David Berning Company - Manufactures a series of vacuum tube audio amplifiers based on impedance converter that replaces the traditional audio-output transformer. Italy. Watford Valves - Suppliers of tested, graded and guaranteed valves to the professional music industry. UK. Variable Voltage Technology Ltd - Transformers power supplies and related equipment for audio equipment. Also EL inverters and magnetron transformers. UK. Audio Kit Mania - Distributor of tube and related products to design and build custom amplifiers and systems. Greenstone USA - Manufacturer and distributor of vacuum tubes and capacitors for audio, broadcast and military applications. THL - Supplier of audio parts and DIY kits. Taiwan. PAEng Design - Devoted to vacuum tubes world applied to audio science. Consulting, design and engineering in audio. Power amplifiers and Hi-End products. Zeitmann tubes - Manufacturers of vacuum tubes such as NOS Tungsram audio tubes. JJ Electronic - Manufacturers of tubes, capacitors for high-end audio amplifiers. Base in Slovak republic and Canada. MagneQuest - Makes tube audio transformers such as air-gapped and parallel feed single-ended transformers. JAC-Music - Provides information about electron tubes products, support, distributor addresses, datasheets and sells tubes parts. MaxiTest, LLC - Digital tube testers for matching and testing power pentode vacuum tubes used in musical equipment amplifiers. Aprilsound, Inc - Audiophile audio transformers and accessories.