DOD / MXR: 250, Distortion + component values table

This is probably where it all started in terms of opamp-driven distortion -  in one side of 741 opamp, out the other with some hard clippers.  

DOD 250 / MXR DISTORTION + SCHEMATIC

Direct link to Google Doc

741 SPECS

Numbers vary slightly by manufacturer and grade, but these headline specs are what give the 741 its “personality” in effects land:

• Supply voltage: works on dual or single supplies (it was designed with ± supplies in mind, but can be run single-supply with proper biasing).

• Gain-bandwidth product: around ~1 MHz class.

• Slew rate: about ~0.5 V/µs typical.

• Input bias current: tens of nA typical (bipolar input).

• Not rail-to-rail: inputs/outputs can’t swing anywhere near the rails, especially on a 9V battery. 

What that means in practice: a 741 can sound/feel a bit rounder and more easily pushed in simple dirt circuits, partly because it’s relatively slow and because it doesn’t have huge clean output swing on a 9V single supply.

LM741 PINOUTS

Note the little tab on the side of the opamp is pin 8.

OFFSET NULL

Offset null is a pair of pins on the 741 that lets you trim out its small DC offset so the output sits where you want it (often 0V on a dual-rail supply). 

In pedals it’s usually ignored because stompboxes are typically single-supply with a virtual ground and AC-coupled stages, so capacitors block DC and the tiny offset isn’t audible.

While the pins aren't used, be sure to isolate them anyway.

GIBSON: GA-30 Schematic

GIBSON GA-30 GUITAR AMPLIFIER SCHEMATICS

This is one of the GA-30 amps - there were a few by the sounds of it.

While they share a similar topology, there are clearly some differences from the one above.

Apart from changing the circuit, their appearance also varies over time - I'm not going to try and document them all, and I'm by no means an expert on the topic.  I'm just using these as a reference for a Bell & Howell conversion.  

Text below is for the first schematic 

6SJ7 MIC PRE

Grid leak 1m

Grid stopper (none)

Ra 470k

Rk 2k2 20u bypass 

Screen 2m2, 50n bypass

10n coupling cap to 500k volume pot.  100k mixing resistor to PI

6SJ7 INST PRE

Grid leak 470k

Grid stopper 47k (forms a voltage divider with the grid leak)

Ra 220k

Rk 2k2 (not bypassed) 

Screen 2m2, 50n bypass

20n coupling cap to 500k volume pot.  100k mixing resistor to PI

470k NFB from OT secondaries, 700pf in series with bypass switch 

6SN7 PHASE INVERTER

Ra 100k on each plate

Rk 1k shared (not bypassed)

50n coupling to 6V6

220k / 4k7 pair to PI grid

Tone 2m pot, mixing 250pf and 50n to grid of PI input 

6V6

220k grid leak

Rk 250 ohms

No grid stoppers or screen resistors 

GIBSON: BR6-F Schematic

6SJ7 preamp, 6SN7 paraphase inverter, 2 x 6V6 with local negative feedback.  

GIBSON BR6-F GUITAR AMPLIFIER SCHEMATIC

Clearly a bit of work has been done on this one - just using the pic as an example of circuit construction.

Low impedance input

6SJ7 PRE

Ra 220k

Rk 2k2 20u bypass 

Screen 2m2, 50n bypass

10n coupling cap to 500k volume pot

6SN7 PI

Ra 100k

Rk 3k3 (not bypassed)

50n coupling to 6V6

100k / 10k pair to PI grid

6V6

100k grid leak

Rk 200 ohms

470k NFB from 6V6 plate to 6SN7 plate

No grid stoppers or screen resistors 

GIBSON: GA-40 Schematic

5879 pentode premap, which is an improved version of the E8F6 - uses a Vox style cut control across the plates of the phase inverter.  Paraphase inverter, driving 2 x cathode biased 6V6.

GIBSON GA-40 GUITAR AMPLIFIER SCHEMATIC

BELL & HOWELL: Model 385 Amplifier Schematic

The original schematic for the 385 was killing me - just too many part numbers listed, so I cleaned it up a bit.  I also removed the projector circuits.

I think there are two versions of this amp - this is the 6V6 version, while the other uses EL84s (which is nearly impossible to find a schematic for).

BELL& HOWELL - FILMOSOUND MODEL 385 SCHEMATIC

After the clean-up, I redrew the schematic in LTspice.  Seems to work as expected, so hopefully no mistakes...

The tone control is unique, as it uses a dual pot for the James EQ instead of separate pots for bass and treble.  Bass is cut while treble is turned up, and vice versa - or you could wire it so both bass and treble go up and down at the same time, leaving the midrange alone...  interesting.  

The dual 3meg pot is not easily found these days - in theory, resistor and capacitor values can be scaled to work with a 1meg pot, however, after spending some time on this, I can't get the response to match.   It's roughly the same, but there are obvious differences.  

The preamp could be simplified a little, and you would most certainly want to get rid of the 1meg resistor on the input.  

Hard not to notice all the little caps rolling off some highs from plates of the 12AX7 gain stages.  Something to experiment with and tune to taste.  

LTspice sim of the eq range

Original pages below

 

UNCLE DOUG CONVERSION VIDEOS

FENDER: Deluxe 5E3 Amplifier

Some reference notes on the Fender 5E3 Deluxe amplifier, possibly the most famous and widely copied variation of the Fender Deluxe, produced from about 1955 - 1960.

TWEED ERA FENDER DELUXE 5E3 SCHEMATIC

POWER

Transformer:  Triad 6452, 380–0–380 V

Rectifier:    5Y3GT

B+ ~360V  

Filtering:    B1 - 16uf,    B2 - 5k / 16uf,    B3 - 22k / 16uf

OUTPUT

OT:   8k P-P,  8ohm secondary.   Triad 108 (pre 60's).   Schumacher 125A1A.   Note the option to add another 8ohm extension speaker, which implies that the amp can handle a 4 ohm load

2 x 6V6

Va ~340 - 350

Cathode biased.  Rk 250hm, 50uf bypass capacitor

Grid stop: 1k5.  Grid leak:  220k

CATHODYNE PHASE INVERTER - 12AX7

First stage, boost:

Ra 100k,  Rk 1k5 ohms / 25uf bypass

22n coupling to grid of inverter

Second stage, invert:   

Ra 56k,   Rk 1k5 in series with 56k 

Rg 1meg to 1k5 / 56k resistor node

100nf coupling to 6V6 from cathode and anode of 12AX7

PREAMP - 12AY7

Ra 100k,  Rk 820 ohms shared / 25uf bypass.   (1,640ohms per cathode)

Grid stoppers 68k (in parallel, for high).   Standard Fender input wiring

Rg 1meg.  100nf coupling caps to volume

Normal and Bright channels

A1M VOLUME & TONE

A1M volume and tone pots.   

In tweed circuits, the volume pots are wired in an older style: the wiper (pin 2) is the input, while the outer lug (pin 3) serves as the output.  Do not change it to the modern pin 3 in, pin 2 out convention, as it will not work the same way.  

In terms of tone control, it is essential to note its position in the circuit.  On the Bright channel, treble can be increased via what is essentially a variable bright cap on the volume pot.  This does not occur on the Normal channel.

On both the Normal and Bright inputs, the treble can be turned down.

A 500 pF capacitor from pin 3 connects back to the preamp’s coupling capacitor (on the bright inputs). This acts like a variable bright cap—its effect diminishes as the volume is turned up.

A 4.7 nF capacitor from pin 1 goes to ground, providing a simple treble cut by shunting high frequencies away.

Pin 2 (the wiper) carries the combined signal forward to the grid of the phase inverter (also connected to the output of the volume pot).

One hallmark of these amps is the interaction between the two volume controls and the tone. Both volume pots connect directly to the grid of the first phase inverter stage, so even if only one channel is in use, adjusting the unused channel’s volume will still alter the response of the active channel.  There are tonal variations all over the place - be sure to experiment with extreme settings.  Mixing resistors were introduced in later models to remove this interaction.

12AY7 x 12AX7

Parameter	12AY7	12AX7
Amplification Factor (µ)	40	100
Plate Resistance (rp)	22.8 kΩ	62.5 kΩ
Transconductance (gm)	1750 µmhos	1600 µmhos
Typical Plate Current	≈ 3 mA @ Va=250 V.  Vg = –2 V	≈ 1.2 mA @ Va=250 V.    Vg = –2 V
Max Plate Voltage	300 V	330 V
Max Plate Dissipation (per triode)	1.5 W	1.2 W
Heater Current	0.3 A @ 6.3 V	0.3 A @ 6.3 V
Typical Bias for ~1 mA	–2 V to –4 V	–1.5 V to –2 V

6V6

Parameter	6V6
Type	Beam power tetrode
Plate Resistance (rp)	~62 kΩ (effective in Class A)
Transconductance (gm)	~4100 µmhos
Typical Plate Current	34–45 mA (idle, Class A)
Max Plate Voltage	350 V (design centre)
Max Plate Dissipation	14 W
Heater Voltage	6.3 V
Heater Current	0.45 A @ 6.3 V
Typical Bias	–12 V to –15 V (Class A)
Typical Load Impedance	8 - 10 kΩ  P-P
Typical Output Power	≈ 14 W (push-pull, Class AB)

5Y3

Parameter	5Y3GT
Type	Full-wave rectifier
Maximum DC Output Current	125 mA
Peak Inverse Voltage	1400 V
Typical DC Output Voltage	~350 V DC (at 100 mA load).
Max Input Capacitance	20 µF (first filter capacitor)
Heater Voltage	5 V
Heater Current	2.0 A
Typical Voltage Drop	≈ 60 V at 125 mA load

CTS Speakers

Chicago Telephone Supply (CTS) originally built drivers for telephone network signaling and tone generation. By the late 1950s they expanded into loudspeakers, supplying the booming tube-amp market. 

Their speakers, often marked with the EIA code "137," were used extensively by Fender, Ampeg, and many other amplifier companies from the 1950s through the 1970s.

Early CTS speakers used alnico magnets (square-back frames) prized for smooth breakup, while later ceramic-magnet versions (round frames) offered tighter lows at the expense of some warmth, but this is of curse a little subjective and varies with a lot of other factors.

Old CTS speakers do pop-up on eBay and Marketplace in Australia, and it can be hard knowing what you're looking at - are they hi-fi or guitar speakers?   The advertisement below might help.

SUPRO: Super Six, Model S6606 Valve Amplifier

Valco made, Supro single-ended valve amp, very much the same as the Gretsch 6150 / 6151

SUPRO SUPER SIX, MODEL S6606

12AX7, 6V6, 5Y3

8" Jensen speaker, 3.2 ohm

OT 2640

PT 3036

B+ 300v

4.5w

GRETSCH: Model G6151 & G6150 guitar amplifiers

Two great little guitar amps made by Valco for Gretsch - probably best to describe them as direct competition for the Fender Champ, but with some fancier appointments for the time.   Pretty sure there's a Supro equivalent that was also made by Valco for these two as well. Supro 1606 for the 6150.

Both small single-ended amps with an 8" speaker. 

Some was nice enough to upload a bunch of detailed photos, schematics and their layouts for the 1606 on their google drive located here

GRETSCH, MODEL G6150 AMPLIFIER SCHEMATIC

12AX7, 6V6GT, 5Y3GT

Jensen 8" speaker 

500k volume control

OT 2640

PT E-3036

GRETSCH, MODEL G6151 AMPLIFIER SCHEMATIC

12AX7, 12AX7, 6V6GT, 5Y3GT

8" speaker

500k volume & tone control

500k speed control

OT 2640 for 3.2 ohm speaker

OT VAC109 for 8 ohm speaker

PT E-3036

FENDER: Tweed Princetons

I'll start with the Fender 5F2A Princeton, as this is generally considered to be "the one" for a lot of people.

Basically take a tweed Champ, and add the classic Fender one knob tone control, which has no insertion loss, and you're in business.  It's hard to go wrong with this formula, which is why it appears in so many amps.  

FENDER TWEED PRINCETON - 5F2A, SCHEMATIC & LAYOUT

• 12AX7, 6V6, 5Y3

• Classic Fender hi / lo input stage

• Standard Fender preamp component values / biasing 

• Preamp is cathode biased

• Volume 1M, Tone 1M

• 22n coupling caps

• 22k negative feedback from OT secondary to second triodes cathode 

• Simple heater wiring - one side grounded

FENDER PRINCETON - 5B2 SCHEMATIC

• Preamp is grid leak biased

• Simple heater wiring - one side grounded

• 6SL7, 6V6, 5Y3

• Jensen P8T

• Volume & tone 1M

• OT 7765

• PT 6500

FENDER PRINCETON - 5C2 SCHEMATIC

• 6SC7, 6V6, 5Y3

• 2 x triodes in preamp - the first feeds the second via a voltage divider to reduce gain

• Shared cathode resistor

• Volume & Tone 1M

• Volume & Tone control is sitting on the grid of the 6V6

• Simple heater wiring - one side grounded

FENDER PRINCETON - 5D2 SCHEMATIC

• 12AX7, 6V6, 5Y3

• Preamp grid leak biased

• Volume & tone 1M - 1meg resistor in series before the volume pot

• No negative feedback

• Simple heater wiring - one side grounded

FENDER PRINCETON - 5E2 SCHEMATIC

• 12AX7, 6V6, 5Y

• Classic Fender hi / lo input stage

• Preamp is cathode biased

• Volume 1M, tone 250k

• Slightly more complicated tone control - tone control sits on the first triode's plate, before the coupling cap

• Simple heater wiring - one side grounded

• Choke on power supply

• 22k negative feedback

FENDER PRINCETON - 5F2 SCHEMATIC

Almost identical to the 5E2 - no bypass cap on the first triode

• 12AX7, 6V6, 5Y3

• Classic Fender hi / lo input stage

• Preamp is cathode biased

• Volume 1M, tone 250k

• Slightly more complicated tone control - tone control sits on the first triode's plate, before the coupling cap

• Simple heater wiring - one side grounded

• Choke on power supply

• 22k negative feedback

FENDER: Tweed Champ Schematics

I know these are everywhere, but humour me - this is mainly for my own reference.

FENDER CHAMP-AMP,  5C1 (WIDE PANEL TWEED)

Note: The Fender Champion 600 and 800 are said to use the same schematic (or at least extremely similar).

Preamp: 6SJ7 pentode, grid-leak biased (no cathode resistor)

• Inputs: two jacks, each 75k in series with 0.02uF (20nF) into the grid

• Grid leak: 5M to ground

• Plate resistor (Ra): 250k

• Screen feed: 2M, bypassed with 0.05uF (50nF)

• Coupling cap to volume: 0.02uF (20nF)

• Volume pot: 1M

Power stage: 6V6 single-ended, cathode biased

• Cathode resistor (Rk): 500 ohms, bypassed with 25uF

• Grid leak resistor = volume pot

Output transformer:  Triad 70189 

Note: I can't find a definitive source of original Champ OT measurements.

• Small single-ended output transformer

• OT Secondary 3.2 ohms 

• OT Primary, usually quoted as ~5k-7k  (some modern replacements offer 5k & 8k primaries). 

Turns ratios

• 46.8:1 = 7k @ 3.2 Ω    15VAC on primary = 0.3205 on secondary

• 39.5:1 = 5k @ 3.2 Ω   15VAC on primary = 0.3797 on secondary

Rectifier: 5Y3

Power supply filtering:

• 8uF reservoir → 500 ohms → 8uF (plate/OT node) → 25k → 8uF (screen + preamp node)

Power transformer:  Triad 6500

Typical Voltages:  Actual voltages vary a lot with wall voltage, rectifier condition, and transformer, but the commonly quoted targets for a 5C1 are roughly:

Rectifier DC output (first B+): 340V

B+ after the 500 ohm dropper / main supply node: 320V

6V6 plate: 300V

6V6 screen: 280V (lower because it’s fed through the 25k drop)

6V6 cathode: 16V across the 500 ohm resistor (so ~32mA cathode current as a rough check)

6SJ7 plate: 130V

6SJ7 screen: 21V

6SJ7 grid: -5V

Heaters:  Simple wiring, one side grounded (not twisted pair, no centre tap)

Speaker (stock): 1x6" alnico - commonly Permaflux 6-L (EIA 395), though other 6" period speakers exist due to Fender’s supplier variability and decades of replacements.

FENDER CHAMP 5D1

FENDER CHAMP 5E1

The classic Fender 12AX7 input stage and preamp values are used.  Negative feedback appears.  Choke on the DC power supply.  Changed to a regular component board, controls now on the top of the amp.

Preamp: 12AX7

Inputs: two jacks in the classic Fender input network.  When using input #1, the effective grid-stopper is about 34k because the two 68k resistors end up in parallel

• Each jack feeds the grid through 68k

• Grid leak: 1M to ground

First gain stage (V1A):

• Plate resistor (Ra): 100k

• Cathode resistor (Rk): 1k5, bypassed with 25uF

• Coupling cap to volume: 0.02uF (22nF)

Volume control:

• Volume pot: 1M (between stages)

Second gain / driver stage (V1B):

• Plate resistor (Ra): 100k

• Cathode resistor (Rk): 1k5, not bypassed

• Negative feedback: from OT secondary back to this cathode via a 22k resistor

• Coupling cap to 6V6 grid: 0.02uF (22nF)

Power stage: 6V6 single-ended, cathode biased

• 6V6 grid leak: 220k

• Cathode resistor (Rk): 470 ohms, bypassed with 25uF

• Screen supply: taken from the main B+ node, after the choke

Rectifier: 5Y3GT

Power transformer: Triad 6500

Power supply filtering:

• On 5E1, the choke sits where the 5F1 uses a 10k dropping resistor

• 8uF reservoir → choke → 8uF (main plate/screen/OT node) → 22k → 8uF (preamp node)

Typical voltages (ballpark):

• Main B+ node often lands around ~305V in many examples (varies with wall voltage and rectifier condition)

• 6V6 cathode: ~19V across 470Ω (≈40mA cathode current as a rough check)

• Preamp supply in many references is around ~260V

Speaker (stock):

Commonly a small 6" alnico in early narrow-panel Champs, often a Permaflux (though other 6" period speakers exist due to Fender’s supplier variability and decades of replacements)

FENDER CHAMP 5F1

Almost identical to the previous model, with a 10k resistor replacing the choke.  No bypass on the input triode - less gain.