What are Sub-Miniature Tubes?
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.
They’re not the smallest generation of vacuum tubes – that distinction probably goes to Nuvistors, but they maintain the classic glass envelope, letting you see the heater glow and internal structure of the tubes. It’s just not right if you can’t see the tubes glow.
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.
|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
Guitar Amp Projects
- JJS Class A submini amp, using 2 6021Ws with SE output stage. Looks very much like the ZVex Nano, and uses a switching power supply.
- JJS Class AB submini amp, using 6112, 6021W, and a dual 5902 PP output stage, with toroidal transformers (sound samples available). Both very impressive!
- The Reduced Princeton (PDF), a class project for UIUC’s Physics of Electronic Musical Instruments course.
- Jon Anderson’s 5902 submini guitar amp, using a 6112 for the preamp and an SE 5902 output stage (sound samples available)
- Micropower subminiature tube guitar amps with reverb and vibrato, a collection of SPICE-modeled circuits replicating classic guitar amps in submini-form. These would be an interesting starting place for a build.
- Mark Lavelle’s PowerMan 6021 amp in a PC power supply.
- Another Powerman build, using a pair of 6021s for output.
- Pencil Pusher
- Pencil Pusher II
- Nabla, with 6N17B, 6N21B, and 6P30B tubes
- Miniature tube amp
- Subminiature Express at Amp Garage. T-Wreck circuit with 5902 output.
- A4 ‘Wreck made with 6112 and 6AQ5, also on Amp Garage. Spiffy.
Hi Fi Amp Projects
- Mini-Amp mit der Miniaturröhre 5902
- Tube CAD Journal – Balanced line stage using subminis, and part 2 — Balanced phono stage
- Kolibri, another 5902 stereo amp.
- 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.
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” guitar speaker. 4 ohm. Appears to be discontinued, but available online from this one vendor.
- Weber Speakers has a couple of 6” models and a couple of 6“x9” models.
- Jensen MOD 6-15
- 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.
- ZVex Amps makes the Nanohead guitar amp, and the iMPAMP stereo amplifier, both using JAN 6021W tubes.
- Audioaero’s Prima amplifier uses 6021Ws (although they’re inside the case, soldered to a PCB board – not nearly as cool if you can’t see them).
- 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
- Subminiature Tube Radios
- 6021W Tube DAC
- Blaupunkt Omniton Hearing Aid, using subminis
- Fred Nachbaur’s Tea Tin Amp