Tube amplifier WS17 LUNA


Description and photos of details by  Andrzej Żółtowski, photos by  Piotr Stanisz Łódź.
[page was updated on April 3, 2005.] 

At the end of the first half of the sixties a small audio amplifier appeared in small electronics stores. It had an avant-garde for those times appearance and shocking pink colored casing.

This was, produced by Warsaw manufacturer called "ELZA", the "WS17 Luna" amplifier designed for ad-hoc sound reproduction in medium-sized rooms: school auditoriums, smaller churches, small clubs etc.

 

  

Amplifier design

The amplifier is placed in a furnace lacquer coated with perforated in the rear part steel casing. Housing is equipped with a detachable (so notoriously lost ...) metal headband enabling transporting the amplifier.

 

 

 

To the top of the cover (on the inside) small, very sharp picture of the diagram device was stuck. Under the cover a rigid chassis bent of cadmium-coated galvanized steel sheet thickness of 0.75 mm is located.

 

A steel plate, with mounted power switch, four potentiometers, two jack signal sockets, ground socket and coaxial microphone jack input, is bolted to the front of the chassis. The plate also includes two control bulb (6.3 V/03A) holders.

 

From the outside, plate is covered with Plexiglas aesthetic descriptions grille with marked sockets, potentiometers, and the name and logo of the amplifier manufacturer. Potentiometers are equipped with a nice plastic knobs like ivory. Mounted on top of the chassis there are two electrolytic capacitors, two transformers - mains and output, a potentiometer for balansing filament voltage of tubes and socket for EZ81 tube rectifier.

Mains transformer is bolted to the chassis by means of spacers.

 

In addition, ont the chassis a steel rectangular screen separating the preamp tubes of inductive components is located.

On the back of the chassis is the mains fuse adapter and the plate with "banana" sockets: three "speaker" and the fourth for the ground wire.

 

 

Inside the chassis the large printed circuit board with soldered: sockets of power tubes, phase inverter tube, preamp tubes and resistors and capacitors is placed.

 

The chassis includes holes through which the tubes partially protrude. It is also an antnoise screen. From the bottom the amplifier is closed by the metal  sheet bolted to spacers. This cover sheet also acts as an antinoise screen.

 

The Electronic Details of the Amplifier

The amplifier output stage is a low voltage circuit working in push-pull class AB, completed on two low power EL84 pentodes. The load is the "economical" (rather small) output transformer with approximately 11cm2 cross section.

 

On the transformer, the board with connectors (to which anode and speaker windings are soldered) is placed.

 

Power tubes are controlled by signals achieved from phase inverter with separate load (cathodyne) providing sufficient symmetry for the mid-range amplifier that is Luna. Grid of the phase inverter is galvanically coupled to the anode of proceeding triode, in which the cathode circuit is connected with negative feedback loop. Tone correction is implemented in the form of Baxandall  system. Input stages amplifying signals delivered by the piezoelectric transducer, radio and microphone are the classical circuits/

Power supply is a classic one-tube full-wave rectifier with RC voltage filtration. Anode supply voltage of power tubes is taken directly from the first electrolytic capacitor incorporated in the cathode of tube rectifier, other circuits are powered from an extensive RC filter. Electrolytic capacitors provide high capacity filtration capabilities sufficient for supplying input stages.

The voltage for heaters of power tubes, phase inverter and preamplifier is taken from a common winding of 6.3 V - (balanced with a potentiomete)r. The second winding 6.3 V supplies rectifier tube heater.

 

Technical parameters:

  • output power: 17W (sinus),
  • frequency range: 50Hz- 12,5kHz,
  • harmonic distortions: 4 - 5% (for 17W),
  • output impedance: 6 i 15 ohms.

Parameters are incomplete, reconstructed from service notes. User manual with technical data of this amp is practically unavailable!

Tubes:

  • ECC83 input amplifiers, 2 pcs.,
  • ECC83 phase inverter, 1 pc.,
  • EL84 output stage, 2 pcs.,
  • EZ81 rectifier, 1 pc.

Amplifier Rating

WS17 "LUNA" is the average quality amplifier of the typical acoustic performance required for the transmission of word-music programs and sound reproduction in small interiors.

Amplifier's sound can be described as undistorted, clean and very dynamic but also in the "midrange" cold and metallic - typical for PP EL84 pentodes amplifiers cooperating with a too small output transformer.

Despite filament symmetrization and a relatively good filtration of anode voltage at the "full" sensitivity and no input signal, mains hum is clearly heard - especially on the mic input. Grounding the amplifier only minimally reduces the unpleasant phenomenon.

In instrumental applications (electric guitar: solo and bass) Luna acquitted itself surprisingly well! The sound was clear, dynamic and, in addition, amplifier mic input allowed to get pretty pretty good "crunch" overdrive! Nevertheless, it was a typical amplifier designed for music and speech reinforcement of the typical sources: radio, turntable and a microphone.

Remarks:

  • Reliable  mechanical assembly in combination with an extremely poorly designed PCB was a kind of sensation! 
  • The printed circuit board made of miserable quality laminate, was the most unreliable element of the amplifier.. 
  • Luna was extremely uncomfortable amplifier to maintain. This was due to difficult access to the components on the circuit board (PCB combined with rigid and short potentiometers connections made virtually impossible for its turn aside from the chassis).
  • Frequent malfunction leads to burning PCB around the anode of EL84 tubes  (photos below).
  •  

 

Electric arc must have been huge - burned wires (orange, blue) and blackened chassis. The previous owner cut off damaged sections of tracks and soldered 6.3 V wires to another place.

 

  • Unintentional disconnecting the speakers (loosing banana sockets contacts) almost always ended with the creation of an electric arc between tracks, burning holes in it and destroying electric paths. 
  • Elimination of firing plate tracks by replacing them with insulated conductors resulted ... in electrical breakdown of unimpregnated output transformer - and ultimately it's destruction and the need of rewiring. 
  • Another disadvantage of the amplifier are changes in operating points of tubes caused by a fatal quality "paper" coupling capacitors. 

 

  • Output tube wear required replacement all EL84 tubes for paired ones. Replacing only one tube resulted in 100Hz hum. 
  • And last, the strangest surprise, revealed during the exploitation of  "Luna" as a guitar amp: the extremely high failure rate of EZ81 rectifier tube.

Made in the amount of only a few thousand copies, amplifier LUNA WS17 is now absolutely unique! The reason for this was probably the extremely high failure rate, problems related to repairs and unavailability of output transformers replacements, probably resulted in scrapping or dismantling of amps, which inevitably led to their complete disappearance.

Written by: Andrzej Żółtowski „ZoltAn”, Gliwice. This email address is being protected from spambots. You need JavaScript enabled to view it.