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Adam's Amplifiers: Subminiature
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Thursday January 5, 2006 by adam

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.

Tube Type Ih (A) Gm mu Ra P
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


Guitar Amp Projects

Hi Fi Amp Projects

  • Kolibri, another 5902 stereo amp.

Tube Data

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.

Commercial Products

  • 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).

Dead Links

Like this article? Add it to!

  1. Hi Adam,

    Not sure if you’ve seen these before but just wanted to mention a couple of links to two 6021 amps I across.


    — Andy c.    Apr 4, 11:54 PM    #
  2. I’ve seen the first one, but not the second – thanks!

    Adam Alpern    Apr 10, 09:38 PM    #
  3. I’ve just finished an initial prototype of a minamp using a 6112 & 5902 submini tubes. A schematic and clips are at:

    Now i just need to fit it in a submini enclosure!


    Jon Anderson    May 18, 01:47 PM    #
  4. Does anyone know where to start in designing/building a low wattage hifi power amp using 2×6021s (per side)? ie. something like the Z.Vex iMP Amp, just twice as powerful (and hopefully a lot less pricey!).

    Ben Wells    May 30, 12:02 AM    #
  5. Check out Chris Siegmund’s use of sub miniature tubes…he’s been doing it since 99 or so

    Mike C    Oct 20, 03:37 PM    #
  6. Hi,

    look at my three-channel stereo-amp with four 6021 an six 5902.

    Greetings, Reinhard

    Reinhard Stenzel    Nov 5, 01:43 AM    #
  7. Hey guys
    I have an Audio Aero Prima mk2 CD player that uses a pair of 6021W. Do any of you know how long I should expect these to last?


    Arthur    Nov 7, 08:09 AM    #
  8. To Arthur

    those 6021 tubes last very long i thought 100000 hours or something like that

    nathan    Dec 29, 03:50 AM    #
  9. That’s 10 years running 24/7

    Phil    May 5, 12:51 PM    #
  10. hey there, i’m in need of some help.

    i wanna build a low-wattage power amp, but i don’t know how to get the high voltage for it (i need 150V for this project).

    i have a 15+15v/300mA transformer here, and i though of using it for getting 30v ac and making a 5x full wave voltage multiplier, to get the 150V i need. but i didn’t find anywhere the values for the capacitors on the voltage multipliers, and i have no idea how to calculate that, so if anyone knows other ways to get the B+ voltages, i’d be deeply thankfull.

    Lucas    Sep 7, 01:56 PM    #
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