Product Overview
Variable Permeability Inductor Forms for Vintage RF and Transmitter Tuned Circuits
Condition
Used condition. Sold as a two-piece pack. Evaluation: Visual inspection only. Function testing not performed. Metal mounting brackets show light surface oxidation and age-appropriate handling wear, with intact tubular bodies, internal slotted tuning slugs, and threaded adjustment studs. Photos are of the actual stock.
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Product Overview
National Radio Company XR-91 variable permeability-tuned coil forms provide adjustable inductance for radio frequency oscillator, amplifier, and filter circuits. Designed around a 3/8-inch tubular body housing an internal iron-powder core, each form permits precise inductance alignment via a slotted threaded adjustment stud. The assembly mounts directly to a chassis plate through a two-hole oval metal bracket and includes an external collar ring to establish winding boundaries.
Key Features
- 3/8-inch outer diameter tubular coil form body
- Internal slotted iron-powder tuning slug for permeability adjustment
- Threaded adjustment stud with locking hardware for stable alignment
- Two-hole oval metal chassis-mounting bracket
- Adjustable collar ring for winding positioning
- Pack of two matched coil form assemblies
Applications
- Building custom VFO and local oscillator circuits for amateur radio gear
- Winding intermediate frequency transformers and RF interstage coupling coils
- Restoring vintage National Radio transmitters, receivers, and communications equipment
- Prototyping tunable filter networks and resonant tank circuits on chassis layouts
Frequently Asked Questions
How is the inductance adjusted on the XR-91 form?
An internal threaded stud moves the slotted iron-powder slug axially through the center of the tubular form. Turning the stud alters core position relative to your winding, shifting circuit inductance into alignment.
What is the outer diameter of the winding area?
The tubular body measures 3/8 inch in outer diameter, providing a cylindrical surface suited for standard enameled magnet wire.
What condition are these coil forms in, and how are they evaluated?
These coil forms are used vintage components with intact bodies, functional adjustment screws, and light patina on the mounting metal. Evaluation is visual inspection only; function testing has not been performed. Photos are of the actual stock.
Winding your own coils on National forms is one of the few bench rituals that connects modern radio builders directly to classic amateur design. Set your turns, adjust the slug, and peak that receiver right where you want it.
