A valve is microphonic when mechanical vibration changes its electrical output. In an audio circuit this can become ringing or feedback. That does not mean every hiss, hum or rattle is a faulty valve: the circuit, mechanical environment and position in the signal path matter.
This guide helps you describe a problem and choose what to ask before buying a replacement. Internal amplifier testing, live tapping tests and bias adjustments belong with a suitably qualified technician. Valve equipment can retain dangerous voltages after it is switched off or unplugged.
What causes valve microphonics?
A valve contains electrodes supported inside an envelope. Vibration can move parts of that assembly and modulate the electrical signal. The resulting response depends on both the valve and the vibration reaching it. Robert B. Tomer’s technical discussion, reproduced by Effectrode, explains why the same specimen may cause trouble in one installation yet work acceptably in another. It is therefore useful to distinguish a physical tendency from an application-specific failure.
RØDE similarly notes that some microphonic response is inherent in valves, while excessive response can interfere with the intended signal. This is why a requirement for low microphonics should be discussed explicitly rather than inferred from a brand, a NOS label or a generic test result.
Why the valve’s position matters
A small unwanted signal introduced near the input may pass through several later gain stages. A mechanically sensitive valve in that position can be more troublesome than the same response elsewhere. Electro-Harmonix identifies early, high-gain stages and speaker-equipped combo cabinets as particularly sensitive situations. This does not establish a universal approved position for a replacement: use the equipment maker’s valve chart and service information.
Ringing, hiss and hum describe different observations
| What you hear | What to report | What it does not prove |
|---|---|---|
| Ringing or a sustained whistle | Whether it follows ordinary sound or vibration during normal operation | Which component is responsible, or which valve to buy |
| Hiss or intermittent crackling | When it starts and whether normal front-panel controls change its level | That emission is low or every valve needs replacing |
| Hum or buzzing | Whether it affects one channel, both channels or a particular input | That it is the same mechanism as microphonics |
The observations above are a way to communicate, not a diagnosis. Tomer’s separate discussion of noise describes how intermittent behaviour can depend on operating voltage, impedance and the circuit in which a valve is used. A single tester indication does not answer every noise question.
What a test can and cannot establish
Emission, insulation, operating current and microphonic response are different characteristics. JJ Electronic lists several such checks in its manufacturing process. Its factory procedures do not describe Glass Audio’s own test method or establish measurements for an individual stock valve.
Glass Audio tests valves before sale. Not every item in inventory has already been tested. No universal matching tolerance, selected low-noise grade or measurement report is promised unless the individual listing supports it. For a sensitive phono or microphone application, describe that requirement before ordering. Read our guide to NOS, old stock, testing and matching.
Useful information to send with an enquiry
- The equipment make, exact model and affected channel or input.
- The complete valve designation, suffix and position from the equipment documentation.
- The symptom in ordinary use, when it began and any recent authorised servicing.
- Whether you require a particular documented condition or selection grade.
Do not remove covers, touch hot valves or reach inside powered equipment to collect this information. If there is smoke, sparking or a burning smell, stop using the equipment and arrange professional attention. Do not repeatedly replace a blowing fuse or fit a different fuse rating.
Questions buyers often ask
Does NOS mean low microphonics?
No. Unused old stock describes condition and history; it is not an application-specific noise measurement.
Does a valve that passes an electrical test have zero microphonics?
No. Different checks answer different questions. Ask what evidence supports any selection claim.
Can I use a 12AT7 instead of a noisy 12AX7?
A shared base does not establish a suitable replacement. Consult our 12AX7, 12AT7 and 12AU7 comparison and the equipment requirements.
For equipment that specifies the exact type, browse the 12AX7 / ECC83 catalogue or E88CC / 6922 catalogue. Current stock and selection claims remain specific to each listing. Contact Glass Audio in Pretoria with your requirements; international postage is calculated before quotation and collection is by prior arrangement.
Technical sources
- Robert B. Tomer: Microphonics, 1960 excerpt reproduced by Effectrode.
- Robert B. Tomer: Noise, 1960 excerpt reproduced by Effectrode.
- RØDE: What is microphonics in a tube?
- Electro-Harmonix: tube FAQ and microphonics discussion.
- JJ Electronic: manufacturing technology and testing parameters.
Original Glass Audio explanatory text, reviewed 28 October 2026. Source descriptions of manufacturer testing are attributed to those manufacturers; no photographs, diagrams or article text have been copied.