Relay-switched overdrive amp with Ratio, PAB and Mid boost, 1980s style

Parts and what they do

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

2.2k, 2.2 kΩ

Dropping resistor, 2.2 kΩ 2 W from B+4 to B+5.

40µ, 40 µF electrolytic

B+5 filter cap, 40 µF 500 V.

1M, 1 MΩ

Input resistor to ground at the Normal jack.

33k, 33 kΩ

Grid stopper into V1b. With the tube’s input capacitance it keeps radio and fizz out.

V1b, 12AX7 / ECC83 / 7025

First stage, CL1 on the drawing. Its plate drives the tone stack directly.

220k, 220 kΩ

V1b plate resistor. It’s also part of the tone stack, since the stack hangs right on the plate. The drawing has 200 V on the plate.

3.3k, 3.3 kΩ

V1b cathode resistor.

5µ, 5 µF electrolytic

V1b bypass cap. Full gain down to about 30 Hz.

.002µ, 2 nF

First treble cap, .002 µF, from V1b’s plate. With the 390 pF after it the two make about 330 pF.

4.7M, 4.7 MΩ

From the two treble caps’ junction to ground. It keeps the junction at 0 V DC, so the Mid boost switch doesn’t pop.

390p, 390 pF

Second treble cap, 390 pF, to the top of Treble. The Mid boost switch shorts it.

150k, 150 kΩ

Slope resistor, from V1b’s plate to the bass and middle caps.

.1µ, 100 nF

Bass cap, to the top of Bass.

.01µ, 10 nF

Middle cap, to Middle.

Middle, 250 kΩ pot, audio taper

Middle, 250 kΩ audio as a variable resistor to ground. Turned up, its resistance rises and less of the mids go to ground.

Treble, 250 kΩ pot, linear taper

Treble, 250 kΩ linear. Its wiper is the stack’s output, the top of Volume.

Bass, 500 kΩ pot, audio taper

Bass, 500 kΩ audio (the drawing notes it measured about 430 kΩ). On Rock with the PAB relay at rest its wiper is tied to its top, so it’s a variable resistor.

10k, 10 kΩ

Under Bass, to ground.

.001µ, 1 nF

Across Bass, .001 µF from its top to its bottom. It lets the treble around the pot, so Bass mostly changes the lows.

22M, 22 MΩ

Across the PAB relay’s first contact, from Treble’s bottom to the Rock/Jazz switch. It keeps the open contact at the same DC level, so the relay doesn’t pop.

22M, 22 MΩ

Across the PAB relay’s second contact, from Bass’s wiper to the Rock/Jazz switch, the same.

.005µ, 5 nF

On Jazz, from Treble’s bottom to ground.

100k, 100 kΩ

On Jazz, from Bass’s wiper to Volume, so bass reaches the volume around Treble. On Rock its switch end hangs open.

Volume, 1 MΩ pot, audio taper

Volume, 1 MΩ audio. It sets how hard CL2 and, with Overdrive on, the whole overdrive path get driven.

220k, 220 kΩ

From Volume’s wiper into V1a’s grid. With the feedback from V1a’s plate it sets how far the feedback cuts this stage’s gain.

500p, 500 pF

Across the 220 kΩ. Above about 1.4 kHz it lets the treble around the resistor, so the feedback cuts the treble less than the rest.

10M, 10 MΩ

Plate-to-grid feedback on V1a, one of four 10 MΩ in series with .05 µF. The 40 MΩ against the 220 kΩ feeds back about 0.5 % of the plate signal.

10M, 10 MΩ

Plate-to-grid feedback on V1a, the second 10 MΩ.

.05µ, 50 nF

In the feedback string, so the plate’s DC stays off the grid.

10M, 10 MΩ

Plate-to-grid feedback on V1a, the third 10 MΩ.

10M, 10 MΩ

Plate-to-grid feedback on V1a, the fourth 10 MΩ, from the plate.

V1a, 12AX7 / ECC83 / 7025

Second stage, CL2 on the drawing. Its output feeds the OD relay.

150k, 150 kΩ

V1a plate resistor. The drawing has 200 V on the plate.

2.2k, 2.2 kΩ

V1a cathode resistor.

5µ, 5 µF electrolytic

V1a bypass cap.

.047µ, 47 nF

Coupling cap out of V1a, to the OD relay.

220k, 220 kΩ

Into the top of OD Trim. With the trimmer it makes a divider, so only a small share of CL2’s output reaches OD1.

OD Trim, 100 kΩ pot, linear taper

OD Trim, 100 kΩ linear, inside the chassis. It sets how hard OD1 is hit. Set at 0.29, since the drawing notes 28.85 kΩ from its wiper to ground, measured on the amp.

68k, 68 kΩ

Grid stopper into V2b.

V2b, 12AX7 / ECC83 / 7025

First overdrive stage, OD1 on the drawing.

220k, 220 kΩ

V2b plate resistor, from B+4. The drawing has 205 V on the plate.

250p, 250 pF

From V2b’s plate to its cathode, 250 pF. In the top end it feeds plate signal back onto the cathode, so the stage’s gain falls off and the fizz comes down.

3.3k, 3.3 kΩ

V2b cathode resistor.

5µ, 5 µF electrolytic

V2b bypass cap, 5 µF.

.01µ, 10 nF

Coupling cap out of V2b.

100k, 100 kΩ

In series with Drive. With the pot it makes a divider, so even at full only about half of OD1’s output reaches OD2.

Drive, 100 kΩ pot, linear taper

Drive, 100 kΩ linear, between the two overdrive stages. It sets how hard OD2 is driven.

180k, 180 kΩ

Grid stopper into V2a. With the tube’s input capacitance it trims the top end going into OD2.

V2a, 12AX7 / ECC83 / 7025

Second overdrive stage, OD2 on the drawing.

150k, 150 kΩ

V2a plate resistor, from B+4. The drawing has 205 V on the plate.

250p, 250 pF

From V2a’s plate to its cathode, 250 pF, as on OD1.

2.2k, 2.2 kΩ

V2a cathode resistor.

5µ, 5 µF electrolytic

V2a bypass cap, 5 µF.

.005µ, 5 nF

Coupling cap out of V2a, .005 µF. Into the 250 kΩ below it, it cuts the bass under about 130 Hz.

150k, 150 kΩ

In series with Ratio. With the pot it makes a divider, so even at full only about 40 % of OD2’s output reaches the Master.

Ratio, 100 kΩ pot, linear taper

Ratio, 100 kΩ linear. It sets how much of the overdrive path reaches the Master, so it matches the overdrive’s level against the clean.

Master, 1 MΩ pot, audio taper

Master, 1 MΩ audio. Its wiper feeds the phase inverter, through the effects send and return, so it sets the level for both paths.

68p, 68 pF

From the Master’s top to its wiper, 68 pF. With the Master down, a little treble skips past the pot.

150k, 150 kΩ

Feeds the FET input from B+5 through 150 kΩ 2 W, which drops most of the voltage. With the 8.2 kΩ below it, the FET’s supply sits around 10 to 15 V.

8.2k, 8.2 kΩ

The bottom of the FET’s supply divider, 8.2 kΩ to ground. It holds the supply low enough for the transistor.

47µ, 47 µF electrolytic

Filter cap on the FET’s supply, 47 µF.

10k, 10 kΩ

The FET’s drain resistor, 10 kΩ.

Q1, NTE452

The FET input’s transistor, an NTE452 on the drawing (modeled as a 2N4416, the part it replaces, with typical values). With its drain on 10 kΩ it has a gain of a few times, and its gate pulls no current, so the guitar sees about 3.3 MΩ.

FET bias, 12 kΩ pot, linear taper

FET bias, a 12 kΩ trim as the source resistor (“Adjust to bias FET” on the drawing). It sets the FET’s current. The drawing reads 10 to 11 V on the drain, and the trim is set so the model reads that.

4.7µ, 4.7 µF electrolytic

Bypass cap on the FET’s source, so the source resistor sets the current without lowering the gain.

3.3M, 3.3 MΩ

From the FET jack’s tip to ground, 3.3 MΩ. It sets the gate’s DC at ground and most of what the guitar sees, about 3.3 MΩ against the Normal jack’s 1 MΩ.

1M, 1 MΩ

1 MΩ from the jack into the gate, with 100 pF across it. The gate pulls no current, so it costs nothing at DC.

100p, 100 pF

100 pF across the 1 MΩ, so the treble gets past it to the gate.

4.7µ, 4.7 µF electrolytic

Coupling cap from the drain to the Trim.

Trim, 10 kΩ pot, linear taper

Trim, 10 kΩ linear, inside the amp. Its wiper goes to the Normal jack’s switch, so it sets the FET input’s level into CL1. The drawing shows no setting; 0.5 of its turn is a guess.

.02µ, 20 nF

Coupling cap into the phase inverter. With the grid resistor it sets how low the inverter goes, around 5 Hz.

V3a, 12AX7 / ECC83 / 7025

12AX7, the driven half of the long-tail pair phase inverter. Its plate swings opposite the input and drives tube A.

V3b, 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.

24k, 24 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.

115k, 115 kΩ

Plate resistor for the driven half. It’s smaller than the other so the two outputs come out about even.

125k, 125 kΩ

Plate resistor for the other half.

4.7k, 4.7 kΩ

The feedback resistor from the speaker. With the 390 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.

390, 390 Ω

The bottom of the feedback loop, 390 Ω to ground. The fed-back speaker voltage arrives here and moves the tail.

2.2µ, 2.2 µF

The presence cap, in series with the pot across the leg. With the pot it sets where the lift starts.

Presence, 2 kΩ pot, linear taper

Presence, a 2k 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.

.1µ, 100 nF

Coupling cap to tube A’s grid.

.1µ, 100 nF

Coupling cap to tube B’s grid.

2.55k, 2.55 kΩ

Grid stopper for tube A. It keeps the tube from oscillating and softens grid current. Each of the two tubes has its own 5.1k. The resistors are drawn as one part, the two in parallel.

2.55k, 2.55 kΩ

Grid stopper for tube B. Each of the two tubes has its own 5.1k. The resistors are drawn as one part, the two in parallel.

V4, 6L6GC

6L6GC, tube A of the pair, cathode grounded, biased from the fixed bias supply. V5 runs beside it on the same grid, plate and screen.

V6, 6L6GC

6L6GC, tube B of the pair. V7 runs beside it.

V5, 6L6GC

6L6GC, the second tube on side A, in parallel with V4 on 100 W. On 50 W its cathode goes through 10k Ω to the LED instead of ground, which cuts it nearly off.

V7, 6L6GC

6L6GC, the second tube on side B, in parallel with V6 on 100 W. On 50 W its cathode goes through 10k Ω to the LED instead of ground, which cuts it nearly off.

10k, 10 kΩ

On 50 W, V5’s cathode resistor to the LED. Its current lifts the cathode until the tube nearly stops conducting. On 100 W the switch grounds the cathode and this carries nothing.

10k, 10 kΩ

The same for V7.

LED, Red LED

The 50 W light. The two switched tubes’ small current runs through it, so it lights on 50 W and adds its drop to their bias.

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.

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.

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.

2.7k, 2.7 kΩ

Dropping resistor from the screen supply node to the node for the inverter, as on the Dumble Overdrive Special #102 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 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 node for the inverter to the node for the preamp, as on the Dumble Overdrive Special #102 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, 2 kΩ to 4 Ω, 20 H primary, 5 mH leakage

The output transformer, 2 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, 352 V ac, 60 Hz

The power transformer’s high-voltage winding, 352 V ac end to end, into a bridge of four silicon diodes. It’s drawn as two sources of 352 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 value is typical of the part, not measured.

PT, 352 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, 40 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, 40 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.

110µ, 110 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, 60 V ac, 60 Hz

The power transformer’s bias tap, 60 V ac, as a sine source at the mains frequency. Typical of the part, not measured.

3.3k, 3.3 kΩ

Series resistor into the bias rectifier, 3.3k Ω. It limits the charging current.

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.

Bias 10k, 10 kΩ pot, linear taper

Bias adjust, 10k Ω linear. Its wiper feeds both output grids through their grid leaks. Toward the diode end the grids go more negative and the tubes idle cooler. Set at 0.53 here, about 39 mA a tube, where crossover distortion on a quiet note is just gone, not measured on an amp.

27k, 27 kΩ

The bottom of the bias divider, 27k Ω to ground.

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

Four 12AX7 triodes, typical of the type, not measured. The drawing names the tubes only by their halves.

Sources

• mdroberts1243, redrawn #102 schematic, Rev 1.0 (2010)