pre-amp for the Wireless World Quality Amplifier

Requirements

  • A stereo  pre-amplifier with balanced outputs to drive a pair of Wireless World Quality Amplifiers
  • Self powered

There's nothing authentically 1930s about a stereo amplifier - although the Disney animated film Fantasia released in 1940 did have a stereo sound track, and the stereo soundtrack had been patented. There had clearly been some development of stereo recording and reproduction in the late 1930s.  However I reckon I can achieve a 1930s feel to this project with rotary controls, the right knobs, and of course a wooden cabinet.

Choices

Keep it small

The two power amplifiers are physically very big, 16in x 8in chassis for each, so it would be good to have a smaller pre-amp.   

Separate power supply

Although I'm making this pre-amp self powered, this isn't something that would have been done between 1930 and 1960, so I'll make the PSU a separate  unit.  This allows me the option of silicon rectification  whilst keeping the pre-amp proper a pure valve design.
Unlike the power amplifiers I'm very tempted to use components that can be easily sourced, so 6.3V heater valves, e.g. EF86 pentodes.
Since starting these notes I've acquired a Sound Sales tuner that also requires power, so a power supply capable of running the pre-amp and the tuner makes good sense.  So I'm building a valve power supply from old radio parts.
This power supply provide HT at 400V.  For the preamp I need approx 5mA per channel at 230V.  So I'll need to drop 170V at 10mA, so that's 17k ohms needed in series with the HT supply. For safety I'll put most of this in the power supply. Say 10k ohms;  at 400V that will be 16W, but at 10mA it will be just 1W, so a 1.5W resistor will also act as a fuse should the HT be shorted for any length of time.

Design notes

The EF86 pentode is electrically equivalent to the earlier octal EF36/EF37/EF37A (the American equivalent is the 6J7).  Both types were used in high-end and professional audio equipment either as pentodes (with NFB) or strapped as triodes.  Replacing, in the 1950s and 1960s, the Mazda SP41 and SP61 that met this need from the late 1930s through the 1940s.
I have several NOS Mullard EF86 and even more Mullard EF37A, so it makes sense to use a single type of valve for the entire design, e.g.  EF37A pentodes for amplification and another as a triode cathode follower for the phase splitter.

Tone controls.  It really wouldn't have a vintage feel without at least one tone control.  I rather like the bass and treble controls on my Leak Stereo 30 amps, so I'll try to achieve something similar.  I won't bother copying the low-pass filter from the Leak. See http://home.comcast.net/~stphkeri/tone_control.html (esp the PDF of Baxandall's 1952 Wireless World article).

Given that I've had good experiences with both the Mullard 5-10 and Mullard 3-3 amplifiers, taking the Mullard pre amplifier design as my starting point seems sensible.  Also it uses two EF86 per channel, so substituting EF37A for these is simple. Details of the stereo version here.

Balance control.  See the method used by Quad http://www.triodeel.com/quad22.gif it's not unlike the way AVC works in valve superhets.

Sourcing components

Though I'll be using valves, valve bases, and some other parts that I have already, it's interesting to identify sources of all the components needed for this project.  Note that when buying in the UK both Farnell and RS have a hefty surcharge on US stock items.

Passives (resistors and capacitors)

Electrolytic capacitors  -  Farnell, RS, Maplin
Polypropylene capacitors  (as substitutes for 0.1uF paper capacitors) - Farnell, RS, Maplin
Silver mica capacitors(*) - Farnell (Cornell Dubilier, US stock), RS.  
Resistors - Farnell (Vishay sfernice 0.5W 5% 200V metal film - qty 10, Mulitcomp 0.125W 1% and Welwyn 0.75W 1% also in 10s, many other types in qty 50+)

* Note. Silvered mica capacitors are very expensive (and always have been), especially for the larger values.  Alternatives might be appropriate since modern plastic film capacitors are pretty good. 

Valves (and bases)

EF86 - Cricklewood

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