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Developing WAD 300B amplifier - page 2

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Developing WAD 300B amplifier
page 2
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Theory of operation

The signal input reaches V1 grid through stopper resistor R2, grid leak R1 setting input Z at 100k Ohms. The first stage applies gain, developing the amplified signal across anode resistor R5. This is coupled through the grid of the 5687 by C3 which passes a.c. but blocks d.c. from the H.T. line. This is a fairly critical component. Although paper-in-oil capacitor construction gives good sound quality, these capacitors can leak. Were this to happen, it would bias the grid of V2 positively and possibly burn out the phase splitter transformer, so C3 has been made a reliable, high voltage Solen audio grade polypropylene capacitor. 


 300b-sept94-small

 


The 5687 driver valve applies gain and develops the audio signal across the primary of the phase-splitter transformer (T1). This supplies signals in anti-phase to the two 3008 output valves (V3,4), so one pushes whilst the other pulls through respective sections of the output transformer primary (T2). 

 
The standing current passing through the output valves develops 95volts across cathode resistors R12,13 which effectively biases the grids negative by this amount, since they are held to ground at d.c. by winding resistance of the driver transformer. This system is known as auto-bias. It has the advantage of automatic compensation of valve characteristics, so valve matching and adjustment aren't necessary, although matched pairs of valves are still recommended. It also gives that sweet, easy sound valves possess. Fixed bias gives more power, but a harder sound. 

 
Since with directly heated valves the heating filament is an electrode carrying 5V a.c. through it, to avoid hum the audio signal is applied in balanced fashion by the resistor networks R14,15,16,17,18, adjustment on one valve allowing any imbalance to be cancelled out by hum cancelling potentiometer R14. 

 
Feedback is applied through R10,C8 from the output back to the input, by developing part of the output voltage across R4 to oppose (i.e. negative feedback) the signal through V1. R1O determines the amount of feedback and C8 the phase shift of high frequencies to avoid instability. The values chosen give a good stability margin and clean square wave response; we don't recommend they are changed. 

 
Feedback has been made available to lower output impedance and improve speaker matching when speakers of strongly varying impedance are used (this is a speaker problem, not an amplifier problem). It isn't used to "improve" the amplifier in any way and, indeed, it doesn't do so. It can be switched off by a switch on the rear panel. 

 
Sensitive valve amplifiers with high input impedance suffer from hum/induction loops more than solid state designs, meaning more care is required in certain respects. 

 
An earth lift resistor (R11) prevents hum occurring (from a hum loop) when other items such as a preamplifier are also earthed and have their metalwork connected to 300B through signal cable screens. Input signal cables (L&R) must be run together alongside their earths too, so twin screened cables should be used.

 
The power supply uses high voltage diodes in bridge rectifier form feeding the smoothing capacitor C9,10 in series.They provide 160uF at  800V giving plenty of safety margin in an H.T. line that runs at 565V. The centre tap ensures that at no time during charge or discharge  will the full H.T voltage appear across either capacitor. The GZ37 series diode acts as a slow start up device, prolonging the life of the relatively expensive 3008 valves by running up H.T. after their heaters. The reservoir capacitors C11,12 are high voltage polypropylene types that hold the line to ground at all a.c. frequencies are specially designed and manufactured for this amplifier. Chokes L1,2 provide smoothing and decoupling of the H.T. lines. 

 
The 4 Ohm/8 Ohm output transformer has been carefully load matched for minimum distortion and to maintain matching (even though triodes are very load tolerant). The 16 Ohm tap is for monoblock working. It allows the channels to be paralleled, increasing output from 28watts per channel to 60watts from one channel. 

 
Separate 100/110/120volt and 220/240volt mains transformers are available for efficiency. For anyone wanting to build this amplifier in its entirety we offer a transformer set comprising mains transformer, two output transformers, two driver transformers and two chokes. We do not sell these items individually.

 

A full kit is also available with welded steel chassis having a special black 'powder coat' finish that is smooth and durable. All parts are provided, including bottom plate, safety gloves and mains test screwdriver. A top cover is not available and since 300B valves run hot and can burn, this amplifier may not be suitable where there are young children or pets. Since valve choice at this level is often personal, the kit comes without valves.  We can supply a set of valves however. All the valves used are commonly available and are likely to remain so. 

 
HI-FI WORLD SEPTEMBER 1994




 

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