PURPOSE: To provide a replacement for the obsolete 6386 tubes in earlier Manley Variable Mu® units or convert later 5670 units to have the same limiting characteristics of the original 6386 for smoother operation.
METHOD: To install a converter board and surrounding modifications to enable usage of two 12BA6 or 6BA6 single pentodes wired in triode mode to take over the duties of a single envelope dual triode 6386 or 5670 (per channel).
INTRODUCTION: The Manley Variable Mu® has been in production for some years now and has become an indispensable tool in studios since its inception. These units operate on the same principle as several top-notch vintage tube limiters, and the original Manley version indeed used the same 6386 input tube as the Fairchild, Gates, and others. This tube is the one that actually performs the gain reduction; it, more than any other component, determines the sonic signature of these units. HISTORY: The 6386 dual triode was designed as a cascode RF/IF gain-controlled amplifier, and as such, had many characteristics ideally suited for use as a gain-controlled audio amplifier. However, it seems this tube was used more in commercial/industrial applications rather than high volume consumer applications (like in radios and TV's). Also, it appears that none of these have been manufactured in recent years. The combination of these two factors means that present supplies of 6386 tubes are quite limited, expensive, and in many cases unusable due to poor matching, high noise, or microphonics (which may not have been issues for the tube's original intended use). Manley decided in June of 1996 to begin using a type 5670 dual triode which were similar, importantly sharing the same pin-out as the 6386 and available in sufficient quantities to allow for testing and matching.
ENTER NEW TUBE TYPES: The 5670 equipped Variable Mu's were well received, and retained many of the characteristics that made the original version of the Variable Mu® popular. They did sound different than the original, especially when pushed past 6db or so of limiting. Even though some people even preferred the new version with the 5670, it was felt that it would be worthwhile to give the users the ability to return to the traditional sound without resorting to use of an esoteric (or unavailable) tube. We also wanted to furnish an alternative replacement for the 6386 in older units. The goal was to find a tube that was 1) nearly identical to the 6386 in performance 2) manufactured (or being manufactured) in large quantities 3) able to be retrofitted into existing units (in the field, if at all possible). This was a tall order, as other available remote-cutoff dual triodes (such as the 6BC8 and 6ES8) are not close enough in characteristics to be used as a direct replacement. Enter the 12BA6, which is a remote-cutoff single pentode. Wired as a triode, this tube has characteristics very close to the 6386 as confirmed by a Tektronix 570 curve tracer, and by A-B listening tests. These tubes were manufactured in large quantities for use as gain-controlled IF amplifiers in common radio receivers. The hitch: the 6BA6 (or 12BA6) is a single-section tube; the 6386 is a dual!