1. The getter mirror has turned white.

The tube has leaked and is no longer usable; it has been scrapped.

2. The filament is not lit.

There are two possible causes for a filament that does not light up: a cold solder joint on the internal filament wiring or a cold solder joint at the tube pins. When measuring resistance across the two filament pins, an infinite reading indicates an open circuit inside the tube, rendering it unusable. The electrode is non-conductive.

Due to poor soldering of the filament connections inside the tube, it must be scrapped; if the issue is poor soldering at the tube pins, re-soldering can resolve it. If the tube still does not light up after re-soldering, use a multimeter to…

III. Open-circuited Electrodes

If the grid (or control electrode) is open-circuited, the plate rapidly turns red and the vacuum tube deteriorates quickly; if the second grid (screen grid) is open-circuited, there will be no anode current and the entire unit will produce no output; if the anode is open-circuited, the screen grid turns red and the vacuum tube deteriorates quickly.

There are two possible causes for electrode discontinuity: poor soldering of the internal lead wire and poor soldering at the tube base. Components with defective internal soldering must be scrapped, whereas those with poor soldering at the tube pins can be repaired by re-soldering.

IV. Short circuits in vacuum tube electrodes

Filament-to-cathode short circuits are generally caused by filament powder shedding, manifesting as loud hum or excessive noise. Cathode-to-first-grid short circuits result from cathode powder clumps, grid deformation, or excessively close lead wire soldering; they cause the anode to rapidly turn red and blow the cathode fuse, often accompanied by arcing and sparking at the cathode due to powder detachment. Cathode-to-second-grid short circuits arise from deflection screen deformation or loosening of the second-grid filament, leading to red-hot second grids and anodes, quickly blowing the fuse. Cathode-to-anode short circuits are caused by deflection screen or anode deformation, resulting in rapid arcing at the cathode. All vacuum tubes exhibiting such short circuits must be scrapped.

V. Low anode current / weak output

During operation, if the anode current drops rapidly or output remains very low, this indicates poor vacuum integrity, inadequate cathode activation, or improper use and aging that has led to cathode poisoning, damage, or burnout.

Due to unstable performance of the vacuum tube, it can be returned to the factory for analysis and handling.

VI. Internal glow discharge inside the tube

There are many types of lighting conditions in the pipe, generally caused by hair spurs in the pipes, poor electrical contact or leakage, and can be returned to the manufacturer to help divide analytical processing.

VII. Noise in the tube

You can return it to the manufacturer for help handling.

There is a hum in the tube, the filament can be changed to DC power or returned to the factory for treatment

IX. Difficulty in inserting tube pins into socket

Check the needle position distribution, such as deformation, with tools available for correction; If the needle welding tin is too large, the welding tin can be removed from the excess part.

X. Foreign objects inside the tube

Some insulating foreign contents, such as mica debris, small glass, and small porcelain sheets, do not affect the use. If larger metal parts are lost, it is best not to use them.

XI. Thorium-tungsten cathode electron tube failure

The spring hook has fallen down, or filaments are seen inside the tube. Generally, the cathode is broken and cannot be used.

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XII. Blue light on the inner top glass of the tube

This is when electrons hit the glass’s fluorescent material to cause light to glow, completely without affecting use. After a period of time, the light will weaken or disappear.

XIII. Black or brown discoloration near mica films

After the electronic tube has been used for some time, the shell becomes black or brown near the mica film. This is a normal phenomenon, because these positions are bombarded by electrons for a long time, and lead oxide in the glass is reduced to lead.

XIV. Electronic tube core deformation

Due to the excessive force of transportation, the electronic tube deformation and even the rupture of the shell, some electronic tubes are overweight and inadequately designed and not adequately evaluated. Consider that the electronic tube support system is caused. It can be solved by contacting the factory.

XV. Octal tube filament flash on power-up

The moment the thumb tube is first inserted, it often finds that the filament suddenly lights up, commonly known as a lighting.

This is an inherent feature of spiral filaments, and this phenomenon is observed in electronic tubes using spiral filament abroad. This flashing phenomenon will not have any impact on the use and life of the pipe.

So why do some filaments flash while others do not or only flicker slightly? This is because the helical filament must be coated with a high-temperature insulating layer via electrophoresis, yet the filament’s tail end must still be soldered to the lead wires. Consequently, there exists a short exposed segment of metal wire between the solder joint and the insulating layer. When voltage is first applied, this exposed metal segment experiences concentrated heat and briefly glows brightly; once the entire filament heats up uniformly, this flash disappears. The length of this exposed segment correlates with the intensity of the flash: the longer the exposed portion, the more pronounced the brightness.

FAQ About Audio Vacuum Tubes

1. What are the common anode materials used in audio vacuum tubes?

Typical anode materials for vacuum tubes include oxygen-free copper, single-sided aluminum-iron coating, double-sided aluminum-iron coating, copper-core aluminum-iron coating, copper-base aluminum-iron coating, nickel-plated iron, carbon-coated nickel-plated iron, spray-carbon nickel-plated iron, and graphite. Among these, carbon-coated nickel-plated iron is the superior material for such vacuum tubes, significantly enhancing both tube lifespan and sound quality; however, it is generally beyond the manufacturing capability of most factories. Currently, only the Shuguang Vacuum Tube Factory can produce this special material.

2. What tests should be conducted on vacuum tubes prior to shipment?

In general, finished vacuum tubes undergo two main categories of testing before leaving the factory: routine tests and life tests. Routine tests include glass heat-resistance stability tests, salt spray tests, vibration stability tests, lead wire strength tests, filament continuity tests, and drop tests. Life tests are performed continuously on dedicated life-test benches in accordance with the product’s detailed specifications; national standards require that continuous life testing for vacuum tubes meet a minimum duration of 500 hours. Most small-scale manufacturers lack the necessary test facilities and have no reliable assurance of their outgoing product quality, leaving everything to chance. Shuguang Vacuum Tube Factory, formerly a state-owned military-industrial enterprise, is equipped with complete testing facilities and strictly adheres to national standards to ensure that all shipped products comply with the company’s detailed specifications.

3. Is inspection required for vacuum tube raw materials upon entry into the factory?

Vacuum tubes are vacuum devices whose manufacturing structures and auxiliary materials are subject to stringent requirements. For all materials entering the factory, it is insufficient to rely solely on suppliers’ certificates of conformity; rigorous incoming inspection is essential. Incoming materials are primarily subjected to physical and chemical analysis to ensure consistency and accuracy across every batch. The reason Shuguang Vacuum Tube Factory can guarantee reliable and stable product quality lies in its tight control over both materials and processes—a capability that small factories simply cannot achieve, as they often skip material analysis and use inputs without verification, making it impossible to trace the root cause when problems arise.

The Effects of Vacuum Equipment on Electron Tube Quality

The impact of vacuum equipment on audio vacuum tubes is primarily reflected in: the operational stability of the tubes, their service life, tube noise, and electrical…

Since 1996, Shuguang Electronic Tube Factory has independently developed and manufactured centerless degassing vehicles. These vehicles achieve a vacuum level two orders of magnitude higher than that of conventional degassing vehicles, representing a global first and a patented technology of the factory. This innovation marks a major transformation in electronic tube manufacturing equipment, ushering legacy products into a new era and making significant contributions to the global recognition of Shuguang’s products.