What is the D3A vacuum tube?

The D3A (D3a) is a pentode vacuum tube designed for preamp / driver applications. German Postamt legend..

What is a D3A substitute or equivalent?

Direct equivalents to the D3A include 7721. 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 D3A?

Amplification factor μ ≈ 80, transconductance gm ≈ 35 mA/V, plate resistance rp ≈ 120.0 kΩ, max plate voltage 220 V, max plate dissipation 4.2 W, heater 6.3 V / 0.315 A.

Is the D3A still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the D3A as Sought-after 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 D3A sound like?

German Postamt legend.. 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.

D3A D3a Pentode Vacuum Tube

Also known as 7721

Small-signal pentode for low-noise voltage amplification

PentodeHeater 6.3 V · 0.315 A9-pin NovalSiemens·Telefunken 1958

Plate dissipation: 4.2W
Max plate voltage: 220V
Amplification factor: 80
Transconductance: 35mA/V
Plate resistance: 120kΩ

D3A

Characteristics

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

HeaterIndirectly heated

Specifications of D3A — Heater
Heater voltageVh	6.3 V
Heater currentIh	0.315 A

Limiting valuesDatasheet maximum ratings — not to be exceeded

Specifications of D3A — Limiting values
Plate voltageVa max	220 V
Screen voltageVg2 max	150 V
Plate dissipationPa max	4.2 W
Cathode currentIk max	30 mA

Mechanical

Specifications of D3A — Mechanical
Base	9-pin Noval
Rarity	Sought-after
Pinout

Small signalDatasheet parameters at the reference operating point — not limits

Specifications of D3A — Small signal
Transconductance	35 mA/V
Plate resistance	120 kΩ
Amplification factor	80

Capacitances

Specifications of D3A — Capacitances
Input	10 pF
Grid–plate	0.035 pF
Output	2.1 pF

Charactereditorial reading, not a measurement

Specifications of D3A — Character
Gain	80%
Power	11%
Drive	0%
Linearity	60%

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

Typical operation

Canonical operating pointModel

B+: 175 V
Load: 240 kΩ
Va: 105 V
Ia: 13.86 mA
Vg: -1 V
Pd: 1.45 W
Gain: 38 dB

At Q: Ayumi Nakabayashi
μ: 82
gm: 31.4 mA/V
rp: 2.6 kΩ

Using this tube

Applications

Voltage amplifier

Generic pentode preamp / driver stage

Replace or compare

Sources

By Jean-Michel Falligan DevergneReviewed May 29, 2026Source : Siemens D3a/7721 datasheet (pearl-hifi mirror)Active model : Ayumi Nakabayashi

Manufacturer datasheets

3 manufacturer datasheets cross-checked for D3a3 verified

D3A DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: Siemens D3a/7721 datasheet (pearl-hifi mirror)
Reviewed: 2026-05-29
Status: DataModelCalc
Model: Ayumi Nakabayashi

Context

The D3A is a indirectly-heated pentode from Siemens·Telefunken, introduced around 1958. It provides a gain factor (μ) of 80 with 120.0kΩ plate resistance. Commonly used in preamp / driver applications. German Postamt legend.

History

Introduced: 1958
Manufacturer: Siemens·Telefunken
Era: Golden Age

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