Pair Mullard EL37 Tubes | Same Blackburn B8T 1958 Factory Code | Kalibr L3-3 Lab

For sale is an ultra-premium, laboratory-certified vintage Matched Pair of Mullard EL37 high-power audio pentodes. 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 (as verified in detail on photos TGTWUW-3.jpg and TGTYWQ-3.jpg).

• 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 during the 4th week of August 1958.

Being authentic birth-mates from the exact same production week, they share identical grid alignments, curved glass glass shape, and internal construction plates. Blackburn-made Mullard EL37 tubes are universally considered the holy-grail upgrade for high-end audio layouts (such as classic Leak TL/12 or premium custom single-ended/push-pull amplifiers), 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 "100%" on the inside tab flap, likely non-original but perfect for secure long-term storage). Meticulously inspected, pins cleaned, and strictly verified on a 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 Balance: 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, 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)

Worldwide shipping with bulletproof packaging. Tracking number provided immediately.