Sub-Miniature Tubes

What are they?

Sub-miniature tubes, also referred to as pencil tubes or hearing-aid tubes, are among the last generation of vacuum tubes designed and built. They’re typically very small, and instead of being socketed they have flexible leads like passive components. Some are designed to be run off of batteries (the hearing aid tubes), with very low heater & B+ voltages, while others were designed for military & industrial use.

I’ve always been interested in small & miniaturized things. I first got interested in small tubes with the 6AK6, which I purchased a bunch of intending to build Doug Hammond’s Pentode Driver, but built the Micro Champ instead. I knew about sub-miniature tubes then, but didn’t get around to getting any until I saw Zachary Vex’s Nanohead amp and at the same time I found a box of 6021W tubes on eBay, which immediately triggered my mania!

Potential 6021W Projects

• Overdrive pedal, Matchless style
• Overdrive pedal—sky’s the limit (dumble style? 3 tubes & 6 gain stages, fuuuullll shred?)
• Tube Buffer, like the Valvulator
• Tube Tremolo pedal, like the Matchless Vibrobox
• Standalone preamps
• Micro amp, like the Nanohead or PowerMan
• Hi-Fi—headphone amp, computer audio amp, preamp
• Tube microphone preamp

High Voltage Sub-Minis

This is a table of some of the sub-miniature tubes which run on 6.3V heaters and high voltage (>100V) B+ and are probably well suited to guitar projects.

Tube	Type	Ih (A)	Gm	mu	Ra	P (W)	Pdiss (W)	eBay	Datasheet
6021	Dual Triode	0.3	5.4K	35	6.5K		1.1	6021	6021
6111	Dual Triode	0.3	5K	20	4K			6111	6111
6112	Dual Triode	0.3	1.8K	70	38K		0.3	6112	6112
6814	Triode	0.15	6K	29	4.8K		2.2	6814	6814
5719	Triode	0.15	1.7K	70	41K		1.3	5719	5719
5744	Triode	0.2	4K	70	17K		1.3	5744
5840	Sharp Cutoff Pentode	0.15	5K	1300	260K		1.1	5840	5840
5899	Remote Cutoff Pentode	0.15	2.5K	650	260K			5899	5899
5902	Beam Power Pentode	0.45	4.2K		15K (Zout 3K)	1	3.7	5902	5902

Low Voltage Sub-Minis

hese, which typically run on 1.25V heaters and 24V B+, would be suitable for building tiny, battery powered amps, such as Fred Nachbaur’s Tea Tin Amp.

Tube	Type	Ih (mA)	Gm	mu	Ra	P (mW)	Pdiss (W)	Eh	Ea	Datasheet
6088	Power Pentode	20				10.5		1.25V	45V	Tung-Sol, Raytheon

Links

• Tube Data
  • WA2ISE sub-miniature tube page (from WA2ISE, which features a rendition in sub-mini tubes of the "All American Five" radio tube set)
  • Raytheon SP-1041 5M-8-61-WEA (cathode subminiature tubes), Raytheon-SP-7073-360-20M (Subminiature tubes).
  • The Virtual Valve Museum can be searched for pictures of many tubes, including some sub-minis. 6021, 6111. It’s also got pictures of just the weirdest tubes, so it’s great to browse.
  • The National Valve Museum also has a few.
• Applications & Misc
  • The "Reduced Princeton" – a class project for UIUC’s Physics of Electronic Musical Instruments course (lucky students!)
  • Tube CAD Journal – Balanced line stage using subminis, and part 2—Balanced phono stage 
  • I found a commercial phono preamp using 6021 and 6112 tubes
  • Pencil tube phono preamp, with 6021W
  • Another RIAA phono preamp with sub-minis
  • Wow, people sure do like to make phono preamps with subminis
  • A Japanese page on submini tubes
  • Subminiature Tube Radios
  • 6021W Tube DAC
  • Audio Aero makes several commercial hi-fi products using the 6021W
  • Blaupunkt Omniton Hearing Aid, using subminis
  • Fred Nachbaur’s Tea Tin Amp
• Small Speakers. Tiny combos need tiny speakers. This is a listing of speakers I’ve found sized 6" and below. The smallest speakers specifically designed for guitar seem to be 6". Below that you need to check frequency responses for something which tails off after 5-6KHz. Sometimes small woofers or midranges have a good response curve for this.
  • Eminence Legend 675. 6", 4 ohm, 75W, 180Hz-4.5KHz (spec sheet, PDF)
  • WeberVST Ceramic Signature 6", 6", 4 ohm, 15W
  • Jensen MOD 6", 6", 4 ohm, 15W
  • BRAVOX 3-1/2” 6 OHM SHIELDED WOOFER from PartsExpress. 3.5", 6 ohm, cheap! (spec sheet, PDF). I made a small cabinet out of two of these.