Harmonized Designators for Fender Tweed and Marshall JTM45 Schematics

Analysis and proposal for a position-stable reference designator scheme

Project: Scan-to-KiCad netlist tooling
Document date: October 1, 2026 (first issued September 26, 2026)
Evidence: the seventeen hand-transcribed schematics in C:\Users\david\finished, read through kicad-cli sch export netlist; sixteen Fender 5E and 5F models and the Marshall JTM45
Status: proposal only; no numbering has been applied to any file
Revision: tube sections now carry universal U designators named by role, and every gain-stage block is keyed to a U role rather than to a tube unit letter (Sections 2.3, 2.7 and 2.8)

Executive summary

Fender's 1950s drawings and Marshall's early drawings carry no reference designators. Every modern schematic needs them, and if each transcription numbers its parts in drawing order the numbers mean nothing across amps. This document proposes one designator scheme for the union of the tweed circuits and the JTM45, so that a designator names a circuit position: the part called R41 is the first-stage cathode resistor on every amp that has one, whatever its value on that amp.

The scheme divides the circuit into functional blocks and gives each block forty numbers. All prefixes share the block map, so R41, C41 and RV200 are read the same way: the number says where in the circuit the part sits, the prefix says what it is. Tube elements are named the same way. Every triode section, pentode and rectifier diode is a U designator named by its role, whatever envelope it shares with another element: U1 is the section jack 1 feeds, U6 is the cathode follower, U20 is the output tube on the phase inverter's inverting side. The V designators of the original drawings, which name envelopes, are kept as a separate record and never reused for sections, so V1A on a Fender drawing and U1 in this scheme cannot be confused. Role numbering also makes a multi-channel amp decomposable: a channel lifted out for characterization on its own is renumbered from its own entry, and the mapping between the two numberings is recorded.

The proposal is verified against all seventeen transcriptions. A script maps every non-tube part on every sheet to exactly one slot, with no slot claimed twice on any amp, and the presence table in Section 4 shows which slots each amp fills. Of 196 catalogued slots, 137 are used by at least one amp. Space is deliberately over-allocated; block widths are compacted, if at all, only after the set is complete, because compaction renumbers every sheet already applied.

1. What the scheme has to do

A designator in this scheme is a claim about circuit position, not about value and not about page position. Two schematics that both carry C520 have a coupling capacitor from the plate of the post-volume gain stage in both, though one may read 0.02 uF and the other 0.1 uF. A part absent from an amp leaves its number unused on that amp; numbers are never reassigned to fill gaps.

This differs from KiCad's own annotation, which numbers in sheet order and will renumber on request. The scheme must therefore be applied as a fixed mapping from circuit role to designator, and KiCad's annotator must not be run over a sheet after the mapping is applied.

The transcriptions carry drawing-order numbers today. Those numbers are treated here as sources of value and topology only. Section 5 gives the old-to-new mapping for every sheet; nothing in the old numbering is preserved once the scheme is applied.

The JTM45 is included because it is a close copy of the 5F6-A Bassman: the same two-channel input, the same cathode follower and treble-middle-bass stack, the same long-tailed pair and fixed-bias push-pull output. Its parts fall into the same slots, and the few Marshall additions (a screen feed resistor, a high-tension fuse, a transistor tremolo) take slots the Fender sheets leave empty. The scheme is therefore meant to apply to old Marshall circuits as it applies to old Fender ones.

2. Rules of the scheme

2.1 Blocks of forty, shared across prefixes

The circuit is divided into functional blocks. Each block owns forty consecutive numbers, and every prefix uses the same block map. The gain-stage blocks are keyed to the U roles defined in Section 2.7; block 14 holds twenty numbers for each of the three gain stages that follow the volume controls.

Forty is generous. The widest block on any sheet read so far (the JTM45's tremolo) uses twenty numbers. Within a gain-stage block the layout is fixed: plate load at the base number, cathode resistor at base plus one, grid leak at plus two, grid stopper at plus three, plate-side mixing resistor at plus four, feedback or bootstrap resistor at plus five; the capacitor with the same number sits on the same node as the resistor (coupling capacitor with the plate load, bypass with the cathode resistor).

2.2 Same number, same node

Where a capacitor sits on the same node as a resistor with a distinct role, the two share a number: R41 is a cathode resistor and C41 its bypass; R40 is a plate load and C40 the coupling capacitor leaving that plate. For a cathode follower the coupling capacitor leaves the cathode, and the follower's cathode load takes the cathode slot.

2.3 Input stages are named by the jack that feeds them

The input always enters U1, the section jack 1 feeds. Where channel 2 has its own first section, fed by jack 3, that section is U2, whether it is the other half of the channel 1 envelope (5E3, 5F6, JTM45) or a tube of its own (5E5, 5E8). Where a jack has a section of its own that is paralleled at the plate with the first section (the Twin's jacks 2 and 4), that section is U3 or U4 by channel. Which physical unit of a 12AY7 holds which role is a fact about the transcription, not about the scheme; where the transcription wired jack 1 to the unit the symbol calls B (the JTM45 and the earlier 5E3 copy), U1 simply names that unit, and the pin numbers come with it.

The consequence is that a gain-stage block always means one role. Under the earlier draft of this scheme the block at 80 meant "the other half of V1", which was the second gain stage on the 5F1, channel 2's first stage on the 5E3 and the paralleled half on the 5E5; three roles under one number. Those three now sit in three places: the 5F1's second stage is U5 in block 14, the 5E3's normal channel is U2 in block 3, and the 5E5's paralleled half shares U1's slots under Section 2.6.

2.4 Output tubes by phase inverter output, not by page position

A push-pull output stage has one tube driven by the phase inverter's inverting output and one by its non-inverting output. In the cathodyne, the plate output inverts and the cathode output does not. In the long-tailed pair, the plate of the section whose grid takes the signal inverts, and the other plate does not. In the paraphase (5E9-A), the first section's plate inverts and the second section, which re-inverts a sample of that output, drives the non-inverting side. In a single-ended amp the driver is a common-cathode stage, so its one output inverts, and only the inverting-side slots are used.

Within the output block, an even number is the inverting side and the following odd number is the non-inverting side: R320 and R321 are the two grid stoppers, R324 and R325 the two screen resistors. A quad output stage takes its second pair ten numbers higher. The tubes follow the same rule: U20 is on the inverting side, U21 on the non-inverting side, U22 and U23 the second pair.

2.5 Everything is numbered by function

The phase inverter lives in the second half of the 12AX7 on the 5E3, in the second half of the post-tone tube on the 5E5-A family and in a tube of its own on the 5F6 family. Its parts have roles (tail resistor, inverting-side plate load) with no counterpart in a gain stage, so it has a block of its own, and the section that hosts it is U10, U11 and U12, or U13 and U14, by inverter type. The same holds for the tremolo oscillator (a whole 12AX7 on the 5E9-A, half of one plus a transistor on the JTM45), which takes block 12 whichever envelope hosts it, and for the tone stack, which hangs on the cathode follower U6 but is not part of it.

Gain stages are numbered by function too. U5 is the stage after the volume controls on every amp that has one: the 5F1's second triode, the 5E3's mixed stage, the 5F6's stage ahead of the follower. U7 is the stage after the tone stack on the 5E5-A family. The earlier draft numbered gain stages by tube section and only the inverter, stack, tremolo and supply by function; that split is gone, and the reason is recorded in Section 6.

2.6 Shared parts are numbered once

Where one physical part serves two sections (the common cathode resistor of a 12AY7 input tube), it takes the lower role's slot and the other role's slot is marked shared, not absent. A section paralleled with another in the same role (both halves of the 5E5's input tubes) has no slots of its own: the role's parts serve both, and the paralleled section is U1A or U2A under Section 2.7.

2.7 Universal section designators

Every tube element is a U designator: each triode section, each pentode or beam tetrode, each rectifier diode. The number names the element's role in the circuit, and the envelope it shares with another element is not part of the name. V1A and V1B on a given amp may be U1 and U2 (the two input stages of a 5F6) or U1 and U5 (the two stages of a 5F1); the letter that joined them on the drawing said only that they share a bottle. The scheme reserves V for the drawing's own envelope designators, which original schematics will always use, so that nothing in this scheme can be mistaken for a reference to the original.

Sequential numbering is not a goal; a gap in the count on an amp that lacks a role is expected, and the gaps between the role groups below are deliberate room for roles no transcribed amp has yet shown.

The follower is U6 on a Bassman and on a Twin, so its parts are R541 and C540 on both, and the paralleled halves of the 5E5's input tube share the U1 block instead of claiming a block of their own. That is the point of the rule.

In KiCad a multi-unit symbol carries one reference, so a U per section means each section is placed as its own single-section symbol. The envelope that joins two sections is a separate fact, carried on each section as a property (V2, pins 6 to 8) rather than as part of the designator. Heaters carry no designator at all: they are wiring, not circuit elements, and a heater drawn only to show the heater string gets no name. The exception is a heater that is part of a heater-elevation circuit, where the heater string is held above ground by a divider or a cathode; that heater is a circuit element and takes the designator H, numbered in block 10 beside the supply parts that elevate it. The library work this needs is listed as an open point in Section 6; the scheme itself does not depend on how the envelope is drawn.

2.8 Decomposition into fundamental circuits

Role numbering is relative to the circuit being characterized. On the whole amp the roles are the ones in Section 2.7. When a channel or a sub-circuit is lifted out to be characterized on its own, it is renumbered from its own entry: the 5E3's normal channel, taken alone, has U1 (the section that was U2 on the whole amp), followed by the shared U5 and U10, and the normal channel's parts move from block 3 to block 2 with them. The same holds for later amps the scheme is meant to reach: the first stage of a Dumble overdrive preamp is V3A on the factory drawing, some higher U number on the whole amp, and U1 in the overdrive circuit characterized by itself.

Two things make this safe. The whole-amp numbering and the sub-circuit numbering are both fixed mappings from role to designator, so each is reproducible from the other and the correspondence is recorded with the sub-circuit, not left to inspection. And the V designators, which name envelopes and never move, stay attached to every section as the thread back to the drawing.

3. Slot catalog

Every slot defined so far, by block. The Used column counts the transcribed amps that fill the slot; a zero marks a reserved slot named so that the block has a stable shape. Blocks 13 (reverb) and the unused parts of every block are open.

3.1 Block 1: Inputs (1 to 39)

3.2 Block 2: U1 channel 1 input stage (40 to 79)

3.3 Block 3: U2 channel 2 input stage (80 to 119)

3.4 Block 4: U3 channel 1 second input section (120 to 159)

3.5 Block 5: U4 channel 2 second input section (160 to 199)

3.6 Block 6: Channel controls (200 to 239)

3.7 Block 7: Tone controls (240 to 279)

3.8 Block 8: Phase inverter (280 to 319)

3.9 Block 9: Output stage (320 to 359)

3.10 Block 10: High-tension supply and bias (360 to 399)

3.11 Block 11: Mains (400 to 439)

3.12 Block 12: Tremolo (440 to 479)

3.13 Block 14: Later gain stages U5, U6, U7 (520 to 599)

4. Presence table

One row per slot that at least one transcribed amp fills. An x marks a part present; s marks a part present but shared with another slot and numbered there; a blank cell means absent. Every non-tube part on every sheet appears in exactly one row, checked by script against the exported netlists.

5. Old-to-new mapping for each transcription

Each table lists the transcription's designator, the canonical designator, the value as transcribed, and the canonical position. The tube line gives the universal section designator for each transcribed tube or tube section.

5E1 (Fender_champ_5e1_schem.kicad_sch)

Tubes: V1A becomes U1; V1B becomes U5; V3 becomes U20; V2 becomes U30.

5F1 (source_5f1_schem.kicad_sch)

Tubes: V1 becomes U1; V2 becomes U5; V4 becomes U20; V3 becomes U30.

5F2-A (Fender_princeton_5f2a_schem.kicad_sch)

Tubes: V1 becomes U1; V2 becomes U5; V3 becomes U20; V4 becomes U30.

5F10 (Fender_harvard_5f10_schem.kicad_sch)

Tubes: V1 becomes U1; V2A becomes U5; V2B becomes U10; V5 becomes U20; V6 becomes U21; V4 becomes U30.

5E3 (Fender_deluxe_5e3_schem.kicad_sch)

Tubes: V1 becomes U1 (bright); V2 becomes U2 (normal); V3 becomes U5; V4 becomes U10; V5 becomes U20; V6 becomes U21; V7 becomes U30.

5E9-A (Fender_tremolux_5e9a_schem.kicad_sch)

Tubes: V1A becomes U1; V1B becomes U2; V2A becomes U13; V2B becomes U14; V3B becomes U15; V3A becomes U16; V5 becomes U20; V6 becomes U21; V4 becomes U30.

Transcription note: V5 plate and cathode are on one net in the transcription (pins 3 and 8 both on T2-PR1); the mapping follows the intended roles.

5E5 (Fender_pro_5e5_schem.kicad_sch)

Tubes: V1A becomes U1; V1B becomes U1A (paralleled); V2 (input) A becomes U2; V2 (input) B becomes U2A (paralleled); V2 (second) A becomes U5; V2 (second) B becomes U10; V6 becomes U20; V7 becomes U21; V4 becomes U30.

Transcription note: The transcription uses V2 for two tubes (annotation error); the channel 2 input tube is written V2 (input) here and the mixed-stage tube V2 (second).

Transcription note: The joined bottoms of the two volume pots form a net with no other connection.

5E5-A (Fender_pro_5e5a_schem.kicad_sch)

Tubes: V1A becomes U1; V1B becomes U2; V2A becomes U5; V2B becomes U6; V3A becomes U7; V3B becomes U10; V6 becomes U20; V7 becomes U21; V4 becomes U30.

Transcription note: The joined bottoms of the two volume pots form a net with no other connection.

5E7 (Fender_bandmaster_5e7_schem.kicad_sch)

Tubes: V1A becomes U1; V1B becomes U2; V2A becomes U5; V2B becomes U6; V3A becomes U7; V3B becomes U10; V6 becomes U20; V7 becomes U21; V4 becomes U30.

Transcription note: The joined bottoms of the two volume pots form a net with no other connection.

5F4 (Fender_super_5f4_schem.kicad_sch)