12J5GT VT135 National Union, triple mica, black plate, NOS/NIB, one pair (2)

Description

One pair (2) National Unionl 12J5GT  VT 135 tubes.

This is the military VT135 special quality version made for the  US forces in the 1940ies.

These tubes feature a triple mica construction and extra long black plates. We have actually never seen any thing like this in a 12J5GT or 6J5GT. Only three pairs available in total. Super rare items!!

The tubes are abolutely new and have not seen daylight for over 80 years. These come in rather torn military boxes but the tubes as such are in perfect working condition and test 100%+ in terms of emission and gm. Sorry, no perfectly matched pairs available due to very limited stock but values are close for each pair.

All tubes are tested and guaranteed to be in fully working condition.

These tubes are guaranteed to be new/NOS and unused from old military stock!

1. General Description

• Tube Type: Triode – single, indirectly heated vacuum tube

• Structure: Single low-mu triode with glass envelope

• Designation: VT-135 = Military version of 12J5 (civilian)

• Amplification Factor (μ): ~20 (low gain)

• Primary Function: Audio-frequency voltage amplification

• Envelope/Base: Glass tubular (“GT”), octal base (8-pin)

2. Audio Applications

Use in Audio Systems:

• Hi-Fi Audio:

  • Rarely used in modern high-fidelity systems due to low gain

  • Sometimes used in custom tube buffer stages or retro gear

• Guitar Amplifiers:

  • Found in a few vintage amp designs or in driver stages

  • Known for smooth, warm tone, not harsh or aggressive

• Studio Gear:

  • Used in vintage signal chains where linearity and warmth were key

Comparison to Other Triodes:

3. Industrial or Other Applications

• Military Electronics:

  • Designed for rugged applications: radios, radar, field equipment

  • Operated reliably across wide temperature and vibration ranges

• Instrumentation:

  • Used in oscilloscopes, signal generators, and analog meters

• Computing:

  • Found in early analog and logic circuits before the transistor era

• Control Systems:

  • Deployed in process controllers and automation systems pre-1960s

4. Comparison to Similar Tubes

🛠️ VT-135 vs 6J5: Identical internally except for heater voltage
🔁 VT-135 = 12J5 = one half of a 6SN7 (same triode section)

5. Key Technical Data

6. Construction and Special Features

• Envelope: Glass (GT = glass tubular); sometimes seen in metal base variants

• Socket Type: Octal (standard 8-pin)

• Military Designation:

  • “VT” = Vacuum Tube (US Army/Signal Corps designation)

  • VT-135 corresponds directly to 12J5 in civilian numbering

• Suffix Meaning:

  • No “A” suffix → standard heater warm-up

  • “GT” = Glass Tubular

• Notable Qualities:

  • Rugged construction for military spec

  • Low microphonics

  • Used where long-term reliability and linearity mattered more than gain

Summary

The VT-135 is the U.S. military version of the 12J5 — a single low-mu triode with a 12.6V heater and octal base. Known for its excellent linearity, ruggedness, and warm audio response, it found use in military communications, instrumentation, and vintage audio gear. While it doesn’t compete with modern high-gain triodes like the 12AX7, it is valued in custom analog circuits, restorations, and low-gain stages where clarity and reliability are key.