EL34 vs 6550

EL34 / 6CA7 versus 6550 — Pentode vs Tetrode compared.

EL34 vs 6550: what differs

The EL34 and 6550 are the two classic rivals of their power class. 6550 has the higher plate dissipation rating (~35 W vs ~25 W for the EL34). Bias points and output transformers differ — never swap blindly; the listening notes stay in their own labelled block below.

Comparison by objective

Choose what the stage has to do: the quantities below are reordered and weighted for that objective. Every value comes from the catalogue and from the datasheet typical operating point; a missing value shows “—” with its reason. No score, no ranking — only measured differences.

Voltage amplification of a small signal — common cathode with a resistive plate load.

Quantity
Difference
Available gain (μ)Weight 3/3 · decisiveμ is the ceiling of the voltage gain of a common-cathode stage.Av ≈ μ · Ra / (Ra + rp) — with Ra = 2–3 × rp the stage reaches about 2/3 to 3/4 of μ. As a follower the gain is close to 1 whatever μ.
EL34
11
6550
8
Differenceratio ×1.4
Load to drive (rp)Weight 2/3 · importantPlate resistance: the lower rp, the lower the load the stage can drive at the same gain and linearity.Ra ≥ 2–3 × rp for a common-cathode stage. As a cathode follower the output impedance is about 1 / gm.
EL34
15 kΩRa ≥ 30 kΩ – 45 kΩ
6550
12 kΩRa ≥ 24 kΩ – 36 kΩ
Differenceratio ×1.3
Available current (Ia typical)Weight 1/3 · to checkPlate current at the datasheet typical operating point — what the stage can deliver into a capacitance or a low load.Read from the published typical point (Va, Vg). No operating point is derived from a model here.
EL34
100 mAat Va 250 V, Vg −14 VTypical point: Svetlana Electron Devices (St. Petersburg, Russia; US offices AL/CA) · 4/97 (April 1997)
6550
140 mAat Va 250 V, Vg −14 VTypical point: Tung-Sol Electric Inc., Electron Tube Division, Bloomfield, New Jersey · 1963 (JULY 1, 1963 / MARCH 1, 1963; PLATE #6755/6679/6680/6681)
Differenceratio ×1.4
Dissipation margin (Pa max − Pd typical)Weight 1/3 · to checkDifference between the maximum plate dissipation and the dissipation at the typical point (Pd = Va · Ia).Pd at idle < 70 % of Pa max.
EL34
0 WPa max 25 W · Pd typical 25 W (100 % of Pa max)Typical point: Svetlana Electron Devices (St. Petersburg, Russia; US offices AL/CA) · 4/97 (April 1997)
6550
0 WPa max 35 W · Pd typical 35 W (100 % of Pa max)Typical point: Tung-Sol Electric Inc., Electron Tube Division, Bloomfield, New Jersey · 1963 (JULY 1, 1963 / MARCH 1, 1963; PLATE #6755/6679/6680/6681)
Differenceidentical
Heater (V × A)Weight 1/3 · to checkHeater voltage and current, hence the power drawn from the heater winding.Same voltage required; the winding must supply the summed current of the tubes in parallel (or the same current in series).
EL34
9.5 W6.3 V × 1.5 A
6550
10.1 W6.3 V × 1.6 A
Differencedifference +0.63 W
Base compatibilityWeight 1/3 · to checkBase and pin-by-pin function according to the catalogue pinouts.Same base and identical pinout → same socket. Same base, different pins → rewiring. Different base → adapter or new socket.
EL34
octalOctal · 8 pins
6550
octalOctal · 8 pins
Differencesame base, pins differ

Rules: Amplifier topologies guide, §07 “Design rules of thumb”

Physical compatibility

EL34
6550
Base
octal
octal
Pinout
Octal
Octal
differs on 4 pin(s): 1: G3 / NC, 2: H / G1, 5: G1 / K, 8: K / H
Sections
1
1
Bulb diameter (max)
—
—
— no mechanical data in the catalogue

Heating

EL34
6550
Heating
Indirect (IHT)
Indirect (IHT)
Heater voltage
6.3 V
6.3 V
same voltage — the same winding can feed both · EL34: no center tap · 6550: no center tap
Heater current
1.5 A
1.6 A
current difference +0.1 A (+7 %)
Heater power
9.45 W
10.08 W

Bias

Datasheet typical operating point (one per tube, SEQ-02) and the cathode resistor it implies.

EL34
6550
Va typical
250 V
250 V
Vg typical
−14 V
−14 V
Ia typical
100 mA
140 mA
Indicative Rk = |Vg| / Ia
140 Ω
100 Ω
Pd typical = Va · Ia
25 W
35 W
EL34 — Typical point: Svetlana Electron Devices (St. Petersburg, Russia; US offices AL/CA) · 4/97 (April 1997) · 6550 — Typical point: Tung-Sol Electric Inc., Electron Tube Division, Bloomfield, New Jersey · 1963 (JULY 1, 1963 / MARCH 1, 1963; PLATE #6755/6679/6680/6681)

Performance for the objective

Catalogue values, weighted as in the objective table (Gain input stage). rp is a design parameter, not a merit.

EL34
6550
Difference
μ
11
8
ratio ×1.4
gm
11 mA/V
11 mA/V
identical
rp
15 kΩ
12 kΩ
ratio ×1.3
Pa max
25 W
35 W
difference +10 W
Va max
800 V
600 V
difference −200 V

Substitution possible under conditions

No direct substitution listed in the catalogue.

What rules it out as a drop-in:

  • same base but pins differ: 1: G3 / NC, 2: H / G1, 5: G1 / K, 8: K / H

Substitutes for EL34Substitutes for 6550

Side-by-side specifications

Side-by-side specifications of EL34 and 6550
SpecEL346550
Full nameEL34 / 6CA76550
TypePentodeTetrode
Primary usePower (PP)Power (PP)
Era19551955
Heatingindirectindirect
Heater voltage6.3 V6.3 V
Heater current1.5 A1.6 A
Amplification factor (μ)118
Transconductance (gm)11 mA/V11 mA/V
Plate resistance (rp)15 kΩ12 kΩ
Max plate voltage800 V600 V
Max plate dissipation25 W35 W
Baseoctaloctal
ManufacturerMullard·PhilipsTung-Sol·GE
RarityCommonCommon

Sonic character

Listening folklore — not electrical data

EL34

British voice. Marshall tone.

6550

American powerhouse.

Need an interactive deep dive?

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