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!
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(Zout 3K)||1||3.7||5902||5902|
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.