Polski (PL) 
English (United Kingdom) 
Design and photographs by Andrzej Żółtowski "ZoltAn", Gliwice,
Note: most photos can be enlarged by tapping on them with your mouse.
In 1963, in PZT Warsaw began producing the WR40/62 (Ampli 40) and WR75/62 (Ampli 75) amplifiers. Very similar to the much earlier WR40 model, they were in fact completely redesigned and modernized acoustic devices. Like their predecessor, they were intended for creating small radio networks in schools and small workplaces, sound reinforcement of large religious buildings, sports halls and stadiums, railway stations and ad hoc megaphony of events organized in the open air.
Amplifier design
Both versions of the amplifier were housed in aesthetic, hammer-gray painted, improved (for better cooling of power tubes and inductive elements) perforated steel covers. Visually, both amplifiers were indistinguishable from each other!
The electronics are assembled on a rigid and stable box-shaped chassis, welded from thick cadmium-plated steel sheet.
The upper side of the chassis is divided by a steel plate, separating the amplifier tubes, which are visible through the mesh part of the housing and emit a lot of heat, from the transformers and electrolytic capacitors.
The EM84 control indicator bracket with base is screwed to the chassis separation plate.
On top of the chassis there are mounted sockets for the preamplifier tubes, phase inverter, power stage and power supply, electrolytic capacitors as well as inductive elements: two transformers - mains and output and a large choke.
Inside the chassis (normally covered from the bottom with perforated sheet metal) there is all the amplifier electronics along with the connections of the individual circuits.
The front of the chassis contains four potentiometers: input signal level (separate for each track) and common tone equalizers, as well as a rotary package switch that acts as a power switch and a standby/operation mode switch.
On the back of the chassis there is an "iron" mains power socket, in the left "recess" fuse sockets - mains and anode, banana "speaker" sockets and a nut for attaching the grounding wire. In the right "recess" there are two "diode" (DIN) signal sockets: "Radio / Turntable" input and microphone input.
Inside the chassis there are two vertical bakelite plates with riveted connectors, one with resistors and capacitors connected by wires to the tube sockets, the other with rectifier diodes. The wires are run in the form of bundles sewn together with string.
Electronic system
The output stages of the amplifiers are medium-voltage, push-pull class B systems, implemented on two (WR40/62) or four (WR75/62) EL34 pentodes, whose load is a large output transformer with a layered winding system - identical (!) for both versions of the amplifier. The cross-section of the central column of the core of this transformer is about 20cm2.
The transformer has a board with three pairs of sockets (switched with a removable jumper) allowing you to set the required output voltage of 30 (i.e. 8 ohms) - 60 - 120V depending on the type of load.
The power tube grids are controlled by a classic phase inverter circuit (based on the ECC85 tube) ensuring a wide and even frequency response and correct symmetry of the EL34 tube grid control signals.
The input stages, implemented on EF86 tubes, are classic resistive amplifiers, connected in series. The first pentode is a highly sensitive microphone preamplifier. The circuit used is characterized by a minimum noise level.
The load of the second stage is a simple tone equalizer that also acts as a grid resistor for the first triode of the phase inverting circuit.
The extended anode power supply system consists of three series-connected diode (germanium) rectifier bridges also supplied from three separate low-voltage (3 x 160V) sections of the mains transformer windings. It provides +420V voltage supplying the anodes of the power tubes and the remaining amplifier circuits.
The output of this rectifier is loaded with a large wire ("glass") resistor - a 20k/50W bleeder - limiting the increase in anode voltage when the power tubes are not driven and discharging the electrolytic capacitors after the amplifier's power supply is turned off.
The negative voltage (bias) polarizing the power tube grids is supplied by a single germanium diode cooperating with a separate winding. It is "rigidly" set by the manufacturer and cannot be adjusted.
Preliminary filtration of the anode voltages occurs in the choke, the winding of which is connected directly to the output of the "upper" diode bridge.
This arrangement is extremely effective in filtering out anode voltage ripples, while also allowing for a significant reduction in the capacitance of the first high-voltage electrolytic capacitor.
The voltages supplying the power and preamplifier tube heaters are taken from two separate 6.3V windings. The connections are made with colored wires grouped in not very aesthetic bundles, the assembly itself is of the "element - element" type.
Design differences
The WR40/62 and WR75/62 amplifiers differ in the number of power tubes and different mains transformers and chokes - and therefore also in their weight.
Amplifier parameters
The difference in parameters comes down to the output power (40 and 75W) and the level of non-linear distortion, which for the WR75/62 model is much lower at lower frequencies than for the WR40/62. The remaining acoustic parameters for both devices are identical: see the user manual (1, 2, 3).
Electron tubes used:
- EF86 preamplifiers, 2 pcs.,
- ECC85 phase inverter system, 1 pc.,
- EL34 power tubes, 2 pcs. (WR40/62), 4 pcs. (WR75/62),
- EM84 electronic drive indicator, 1 pc.
Amplifier Rating
The WR40/62 and WR75/62 - "Ampli40 and 75" broadcast amplifiers are mid-range acoustic amplifiers with typical electro-acoustic parameters required of devices intended for transmitting spoken-music broadcasts and for sound reinforcement of rooms.
Very solid assembly with the use of reliable components ensured that both models had an exceptionally low failure rate and astonishing parameter stability: most of the devices found are still fully functional despite the fact that more than thirty years have passed since their production was discontinued!
An identical - as in the previous WR40 model - "shortcoming" of the amplifier was the use of a metal chassis for grounding and - instead of symmetrization - one-sided grounding of the tube filament windings!
The effect was the formation of local "ground loops" and thus annoying mains hum - the amplifiers buzzed even with the gain potentiometers turned down to the minimum.
The sound of the amplifiers can be described as glassy-clean, undistorted and very dynamic, but definitely hard ("pentode") - unfortunately disturbed by clearly audible mains hum. A characteristic feature of both models is the minimal level of "white" noise coming from the input circuits - barely audible even with the microphone input potentiometer turned up to maximum.
In instrumental applications (electric guitars: solo and bass, and organ) the WR40 (75)/62 performed admirably well! The sound was powerful and clear, and at maximum output it allowed for a "crunch" effect!
The advantage of both models was the possibility of continuous operation with full control of the power stage. Like their predecessor WR40 (on 6P3S tubes), these were classic broadcast amplifiers, which were primarily required to ensure intelligibility of transmission even at full power and in poor acoustic conditions.
Comments
- The photos show two amplifiers from my collection. They are fully functional.
- The WR40/62 and WR75/62 amplifiers are completely unsuitable for modification!
The assembly method used makes it virtually impossible to introduce even limited modifications to improve their parameters (e.g. eliminating mains hum by symmetrizing the filament or eliminating the ground loop) - this would practically require significant disassembly of the device. The only solution would be to completely unsolder the electronics and reassemble the device - in accordance with the rules applicable to the construction of this type of amplifiers:
- while maintaining the current electronic system - the parameters of these amplifiers are so good that it is worth undertaking such work,
- expanding the input circuits and tone equalizers to give it the features of a classic guitar amplifier (jazz, classic rock or blues) or microphone amplifier.
Operation and service notes
Since both amplifier models are still widespread and are still eagerly used as instrumental amplifiers, it is worth following a few recommendations that effectively reduce the possibility of failures and significantly extend their service life:
- it is absolutely necessary to adhere to the principle of switching on the anode voltage only after the power tubes have completely warmed up (after approximately 2 minutes from switching on the mains voltage),
- when turning off the amplifier, first turn the switch to the "stand by" position and then set it to the "off" position,
- in case of frequent burnout of the anode fuse, it is absolutely necessary to check the resistance of the "bleeder" R40 (20k/50W) which limits the increase in anode voltage,
- it is unacceptable to use an anode fuse with a value higher than recommended(!!!),
- it is recommended to replace the original germanium rectifier diodes DZG7 with silicon diodes 1N4007 or similar,
- it is worth replacing the two-section electrolytic capacitor C12 + C17 (2x16uF/450V) with a capacitor 2x 50uF/450V with the following addition: the C17 capacitor section should be supplemented with an additional 100uF/350V capacitor connected in series. This will avoid a serious failure of the device in the event of a break in the "bleeder".
The capacitor series replacing the previous C17 should be bypassed with a series of 220 - 240k/1W resistors - according to the rules for high-voltage anode voltage filters. This additional (well isolated from ground!!!) "upper" capacitor should be mounted in the unused hole in the chassis - as in the pictures.
- used EL34 tubes should be replaced with paired (for WR40/62) or quadrupled (for WR75/62). At least they should be tubes from the same production series (applies to NOS tubes manufactured by RFT or TESLA) – which will ensure their sufficient parameter compatibility.
- the ECC85 tube in the phase inverter should be selected with respect to the symmetry of both triode systems, but it is beneficial to replace it with the ECC83 or ECC81 tube, which is better in this system (it requires a change in the filament circuit!).
[page updated on April 18, 2005.]
(prepared by: Andrzej Żółtowski „ZoltAn”, Gliwice,
A copy of the work originally placed in the Fonar Gallery run by Grzegorz Makarewicz