Amplifiers WR40/62 and WR75/62 (Ampli 40 i Ampli 75)
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- Published: Tuesday, 09 January 2018 09:49
- Written by Grzegorz Makarewicz
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In 1963, the State Teletransmission Company in Warsaw started the production of amplifiers marked as WR40/62 (Ampli 40) and WR75/62 (Ampli 75). Very similar to much earlier model WR40 in fact they were improved and completely redesigned acoustic devices. Like their predecessor, amplifiers were intended to create a small broadcasting networks and to use in schools, small factories, large sacred halls, sports stadiums, train stations and events held in the open air.
Both versions of the amplifier are placed in aesthetic, painted with gray hammer paint, perforated steel housings. Visually, there is no difference between them..
Electronic components are mounted on a rigid and stable box-chassis made of cadmium-coated galvanized sheet-steel.
From the top, the chassis is divided by steel sheet, separating visible through the mesh of the case and give off a lot of heat electron from and transformers and electrolytic capacitors.
The EM84 magic eye bracket is screwed on to the separation plate.
At the top of the chassis, preamplifier and phase inverter tube sockets, the supply electrolytic capacitors and inductive components: two transformers - output and mains and large choke are mounted.
Inside the chassis (shielded from the bottom by the perforated plate) electronic elements and wiring are located.
In front of the chassis there are four potentiometers: the level of input signals (separate for each track), and joint tone equalizers and rotary packet switch acting as a power switch and mode switch (Stand by/On).
On the rear panel of the chassis there is "retro" type mains socket. In the left "niche" fuse socket (power and anode), banana speaker sockets and nut to attach the ground wire are located. In the right "niche" there are two DIN input sockets : "Radio/Turntable" and "Microphone".
Inside the chassis, there are two vertical bakelite plates with riveted terminals, On the first plate, resistors and capacitors wired to the tube sockets are located. On the second plate we cen see rectifier diodes. Wires are conducted in the form of bundles.
The electronic system
Output stages are designed as a medium voltage push-pull class B circuits, realized in two (WR40/62) or four (WR75/62) EL34 pentodes loaded with a output transformer - the same (!) for both versions of the amplifier. Middle column cross-section of the core of the transformer is approximately 20cm2.
The plate with three pairs of jumper switchable slots are attached to the transformer. Using jumpers it is possible to select different output voltage ranges (30V for 8ohms - 60 - 120 V), depending on the load value.
Power tubes grids are controlled by the classical phase inverter(the ECC85 tube) providing a broad and uniform frequency response and the correct symmetry of the signals controlling EL34 grids.
Input stages are made of the EF86 tubes. They are simple load resistance amplifiers, connected in series. The first pentode forms a high sensitivity microphone preamplifier. The applied circuit ensures a minimum noise level. The load of the second stage is a simple tone equalizer, performing at the same time the function of the grid resistor of first triode of the phase inverter stage.
Complex anode power supply is composed of three series connected germanium (Ge) power rectifier bridges powered by three separate low voltage (3 x 160V) transformer winding sections. It provides 420 V output to supply the anodes of output tubes and other circuitry.
The output of the rectifier is loaded with a high power wire resistor (Bleeder) 20k/50W - limiting the growth of anode voltage when the amplifier is powered on (heaters of the output tubes are cold), and discharging capacitors when the amplifier is powered off.
Negative voltage (bias) polarizing the grids of power tubes is supplied by a single germanium diode connected with a separate transformer winding. It is set by the manufacturer and can not be adjusted.
Preliminary filtering of anode voltage is realized by the choke, which is connected directly to the output of "top" rectifier bridge.
Such a system very effectively filters out the anode voltage ripple, while allowing a significant reduction of vthe capacity of the first high-voltage electrolytic capacitor.
Power and preamplifier's tubes filaments are powered by voltages taken from two separate windings of 6.3 V.
Connections are made with colored wires grouped in not very aesthetic bundles. The wiring itself is made using "point to point" method.
Basic differences between amplifiers
Amplifiers WR75/62 and WR40/62 differ in the amount of power tubes and other power transformers and chokes - and therefore also the weight.
The difference in parameters comes down to the power output (40 and 75W) and the level of distortion that for the model WR75/62 are much smaller at lower frequencies than for WR40/62. Other electroacoustic parameters for the two amplifiers are identical: see manual (1, 2, 3).
- EF86 input amplifiers, 2 pcs,
- ECC85 phase inverter, 1 piece.,
- EL34 loutput tubes, 2 pcs. (WR40/62), 4 pcs. (WR75/62),
- EM84 magic eye, 1 piece.
WR40/62 and WR75/62 - "Ampli40 and 75" broadcast amplifiers are mid-range acoustic amplifiers with typical electroacoustic parameters required for the transmission of word-music programs.
Reliable assembly of components using provides an extremely low failure rate and a remarkable stability of the parameters: the majority of existing units is still fully functional despite the lapse of more than thirty years after the cessation of production!
Same - as in the previous model WR40 - defects were: the use of a metal chassis to "ground distribution" and lack of filament symmetrization.
The result was the formation of local "ground loops" and so annoying mains hum, audible even when volume potentiometers are set to a minimum gain.
Subjectively amplifiers sound can be described as glassy-clear, undistorted and very dynamic, but definitely hard ("pentode - type") - unfortunately disrupted clearly audible hum. A distinctive feature of both models is the minimum level of "white" noise from the input circuits - barely audible even at the maximum position of the potentiometer assigned to microphone input.
W zastosowaniach instrumentalnych (gitary elektryczne: solowa i basowa oraz organy) WR40 (75)/62 spisywał się nad podziw dobrze!
In the musical instruments applications (electric guitar: solo and bass and organ) WR40 (75) / 62 amplifiers behaved surprisingly well! The sound was powerful and clear, with a maximum gain settings allow the power to get the effect of "clipping" (crunch)! The advantage of both models were ability to work continuously with full output power.
Just like their predecessor WR40 amplifier (6P3S output tubes) described devices are classic broadcast amplifiers, from which most of all was required the transmission clarity even at full power and bad acoustic conditions.
- Photos show two amplifiers from my collection. They are fully functional.
- WR75/62 WR40/62 amplifiers and wholly unsuited for modification!
The method of elements installation makes impossible even the slightest modifications, for example to improve their performance (elimination of hum through filament symmetrization or ground loop elimination) - it would practically need to significantly dismantle the device.
The only solution would be to completely disassembly and then assembly electronic parts again - according to the rules applied for the construction of this type of amplifiers:
- maintaining the existing electronic system - the parameters of these amps are so good that it is worth to take such a job,
- expanding the input circuits and tone correction, in order to give it the characteristics of a classic guitar amp (jazz, classic rock and blues), or microphone amp.
Operational and Service Notes
Since the two amplifuer models are used as instrumentation amplifiers it is worth to follow a few guidelines to effectively reduce the possibility of failure and significantly prolonged their life:
- switch on the anode voltage only after complete warming of power tubes (after about 2 minutes delay),
- when powering off the amplifier, first turn the amp off switch to "stand by" and then set it to "off" position,
- in the case of repeated fuse blowing, check "bleeder" R40 (20k/50W) resistance,
- it is unacceptable to use anode fuse with a rating greater than recommended (!),
- it is recommended to replace original DZG7 germanium diodes by silicon rectifier diodes 1N4007 or similar,
- consider replacing electrolytic capacitor C12 + C17 (2x16uF/450V) by another capacitor 2x 50uF/450V supplemented as follows: C17 capacitor connect in series with an additional capacitor 100uF/350V. This is to avoid a problem in the event of a break in the "bleeder" resistor.
Two capacitors replacing C17 should be bypassed by 220 - 240k/1W resistors - according to the rules governing the high-voltage anode filters. This additional (well isolated from the ground!) "upper" capacitor must fit into the unused hole in the chassis - like the photos below.
- used or faulty EL34 tubes should be exchanged by matched pair (for WR40/62) or quartet (for WR75/62), or at least by tubes from the same manufacturer - which will provide them with sufficient parameters matching.
- ECC85 tube used as a phase inverter should be selected in terms of the symmetry of the two internal triode systems. It is possible to replace it with a better tube, for example ECC83 or ECC81 (requires a change in the circuit filament!).
[Last updated in 18.04.2005]