets the bias, about 1.5 V.

R203, 7.5 kΩ

To ground, 7.5 kΩ. The follower’s current through the 56 kΩ also flows here, which lifts the whole cathode to about 22 V on the drawing.

R204, 220 kΩ

Plate resistor, 220 kΩ. The drawing reads 192 V.

C202, 10 nF

Coupling cap, .01 µF, to the second section.

R205, 1 MΩ

Grid leak, 1 MΩ.

V201b, 12AX7 / ECC83 / 7025

Second section of the 6K11. Its plate drives the follower’s grid directly.

R206, 3.3 kΩ

Cathode resistor, 3.3 kΩ, not bypassed. The drawing reads 1.26 V.

R207, 470 kΩ

Plate resistor, 470 kΩ. Its plate sits at the follower’s grid, a volt or two above the follower’s cathode. The drawing reads 178 V, which doesn’t fit the 140 V it shows on the follower’s cathode.

V201c, 12AX7 / ECC83 / 7025

Third section of the 6K11, a cathode follower. It drives the midrange network, the next stage and, through 56 kΩ, the first section’s cathode. From the first section’s grid to here the signal is turned over twice, so that path is negative feedback.

R211, 47 kΩ

The follower’s load, 47 kΩ, above the 6.8 kΩ. The signal goes on from their middle, about an eighth of the follower’s swing.

R210, 6.8 kΩ

Bottom of the follower’s load, 6.8 kΩ.

R208, 56 kΩ

The midrange loop’s feedback, 56 kΩ from the follower back to the first section’s cathode. It also carries DC, about 2 mA, which is why that cathode sits at 22 V.

R209, 470 Ω

From the first section’s cathode to the midrange pot’s boost end, 470 Ω.

C203, 330 nF

.33 µF to the midrange pot’s boost end. With the inductor it sets the band’s center.

R212, 620 Ω

From the follower’s output to the midrange pot’s cut end, 620 Ω.

C204, 330 nF

.33 µF to the midrange pot’s cut end.

Midrange, 50 kΩ pot, linear taper

Midrange, 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 the first section’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.

L101 4–3, 93.7 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.

L101 3–2, 98.6 mH

The inductor from tap 3 to tap 2. Typical, not measured.

L101 2–1, 406 mH

The inductor from tap 2 to its foot (terminal 1). Typical, not measured. The whole winding is about 1.58 H.

K, coupling factor 0.990

Coupling between the inductor’s top two sections, wound on one toroid. 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.

R102, 220 kΩ

From tap 3 to ground, 220 kΩ. It keeps the .033 µF from sitting charged when it’s switched in.

R103, 220 kΩ

From tap 2 to ground, 220 kΩ, the same for the .15 µF.

C106, 33 nF

.033 µF from tap 3 to the select switch. On position 3 it’s grounded, so the top section of the inductor and this cap tune the band near 3 kHz.

C107, 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.

C205, 100 nF

Coupling cap, .1 µF 250 V, from the follower’s divider toward the mixing point. Joe Piazza’s 1970 redraw shows 1 µF.

R16, 180 kΩ

The dry signal’s mixing resistor, 180 kΩ.

R17, 270 kΩ

The reverb’s mixing resistor, 270 kΩ. With the reverb return behind it, it loads the mixing point, about 4 dB off the dry signal.

R104, 150 kΩ

From the reverb return to ground, 150 kΩ.

C105, 2.2 nF

.0022 µF from the reverb pot’s top. It keeps the lows out of the reverb.

Reverb, 500 kΩ pot, linear taper

Reverb, 500 kΩ linear. The recovery stage drives its wiper, and its top goes through .0022 µF to the reverb’s mixing resistor. At 0 the top is grounded, so nothing from the springs reaches the mix. Turn it up for more of the springs.

C212, 5 nF

.005 µF from the recovery stage’s plate to the reverb pot’s wiper.

R221, 270 kΩ

The recovery stage’s plate resistor, 270 kΩ. The drawing reads 115 V.

V203, 12AX7 / ECC83 / 7025

The reverb recovery stage, half a 12AX7. Its grid sits on the reverb unit’s output coil through 10 kΩ. Its plate drives the Reverb pot’s wiper.

R220, 1 kΩ

Cathode resistor, 1 kΩ. The drawing reads 0.76 V.

C211, 10 µF electrolytic

Bypass cap, 10 µF 25 V.

R218, 10 kΩ

Grid stopper, 10 kΩ, from the reverb unit’s output.

R219, 22 kΩ

Across the reverb unit’s output coil, 22 kΩ.

C210, 5 nF

.005 µF from the reverb unit’s output to V203’s cathode, which the 10 µF holds near ground. It trims the springs’ highest treble.

V202a, 6SN7

First half of V202, a 6CG7 (modeled as a 6SN7, the same triode on an octal base), the reverb send’s gain stage. Its grid sits on the tone network’s output with no grid resistor of its own.

R213, 120 kΩ

Plate resistor, 120 kΩ. The drawing reads 112 V.

R214, 2.2 kΩ

Cathode resistor, 2.2 kΩ, not bypassed. The drawing reads 4.1 V.

C206, 10 nF

Coupling cap, .01 µF, to the output half. Into 22 kΩ it passes only from about 700 Hz up, so the springs get mostly mids and treble.

R215, 22 kΩ

Grid resistor for the output half, only 22 kΩ.

V202b, 6SN7

Second half of the 6CG7, the reverb driver. Its plate drives the reverb unit’s input coil through .47 µF, with no transformer.

R217, 10 kΩ

Plate resistor, 10 kΩ 5 W, from node C (400 V on the drawing). The drawing reads 270 V on the plate, about 13 mA.

R216, 330 Ω

Cathode resistor, 330 Ω 2 W. The drawing reads 5.6 V, which is about 17 mA. That doesn’t fit the plate reading’s 13 mA.

C207, 10 µF electrolytic

Bypass cap, 10 µF 25 V.

C208, 22 nF

.022 µF from the driver’s plate to ground. It rolls off the top end going into the reverb unit.

C209, 470 nF

.47 µF from the driver’s plate to the reverb unit’s input coil. It keeps the plate’s DC off the coil.

Unit in R, 155 Ω

Resistance of the reverb unit’s input coil. The current through it drives the springs, which play from a measured tank’s impulse response. The drawing shows only Ampeg’s part number. A plate driving the coil through a cap needs a high-impedance input coil, so this is a typical 1.5 kΩ coil, scaled from a measured long tank by the turns. Not measured.

Unit in L, 210 mH

Inductance of the reverb unit’s input coil, 0.21 H, about 1.3 kΩ at 1 kHz. Typical, not measured.

Unit out L, 350 mH

The reverb unit’s output coil, from the 10 kΩ and 22 kΩ to ground. The springs come back in series with it. Its figures are a measured long tank’s, typical for this unit, not measured on it.

Unit out R, 181 Ω

Resistance of the reverb unit’s output coil, a measured long tank’s figure.

C8, 10 nF

Coupling cap, .01 µF, from the mixing point to the cathode follower.

V3a, 12AU7 / ECC82

Cathode follower, the low-gain half of a 12DW7 (pins 1, 2 and 3, on the 12AU7 model, as that half is the same). It drives the Ext Amp jacks and the last stage.

R18, 1 MΩ

Grid leak, 1 MΩ, to the middle of the cathode resistors, so the grid sits 1 kΩ’s drop below the cathode.

R19, 1 kΩ

Under the cathode, 1 kΩ. Its drop is the follower’s bias.

R20, 47 kΩ

The follower’s load, 47 kΩ 1 W. The drawing reads 200 V at the cathode, about 4 mA.

C9, 470 nF

Coupling cap, .47 µF, from the follower.

R22, 10 kΩ

10 kΩ to the Ext Amp jacks (not modeled).

R21, 100 kΩ

From the Ext Amp jacks to ground, 100 kΩ.

R23, 10 kΩ

Grid stopper, 10 kΩ, into the last stage.

V3b, 12AX7 / ECC83 / 7025

The last stage before the inverter, the high-gain half of the 12DW7 (pins 6, 7 and 8, on the 12AX7 model). The amp’s feedback goes to its cathode, so the loop runs from here to the speaker.

R24, 220 kΩ

Plate resistor, 220 kΩ. The drawing reads 205 V.

R25, 3.3 kΩ

Cathode resistor, 3.3 kΩ, not bypassed. The drawing reads 1.8 V.

R26, 33 Ω

The bottom of the feedback loop, 33 Ω under the cathode resistor. The feedback from the 8 Ω tap connects at the top of it.

R34, 4.7 kΩ

The feedback resistor, 4.7 kΩ from the output transformer’s 8 Ω tap. With the 33 Ω it sends about 0.7 % of the tap’s voltage back to V3b’s cathode. With Preamp only or another power amp, its far end is grounded.

C14, 1 nF

.001 µF across the feedback resistor. It feeds back more of the far top end, which keeps the loop steady.

C10, 10 nF

Coupling cap, .01 µF, into the inverter’s grid.

V4a, 12AU7 / ECC82

12AU7 (ECC82), the driven half of the floating paraphase 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, 12AU7 / ECC82

The other half. Its grid is fed from the middle of the divider, so its plate swings the other way and drives tube B. The shared cathode isn’t bypassed, so it’s also driven through the cathodes, as in a long-tail pair.

1M, 1 MΩ

Grid resistor for the driven half, to ground.

1M, 1 MΩ

Grid resistor for the other half, to ground.

1.5k, 1.5 kΩ

Cathode resistor for both halves. It sets their bias, a couple of volts. It isn’t bypassed, so each half’s signal current runs through it and moves the other’s cathode.

47k, 47 kΩ

Plate resistor for the driven half.

47k, 47 kΩ

Plate resistor for the other half.

820k, 820 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.

1M, 1 MΩ

Bottom of the divider, to tube B’s plate.

.1µ, 100 nF

From the middle of the divider to the second half’s grid. It keeps the plates’ DC off that grid.

.33µ, 330 nF

Coupling cap to tube A’s grid.

.33µ, 330 nF

Coupling cap to tube B’s grid.

23.5k, 23.5 kΩ

Grid stopper for tube A. It keeps the tube from oscillating and softens grid current. Each of the two tubes has its own 47k. The resistors are drawn as one part, the two in parallel.

23.5k, 23.5 kΩ

Grid stopper for tube B. Each of the two tubes has its own 47k. The resistors are drawn as one part, the two in parallel.

V5, V6, 6L6GC

6L6GC, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply. Two tubes in parallel, drawn as one.

V7, V8, 6L6GC

6L6GC, tube B of the pair. Two tubes in parallel, drawn as one.

5, 5 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 two tubes has its own 10. The resistors are drawn as one part, the two in parallel.

5, 5 Ω

Plate resistor for tube B, the same. Each of the two tubes has its own 10. The resistors are drawn as one part, the two in parallel.

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.

235, 235 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 470. The resistors are drawn as one part, the two in parallel.

235, 235 Ω

Screen resistor for tube B. Each of the two tubes has its own 470. The resistors are drawn as one part, the two in parallel.

470, 470 Ω

Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.

20µ, 20 µF electrolytic

Filter cap on the screen supply.

3k, 3 kΩ

Dropping resistor from B+ to the node for the reverb driver, as on the Ampeg V-4 and VT-22 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 reverb driver. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.

2.7k, 2.7 kΩ

Dropping resistor from the node for the reverb driver to the node for the inverter, as on the Ampeg V-4 and VT-22 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.

2.2k, 2.2 kΩ

Dropping resistor from the node for the inverter to the node for the preamp, as on the Ampeg V-4 and VT-22 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.

Balancing, 200 kΩ

The reservoir caps’ two 100 kΩ 2 W balancing resistors, in series across B+. The reservoir is two caps in series on each side, so these hold the middle point at half of B+. Together they pull about 2.7 mA.

OT, 2.5 kΩ to 8 Ω, 20 H primary, 5 mH leakage

The output transformer, 2.5 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, 407 V ac, 60 Hz

The power transformer’s high-voltage winding, 407 V ac end to end, into a bridge of four silicon diodes. It’s drawn as two sources of 407 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, 407 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.

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.

35µ, 35 µF electrolytic

Another reservoir cap, in parallel with the first.

Pump, 223 V ac, 60 Hz

Stands in for the bias pump with the part below it. On the drawing, a .047µF cap from one leg of the high-voltage winding, 56k Ω to ground and the diode make the bias. The operating point can’t be solved through a cap in series, so this 222.68 V ac source and its resistor replace it. They were fitted to a simulation of the real pump on this winding: the same bias, and the same shift when the grids pull current on a hard note. It moves with the wall voltage, as the pump does.

pump R, 81.2 kΩ

Stands in for the pump with the source beside it, 81.2k Ω, fitted. It sets how far the bias moves under grid current. The amp has a .047µF cap here and 56k Ω from the diode end to ground.

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.

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.

10µ, 10 µF electrolytic

Second filter cap, on the bias line, its plus side to ground.

75k, 75 kΩ

From the bias line to ground, 75k Ω. With the 10k Ω and 81.2k 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, 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. With no feedback, the voltage at the speaker rises and falls with its impedance.

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. With no feedback, the voltage at the speaker rises and falls with its impedance.

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.

Versions

V-4 head

Ampeg’s drawing for the V-4 and VT-22. The 6K11’s three sections run on the 12AX7 model and the 12DW7’s halves on the 12AX7’s and 12AU7’s. The tone network’s and the inductor’s values aren’t on the drawing and are typical. The tube models are typical of each type, not measured.

VT-22 combo, modeled after the Ampeg VT-22

The same drawing as the V-4: the VT-22 is the V-4’s chassis in a combo with two 12 in speakers. Their two 8 Ω speakers in parallel are a 4 Ω load on the 4 Ω tap, the same match as one 8 Ω load on the 8 Ω tap, so the circuit here is the same.

1970, Piazza

Joe Piazza’s redraw of a 1970 V-4: no 1 kΩ grid stoppers or 50 pF caps at the inputs, and 1 µF where the factory drawing has .1 µF out of the midrange loop. The rest of the preamp is the same.

Sources

• Joe Piazza, redrawn V-4 preamp and power amp schematics, 1996