# SBM-21 compact, reversible contacts - 9 September 2026

**PCB ORDER HOLD.** The current nominal geometry does not establish a secure, adequately insulated mount. The PCB coordinates remain the previous layout for review; they have not been cosmetically moved to make the illustration appear seated.

### Axial fit finding - 9 September 2026

The model's 21 mm tube spans x = 23.445–44.445 mm. Its 16 mm can ends at x = 39.445 mm. The illustrative clip stop has its inner face at x = 22.095 mm, leaving **1.350 mm** axial gap. This stop location is a modeling assumption, not a dimensioned manufacturer datum. The socket body width is also corrected to the manufacturer’s 0.147 inch (3.734 mm), rather than the neighboring socket’s 0.156-inch width. The cap previously extended the model by 0.1 mm; it is now contained within the specified can length.

The socket entrance is x = 42.3585 mm, giving **2.0865 mm insertion into the housing envelope**. This is not measured spring engagement. The manufacturer's sectional drawing shows a spring constriction inside the housing but does not specify the minimum required insertion depth. Moving the tube left to the modeled clip stop would reduce envelope insertion to **0.7365 mm**.

The unused forward anode pad has centre x = 42.460 mm and radius 1.100 mm. With the modeled can bottom 0.500 mm above the PCB, its shortest distance to that pad is **1.979 mm**, below the project's **2.8 mm** clearance target. This is a 3D metal-to-metal geometry result; ordinary planar DRC does not test the elevated tube can. It excludes solder fillets and dimensional tolerances, which cannot improve the guaranteed minimum. Mirroring the assembly gives the same result at the opposite site.

For comparison only, moving the tube right to obtain 3.5 mm envelope insertion would leave just **0.708 mm** to that pad, and increase the clip-stop gap to **2.764 mm**. 3.5 mm is an illustrative trial depth, not a Keystone specification or an approved design. Moving the contacts together would also require relocating JP1 or changing its layout to preserve copper clearance. Neither trial has been applied to the PCB.

Before changing holes: measure actual can length/diameter, exposed pin length/diameter, overall length, loaded clip stop/axis, socket engagement and contact retention. Choose a mount that provides adequate engagement and insulation together. The nominal 6 mm can is smaller than the clip's specified 6.3/6.35 mm fuse size; the nominal 2.03 mm socket does not cover the factory's entire 2.01–2.15 mm pin envelope. A sample fit or a different contact/insulating support is necessary.

See [dimensioned side view](mechanical/tube-fit.svg) and [calculated fit report](tube-fit-checks.json). The new fit checker blocks order approval independently of clean bare-PCB DRC.

Selected prototype parts: **one Littelfuse 01000058Z body clip and one Keystone 6049 pin socket**. The clip is 5.84 mm tall and 7.11 mm along the tube, replacing the rejected 12 × 12 mm Keystone 3562. Both tube-end sites now share one identical footprint. This is a sample-fit design, not a qualified holder assembly.

## Published dimensions

[PocketMagic](https://www.pocketmagic.net/tube-sbm-21-%D1%81%D0%B1%D0%BC-21-miniature-geiger-muller/) gives a **16 mm long × 6 mm diameter can, 5 mm long × 2 mm diameter pin, and 21 mm overall length**. These dimensions control the tube model, not the pad spacing. The [earlier factory label](https://www.gstube.com/scan/2399_1.jpg) gives a 2.01–2.15 mm pin envelope. Measure the actual surplus tube; the rounded 2 mm nominal is not a fit guarantee.

| Contact | Purchase | Manufacturer dimensions | Remaining fit check |
|---|---|---|---|
| Body clip | [Littelfuse 01000058Z, DigiKey F4186-ND](https://www.digikey.com/en/products/detail/littelfuse-inc/01000058Z/2518627), quantity 1 | 7.11 mm axial length, 7.40 mm transverse envelope, 5.84 mm height above seating plane; 5.08 mm transverse hole pitch, 1.32 mm recommended holes | Specified for 6.3/6.35 mm fuses: verify spring retention and contact resistance on the 5.9–6.1 mm tube can without denting it |
| Pin socket | [Keystone 6049, DigiKey 36-6049-ND](https://www.digikey.com/en/products/detail/keystone-electronics/6049/316473), quantity 1 | Red nylon, nominal 2.03 mm pin; 5.283 × 3.734 × 5.156 mm body; 5.08 mm axial lead pitch; 3.175 mm pin axis above seating plane | Actual pin acceptance, insertion force, spring engagement and insulation |

Sources: [Littelfuse manufacturer drawing](https://www.littelfuse.com/assetdocs/100-58-series-datasheet?assetguid=5b808640-bf54-4935-9350-a02381dfbaf8), [Keystone socket catalogue](https://www.keystone-europe.com/wp-content/uploads/2025/08/pins-plugs-jack-sockets.pdf). The Littelfuse distributor lists a 125 V fuseholder rating; neither contact is being claimed as a qualified 400 V tube assembly. Installed insulation must be established separately. No parts have been ordered.

## Identical footprints at both tube ends

Both sites use `GC1_RevD:SBM21_Interchangeable`: four **1.40 mm finished plated holes with 2.20 mm lands**. Relative hole centres are **(0, −2.54), (0, +2.54), (−2.54, 0), (+2.54, 0) mm**. Each opposing pair is 5.08 mm apart. All four holes at a site are duplicated pad 1 on the same site net.

The body clip uses the transverse pair; the pin socket uses the axial pair. Populate only one contact at each site. The common 1.40 mm drill enlarges Littelfuse's 1.32 mm recommendation to accommodate the approximately 1.20 mm socket leads; check solder fill and mechanical fit. The unused pair remains plated.

| Site | Centre, top-view PCB coordinates | Site net |
|---|---|---|
| MC1 | (25.50, 4.20) mm | TUBE_LEFT |
| MC2 | (45.00, 4.20) mm | TUBE_RIGHT |

Both sites receive the same 2.8 mm clearance rule to other nets, because either can become the anode. H4/H5 screw holes are removed; H1–H3 enclosure holes remain. The tube ground-pour exclusion and nearby ballast/test-pad positions follow the new layout.

## Reversing orientation

JP1 has four numbered underside solder pads in a 5 mm square. Fit **two short 0.3–0.5 mm tinned solid-wire links** close to the board. Use the pad numbers: viewing the underside mirrors the drawing.

| Assembly | MC1 left site | MC2 right site | JP1 links |
|---|---|---|---|
| Default, shown in schematic and model | Body clip / can (−) | Pin socket / isolated pin (+) | **1–3 and 2–4** |
| Reversed | Pin socket / isolated pin (+) | Body clip / can (−) | **1–2 and 3–4** |

JP1.1 = TUBE_LEFT, .2 = GM_ANODE, .3 = GM_CATHODE, .4 = TUBE_RIGHT. The schematic shows the default removable-tube pin assignment; alternate assembly changes the tube orientation and the two links together. The native PCB netlist keeps the configurable links electrically separate; assembly continuity is checked for each configuration.

1. Disconnect power and discharge the HV storage capacitor before servicing.
2. Remove the tube and both existing JP1 links before changing configuration.
3. Swap the body clip and pin socket between the sites, rotating each 180° so the socket faces the tube. Trial-fit without loading the insulating seal; remove the tube before soldering.
4. Fit exactly the two links for the chosen orientation. Never mix link configurations or short all four pads.
5. After cooling, verify can-contact to **GM_CATHODE** continuity and pin-contact to **GM_ANODE** continuity, with no short between them. GM_CATHODE is the pulse pickoff, not board ground.

## Assembly limits

The model uses a provisional **3.50 mm loaded tube axis** and **0.325 mm socket stand-off**. The can is 1.350 mm inboard of the illustrative clip stop; this is an unresolved fit defect. Geometric overlap with the socket envelope is not proof of spring engagement. Verify alignment, retention, repeated insertion/removal and electrical contact in both orientations.

**Assembled HV insulation remains unresolved.** In particular, inspect the anode-facing socket tail and the nearby end of the cathode can, unused pads, underside links and enclosure. Bare-PCB DRC cannot establish the clearance of those elevated metal surfaces, the nylon socket's insulation, or the tube seal. Revise the placement, insulation or contact choice if sample checks fail; do not treat the rendering or prototype Gerbers as an assembly release.

`mechanical/tube-mount.json` is the shared geometry record; the PCB, website and renderers consume it. `tube-mount-bom.csv` lists both contacts plus the two manual wire links. Previous carrier, screw-clip and tall-clip designs remain explicitly superseded history.