What is the 27 vacuum tube?

The 27 (27 / UY-227) is a triode vacuum tube designed for preamp applications. First indirectly heated triode. Ancestor of 6SN7..

What is a 27 substitute or equivalent?

Direct equivalents to the 27 include VT-29. These share the same pinout and operating envelope, so they can usually be swapped without circuit changes — though batch-to-batch variations between manufacturers (NOS vs current production) may shift the sonic character noticeably.

What are the key specifications of the 27?

Amplification factor μ ≈ 9, transconductance gm ≈ 0.975 mA/V, plate resistance rp ≈ 9.3 kΩ, max plate voltage 275 V, max plate dissipation 2 W, heater 2.5 V / 1.75 A.

Is the 27 still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the 27 as Collector availability. NOS (new-old-stock) examples from RCA, Mullard, Tung-Sol, GE, Sylvania and Telefunken trade among collectors; modern production where it exists comes mostly from JJ Electronic, Electro-Harmonix, Tung-Sol (New Sensor), Genalex, and a handful of small-batch makers.

What does the 27 sound like?

First indirectly heated triode. Ancestor of 6SN7.. Sonic character also depends heavily on the circuit operating point, the surrounding tubes in the stage, and the output transformer when present — use the interactive loadline calculator on this page to explore the trade-offs.

27 UY-227 Triode Vacuum Tube

Also known as VT-29

Low-mu triode with excellent linearity for driver and line stages

TriodeHeater 2.5 V · 1.75 AUX-5RCA·GE 1927

Plate dissipation: 2W
Max plate voltage: 275V
Amplification factor: 9
Transconductance: 0.975mA/V
Plate resistance: 9.25kΩ

Characteristics

Datasheet values, once each. Every value cites its sheet.

HeaterIndirectly heated

Specifications of 27 — Heater
Heater voltageVh	2.5 V
Heater currentIh	1.75 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of 27 — Limiting values
Plate voltageVa max	275 V
Plate dissipationPa max	2 W
Heater–cathode voltageVhk max	90 V

Mechanical

Specifications of 27 — Mechanical
Base	UX-5
Rarity	Collector
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of 27 — Small signal
Transconductance	0.975 mA/V
Plate resistance	9.3 kΩ
Amplification factor	9

Capacitances

Specifications of 27 — Capacitances
Grid–cathode	3.1 pF
Grid–plate	3.3 pF
Plate–cathode	2.3 pF

Charactereditorial reading, not a measurement

Specifications of 27 — Character
Gain	9%
Power	5%
Drive	91%
Linearity	95%

datasheet cited · model or editorial reading · catalogue value, no citation attached yet · nothing invented

Typical operation

Canonical operating pointModel

B+: 225 V
Load: 19 kΩ
Va: 135 V
Ia: 2.35 mA
Vg: -11 V
Pd: 0.32 W
Gain: 14.6 dB

At Q: Ayumi Nakabayashi
μ: 8.9
gm: 0.69 mA/V
rp: 13 kΩ

Using this tube

Applications

Voltage amplifier
Line stage

Generic triode preamp stage

Hi-Fi stereoHeadphone amp

Replace or compare

Sources

By Jean-Michel Falligan DevergneReviewed May 29, 2026Active model : Ayumi Nakabayashi

Manufacturer datasheets

5 manufacturer datasheets cross-checked for 27 / UY-2275 verified

27 DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: RCA·GE
Reviewed: 2026-05-29
Status: DataModelCalc
Model: Ayumi Nakabayashi

Context

The 27 is a indirectly-heated triode from RCA·GE, introduced around 1927. It provides a gain factor (μ) of 9 with 9250Ω plate resistance. Commonly used in preamp applications. First indirectly heated triode. Ancestor of 6SN7.

History

Introduced: 1927
Manufacturer: RCA·GE
Era: Pre-War

METRIX TESTER