WAD 300B amplifier - Part 2

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WAD 300B amplifier
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Part 2
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300B Valve Amplifier Part II, DIY Supplement No5, October 1993 issue.

The final part of the 300B valve amplifier project contains finishing details. A transformer set is now available. Noel Keywood explains.


So here's Part II of our 300B amplifier project - and thank heavens for Part II! A second part gives us the opportunity to sort out errors or omissions arising in the first part, as well as time to finalise component choice, transformer specification and what have you. For the transformers are hand wound in prototype form first, tested in the amplifier to see if they meet expectations, then machine wound to specification by our supplier in second prototype form and re-tested to ensure that production items will perform at least as well. All this takes time of course, which the time period up to Part II gives us. 

Of errors in the original published circuit there were thankfully few of consequence and as always, eagle eyed readers did a useful job of spotting them. An obvious anomaly - in retrospect - was my incomplete eradication of a SY3 rectifier valve which appeared in the basic circuit on p24 of Supplement No4, but not in the more detailed power supply circuit (drawn later) on p23. This was in an early prototype, but the appearance of blue sparks reminded us that its ratings weren't adequate for the task. The GZ34 has a cathode too, which wasn't shown, but since this is connected to pin 2, together with one side of the heater, the circuit remains operationally correct. 

One GZ34 is used per channel, a single shared H.T. winding feeding them. After rectification, the pi section low pass hum filter, one per channel, uses high voltage 'motor nun' 25µF polypropylene capacitors in conjunction with compact 5H chokes, to effectively suppress hum and its harmonics. These capacitors have a very high performance and are safe for DIY, since they discharge completely on power down. 

Surge limiting resistors should be fitted for safety, although we have had no problems without them when using GZ34s and Tim insists they are not essential with the low value of C 12 used, made possible by the inclusion of chokes. A value of 175 ohms minimum is recommended for this rectifier, but since the secondary winding of our mains transformer contributes around 50 ohms per arm, a 100 ohm, 3W resistor in series with each anode should be satisfactory if constructors want to fit them. The chokes have a low d.c. resistance, so they contribute little to switch-on/off surge suppression. 

The mains transformer is something of a monster, since it needs no fewer than six 5V windings, one 6.3V winding and a centre tapped secondary. To maintain efficiency and reduce the number of leadouts, in order to lessen the likelihood of connection errors, either I I 0/ I 20V or 220/240V primaries will be made available by us. Twin tapped primaries are not the most efficient we were warned Our transformers are super-efficient. to nun cool and quiet (5% regulation). We didn't like the idea of compromising this property. 

Anyone attempting to put together their own power supply will need a transformer with a 490-0-490V secondary. The 5V windings must be isolated; you cannot use one high current winding to feed all six 5V valve heaters. 

One other small modification has been to raise the voltage rating of the cathode capacitors (C8, 9) to 250V. Otherwise, the circuit published in Supplement No4, accompanying our August issue, can be used as published. 



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