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Professional Tube Laboratory Test Report

Every tube listed in my shop undergoes a rigorous testing procedure to ensure the highest fidelity and performance for your equipment.

Testing Equipment: Professional Kalibr L3-3 Tube Tester.
Precision Measurement: Measurements are verified using a calibrated Metrix MX 51EX digital multimeter.
Transparency: All results are provided in the product photos, displaying specific Anode Current (Ia) and Transconductance (S) values compared to manufacturer specifications.

Why buy from us? Validated Quality (no guesswork, just documented performance) | Safe Shipping (double-boxed with anti-vibration materials) | Expert Service (specializing in high-end vintage electronics).

For sale is an ultra-premium, laboratory-certified NOS (New Old Stock) Matched Pair of vintage Mullard EL37 high-power audio pentodes (Internal References: TGTWUW & TGTYWQ). These magnificent British specimens carry the iconic etched factory markings and represent the absolute pinnacle of mid-century United Kingdom thermionic engineering.

Legendary Mullard Blackburn Heritage (Identical 1958 Factory Codes)

Both tubes feature the physical etched codes permanently embedded into the glass near the base layout: R61 / B8T.

R61 is Mullard's official structural formula code for the premium EL37 large-bulb envelope design.
B guarantees its birthplace from the revered Mullard production facility in Blackburn, England.
8T chronological batch tracking: manufactured precisely during the 4th week of August 1958.

Being authentic birth-mates from the exact same production week, they share identical grid alignments, curved glass shape, and internal construction plates. Blackburn-made Mullard EL37 tubes are universally considered the holy-grail upgrade for high-end audio layouts, celebrated for their vast holographic soundstage, rich lower-midrange warmth, and stunning three-dimensional transparency.

Exemplary Laboratory Matching Profile

On our reference laboratory test bench, this pair showcases a magnificent electrical harmony. The crucial dynamic measurement—the mutual transconductance S—tracks with an exceptionally tight 2.7% cross-matching variance between the two tubes, while the idle emission current matches at an elite 1.1% variance. This professional level of symmetry prevents channel drift and ensures true high-fidelity stereo balance.

Both tubes come housed inside excellent Mullard cardboard boxes (vintage boxes showing clear signs of handling with pen markings on the inside tab flap, perfectly suitable for secure long-term storage). Meticulously inspected, pins cleaned, and strictly verified on our professional, calibrated laboratory-grade Kalibr L3-3 master tube tester.

Key Features

Brand / Label: Mullard Great Britain (Classic Shield Stencil Logo Print)
Custom Labels (SKUs): TGTWUW & TGTYWQ
Provenance: Genuine British Production (Identical Etched B Blackburn Codes - 1958)
Design: Large "Fat Bottle" Glass Profile / Robust Dual-Mica Top Assembly / Metal Base Ring Accents
Condition: Vintage Tested Strong (Highly consistent operational curves)
Packaging: Complete with protective vintage replacement boxes
Safety: Strictly audited, NO shorts, flawless internal insulation profile.

Detailed Test Results (Kalibr L3-3)

Tested under reference operational conditions: Heater Voltage (Vh) 6.3 V | Control Grid Voltage (Vg1) -13.5 V | Anode Voltage (Va) 250 V | Screen Grid Voltage (Vg2) 250 V
(Factory specification for a 100% brand new reference tube: Ia = 100.00 mA / S = 11.00 mA/V)

Tube 1 (SKU: TGTWUW):
• Anode Current (Ia): 78.80 mA (78.8% of factory spec)
• Transconductance (S): 8.55 mA/V (77.7% of factory spec)

Tube 2 (SKU: TGTYWQ):
• Anode Current (Ia): 77.90 mA (77.9% of factory spec)
• Transconductance (S): 8.32 mA/V (75.6% of factory spec)

🌟 Seller Note on Performance: An outstanding audiophile grade match. The dynamic transconductance S—the core metric for audio signals—is incredibly tight, tracking at 8.55 mA/V vs 8.32 mA/V (only 2.7% deviation). With emission tracking near 78% of nominal target (78.8% and 77.9%), this healthy, highly stable 1958 Blackburn set is fully optimized for flagship stereo audio blocks.

Advanced Structural & Pentode Insulation Tests Passed

(Both specimens have been individually audited and successfully passed all laboratory insulation criteria)

Insulation test between Cathode (k) and Heater (H): PASSED for both tubes (No leakage)
Insulation test between Cathode (k) and Control Grid 1 (g1): PASSED for both tubes (No shorts)
Insulation test between Control Grid 1 (g1) and Screen Grid 2 (g2): PASSED for both tubes (No leakage)

About our testing methodology

To provide full transparency, our laboratory testing uses a professional Kalibr L3-3 tube tester. Since this device uses specific internal scales rather than direct mA/V readouts, each value is physically calculated to ensure precision.

Example of our calculation (Transconductance S):
Using a raw Kalibr reading of 85.5 on a standard scale of 150 with a multiplier of 15:
(85.5 x 15) / 150 = 8.55 mA/V
Percentage vs. Factory Specs:
We then benchmark this result against the factory nominal value for a new tube (e.g., 11.00 mA/V for an EL37):
(8.55 / 11.00) x 100 = 77.7% of factory specification.

Worldwide shipping with bulletproof packaging. Tracking number provided immediately.