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Developing WAD 300B amplifier

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Developing WAD 300B amplifier
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DEVELOPING WORLD AUDIO DESIGN 300B VALVE AMPLIFIER 

(See WAD 300B AMPLIFIER for original design concept etc)

 


 

This is an unaltered copy of the original 1994 article, re-published for the information it contains. We do not now supply kits, or parts, and cannot help with supply or repair.

See SP Wound Components (http://www.spwoundcomponents.co.uk/) for transformers and the World Design forum for information. SP Wound tell us they still (2011) supply transformer sets. Price is  currently £495 per set  (2 outputs, 1 mains, 2 inter-stage, 2 chokes)  inc VAT and shipping in the UK.

 


 

300b-amp

from Hi-Fi World, September 1994 issue (not the Supplement).


We didn't know how our original DIY 300B valve amplifier design would be received. In the event we were swamped by interest. Rare and esoteric this amplifier may be, unpopular it was not, especially in the Far East and the States. The 300B valve has more of a reputation than we knew. 

 
Our first prototype worked well, but production engineering a kit is another matter altogether. Long term reliability and safety have to be taken into account, which brings in a host of extra considerations. It took us roughly six months to overcome some of the most peculiar and intractable problems I have ever come across. Here's a brief history explaining the delays, why the design has changed and what our 300B kit, now in stock, has to offer. 

 
I thought the original driver transformer screening cans were Russian surplus tank parts. Made from steel 3mm thick, I'd never seen such construction. "Why does 300B have to survive the next nuclear war?" I asked their designer facetiously. "Good driver transformers are very difficult to design" I was told. "Every so-and-so will try to copy mine so I want you to pot them in cans that are impossible to saw open. Put steel ball bearings in the potting compound so that if they get through, the balls will break the blade". It sounded like something from Raiders of the Lost Ark. 

 
Andy was down on his luck. "Can you design driver transformers?" I asked. "Yes. I've done quite a few". ''Then I've got a job for you". 

 
All designers have their own ideas and Andy Grove had his. For the production version he felt things could, with benefit, be done differently. The ECC83 front-end went out, replaced by a 6072 which is designed for low noise and microphony. "It's similar, but it has a clearer sound". 

 
The ECC82 driver valve was replaced by a 5687 which dissipates 4watts per channel. "It's dead linear, needs less drive and has a low output impedance". 

 
The all-important driver transformer posed some interesting and difficult problems. Getting a symmetrical square wave out of the thing was the crucial test and finally, with a particular winding arrangement, we got it, together with good audio bandwidth - without resorting to feedback. The use of driver transformers is a major difference between our 300B amplifier and most others. 
We got the new prototype up and running, but with large amounts of heat being dissipated in a relatively compact chassis, running tests showed that after 12 hours or so the chassis was getting hot. The chassis was redesigned. 

 
The new chassis was fine, but during soak tests a GZ34 rectifier started arcing internally, making some fearful cracking sounds; then another went. Whilst original Mullard GZ34s, rated at 550V, had worked perfectly, Chinese and Russian versions, we found in these tests, had a short life of just weeks. The original Mullard GZ34 was a very precisely made rectifier with small clearances, in order to make it compact, minimise losses and reduce heat production. Modem versions do not match it, so we chose instead a bridge rectifier for reliability and to cut electrical losses and heat production. A GZ37 was employed as a slow start series diode. From that moment on, our power supply problems vanished. There are plenty of original GZ37s around, beautifully made to military spec. and priced at just £5 or so. 


 300b-layout-underside

 
The first smoothing capacitors needed to be increased in value to reduce ripple on the H.T. line, so we resorted to electrolytics, retaining polypropylenes for use after the choke, where they were needed most to pull the H.T. line down to ground with respect to a.c. The polyprops are manufactured to our own specification using an audio grade metallised film. 

 
Finally, we ran long listening tests on the new design to optimise subjective component balance. Here we found, for example, that although we all liked the sound of modern paper-and-oil capacitors, Solen audio grade polyprops got close but, by not being leaky, were potentially more reliable. Reliability and safety are both very important of course. Experimenters can substitute components of their choice at a later date. 

 
The re-design and re-prototyping delayed 300B for six months. However, the final design is now so linear that feedback has been removed and made optional - it can be switched in and out on the back panel. I prefer it switched out, finding this gives greatest stage depth and most 'freedom' in the sound. But with loudspeakers having a strongly varying impedance characteristic it is best switched in, to reduce the amplifier's output impedance from around 6 Ohms to 2 Ohms or so. 

 
This amplifier is a fine match for electrostatic loudspeakers, delivering a good 28watts per channel, or 60watts in monoblock form. With 280mV input sensitivity (feedback in) 300B needs no preamp, just a passive volume control, except with LP of course. 


 300b-underside

 
Although built like the Forth Bridge, possessing no fewer than seven transformers bolted down to a rigid welded steel chassis, this sort of amplifier is simple and elegant in its circuit topology. There are just four active devices and no feedback - how much simpler can an amplifier get? It capitalises on the innate strengths of the 300B power triode to give a super smooth sound with superb sound stage dimensioning, offering a standard of reproduction not really available from today's overly complex, feedback-reliant solid state circuits. 

 


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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