About Valves (Vacuum Tubes)

London-Series Valve types

The Phædrus Audio PHAB and PHILTER employ an EF86 pentode input valve and an ECC88 valve as the output device. Positions for the valves are very clearly marked on the PCB silkscreen as indicated in the illustration. The PHI employs the very well know dual-triode 12AU7 tube (European name is ECC82), as does the PHAMULUS which also uses the 6BC8 or the 6BZ8 as the all-important vari-mu (or gain-control) element.

The valves (tubes) that are installed in Phædrus Audio’s products during assembly are selected to give the best possible performance. We offer versions of all products with NOS (rather than current production) valves. For example a Mullard NOS tube is shown for comparison with our standard fit EF86 in the picture (left). But NOS tubes must be chosen carefully, and be pre-screened. Replacing the supplied valves (tubes) with different, collectible, or "high-end" valves (tubes) types may not affect any improvement and might cause deterioration of performance. So, please contact us for further details of NOS options.

Valve (tube) types

EF86

The EF86 is a low noise pentode, originally manufactured by many different factories and branded by Philips, Mullard, Telefunken, Valvo, and GEC, TESLA and Siemens/RFT. Many NOS valves are still available. Equivalents include the CV2901 (the UK service designation for the EF86) and the CV4085; a special quality version for military use. The EF86 tube is still being produced in Russia and in the Slovak Republic under JJ Electronics brand name (formerly Tesla). Chinese close equivalents are also still being manufactured.

ECC88

The ECC88 is a frame-grid, sharp cut-off, twin (dual) triode with an internal, separating screen. The valve was originally designed for service in the front ends of VHF radio receivers. The ECC88 (E88CC) tube type is equivalent to the North American 6DJ8 tubes and the (rare) military branded CV2492. The transposition of the figures from ECC88 to E88CC was Mullard's way of denoting a special quality build and is equivalent to the US 6922. Russian 6N1P tubes and the Chinese 6N1 tube are NOT equivalent. NEVER install a 6N1(P) in the Phædrus Audio PHAB or PHAME product. The ECC88 valve is sill being manufactured in China and in the Slovak Republic under JJ Electronics brand name (formerly Tesla).

12AU7 or ECC82

The 12AU7 is a popular, miniature 9-pin medium-gain dual triode valve (tube) and is used in many instrument and hi-fi amplifiers. Both triodes are used, in cascade in the Phædrus Audio PHI product. The 12AU7 is also known in Europe under its Mullard-Philips tube designation ECC82. This tube is widely available both new and NOS and a number of special quality equivalents are available. Current production of 12AU7 takes place in Russia, Slovakia, and China. Phædrus Audio selects and recommends the long anode (plate) version of the ECC82 (ECC802S) or 12AU7 valve for application in the PHI DI-Box.

6BC8 and 6BZ8 tubes

The tube chosen for the "vari-mu" (δμ) element in the Phædrus Audio PHAMULUS is the 6BC8/6BZ8 tube. This was the tube in the original Altec circuit. The 6BC8/6BZ8 tubes are meduim-mu, semi remote cutoff twin triodes, originally intended for applications in television receivers. Because theses tubes arrived late in the "age of thermionics" and were employed in the most sucessful consumer product of all time (the television), there exist many, many of these tubes as NOS (new old stock) at reasonable prices. This underlies Phædrus Audio's decision to adopt this tube.

Valve (tube) lifetime

You should replace the valves in the tubes in the Phædrus Audio products only when you start to notice changes in the sound quality. If the gain of the preamplifier decreases noticeably, then this is certainly evidence of the onset of valve (vacuum tube) failure. Before this, the tone may become "dull" and transients may be become "blunted".

That said, the lifetime of a valve (tube) is largely determined by the lifetime of its cathode emission and the small-signal valves (tubes) used in the PHAB, PHAME and PHI use oxide cathodes, which can provide adequate cathode emission for 100,000 hours or more. That's over eleven year’s continuous use. So do not replace valves (tubes) just because they have seen a few years service. It is also perfectly acceptable to leave Phædrus Audio London-Series components energised at all times. In fact, it may even be preferable, as research at the end of the valve era proved that valve lifetime was improved by leaving valves energised at all times.

Phædrus Audio Ltd. can provide suitable valves (tubes) as spares which, after a burn-in period, are screened for best performance in your Phædrus Audio product. These are available as line items:

PHAE-12AU7 – Selected 12AU7/ECC82 type valve
PHAE-ECC88 - Selected ECC88 type valve
PHAE-EF86 - Selected EF86 type valve

Please contact Phædrus Audio Ltd. for current prices.