NOS, old stock, tested and matched describe different things. A useful valve listing separates manufacturing history, evidence of use, present electrical condition and the quantities being sold. None of these labels, on its own, establishes compatibility with an amplifier.
What does NOS mean?
NOS means “new old stock”: older production represented as unused. Age alone does not establish unused condition. An original-looking box, a clean envelope or a familiar brand is not conclusive evidence of a valve's history. Ask what supports the description and whether the photograph shows the actual item.
OS means “old stock”; it does not necessarily establish that the item is unused. If previous use is unknown, that uncertainty should remain explicit. A valve may have useful electrical characteristics without qualifying as NOS. Conversely, an unused valve is not automatically free of storage-related or manufacturing defects.
New production is a separate description
A currently manufactured valve should be identified by its exact maker and model, including suffixes. A modern model carrying a historic brand name is not automatically the same construction as an earlier valve bearing that brand. Consult the applicable manufacturer's data rather than transferring ratings or reputation from an unrelated version.
What does “tested” establish?
A test result has meaning in relation to the instrument, method, settings and acceptance criteria used. Checks for shorts, leakage, emission, transconductance, current, noise and microphonics answer different questions. A single “good” indication does not describe all of them, prove unused condition or guarantee remaining operating life.
Glass Audio tests valves before sale. Not every valve currently held in stock has already been tested. No universal test method, individual measurement report, low-noise selection or matching tolerance is promised by that statement. If your application needs particular evidence, confirm it before ordering.
Matched pairs, quads and balanced sections
A matched pair contains two valves selected against stated criteria; a matched quad contains four. Matching is selection, not a change that makes arbitrary specimens identical. Ask which electrical quantities were compared, at what operating conditions, and with what tolerance. Matching at one operating point does not establish identical behaviour throughout an amplifier's signal swing.
For a dual triode, similarity between the two internal sections is a different question from matching separate bottles. Neither description automatically establishes a noise or microphonics grade. Follow the amplifier manufacturer's requirements for sets and adjustment. Buying two individual valves does not, by itself, specify a matched pair.
As an example of transparent retail practice, The Tube Store publishes its own current-based matching system. That supplier's method and tolerances must not be attributed to Glass Audio or to every seller.
Noise and microphonics are different
Noise can include electrical hiss, crackling or intermittent disturbances. Microphonics describes mechanical vibration producing an unwanted electrical response. Circuit gain, valve position and the equipment's mechanical environment affect whether that response is troublesome. A valve acceptable in one position may be unsuitable in another.
Do not tap hot valves or reach inside an operating amplifier to investigate a noise. Describe the symptom, affected channel and operating circumstances to a qualified technician. A hum or crackle is not proof that a valve is the cause; power supplies, connections and other components can produce similar symptoms.
How long do amplifier valves last?
There is no universal service-life figure for all valve types and equipment. Operating conditions, heater supply, temperature, duty and the individual specimen matter. A glowing heater only establishes that it is glowing; it does not demonstrate correct emission, amplification or safe amplifier operation. Replacing valves on an arbitrary calendar cannot substitute for the equipment manufacturer's guidance and competent diagnosis.
A practical buying checklist
- Record the equipment manufacturer, model, valve position and required designation.
- Compare the complete suffix and approved replacement information.
- Check whether the price is for one valve or a specifically described set.
- Clarify NOS, old-stock or new-production status and any uncertainty about use.
- Confirm any measurement, noise-selection or matching requirement before purchase.
- Ask for a destination-specific international postage quote, or arrange collection before visiting Glass Audio.
Further reading and sources
For the underlying valve principles, consult US Navy NEETS Module 6, Introduction to Electronic Emission, Tubes, and Power Supplies, September 1998, NAVEDTRA 14178. The commercial matching example above describes that retailer's practice only. This article does not report measurements of Glass Audio stock.
Continue with reading a valve datasheet, how valves work and the manufacturer reference library. Contact Glass Audio with the exact type and required quantity.
Electrical safety: valve amplifiers can retain potentially lethal stored voltages after unplugging. Internal inspection, testing, repair and bias adjustment require a qualified technician.