The "textbook Heerlen sound" has been referenced in several Soundlumia ECC83/12AX7 listings, and many tube enthusiasts asked what all that means. So here is the answer: the late 1950s and very early 1960s tone, the beginning of over a decade's excellence in sound, the classic tonal template for all the great (and greater) Amperex/Philips/Valvo/Miniwatt tubes to be made in the years to come, the timeless Philips radio tone that many of us grew up listening to.
Sound Characteristics
The most distinctive sound feature of the Heerlen ECC83's is the transparent, natural and expansive midrange presentation, with a slight touch of warmth that illuminates your music. Every piece of music you play through these tubes sounds like from a vinyl record, whether your sound source is phono or not. The sound stage is spacious, deep and structured. Bass is incredibly deep but well controlled and feels as accurate as on the high end German ECC83's (Siemens, Telefunken, Valvo etc.) We would recommend these as an excellent choice if you are seeking the high end ECC83 tubes that can deliver the legendary Philips house sound flawlessly. Thanks to the super rare foil d-getter on this pair, you get with fast response (as in the solid-state fast), overwhelming detail in sound and the realistic bass - this is truly the Holy Grail sound of Philips ECC83 that one should not miss!
Professional Tube Testing
NO shorts, leakage or gas. NO noise or microphonics.
Both tubes tested NOS (100%+ nominal mutual conductance) and tightly matched on a reliable and recently calibrated Mercury 2000 tube tester with the following results for mutual conductance:
Tube 1: 1,250/1,210 (125%/121%)
Tube 2: 1,200/1,200 (120%/120%)
Good tubes start at 700 and new tubes (incl. NOS/NIB) 1,000, and the Mercury 2000 tester's rated mutual conductance (nominal) for new tubes is 1,000. Test results can be seen in the pictures. In case you wonder what all these numbers mean to your sound experience, it's actually quite straightforward: mutual conductance directly translates to a tube's ability to amplify sound, so the higher these numbers the stronger.
Appearance & Cosmetic condition
This pair stands out aesthetically among the vintage tubes with pristine cosmetic condition. Nearly 100% of the silkscreen is well preserved and the IDENTICAL acid-etched Heerlen date codes are perfectly visible on both tubes:
mC2 ⊿7A (mC2 = ECC83 Long Plates production gen #2, ⊿ = Heerlen, 7 = 1957, A = January)
