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WAD 300B amplifier

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WAD 300B amplifier
page 2
Part 2
page 4
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WORLD AUDIO DESIGN 300B VALVE AMPLIFIER


Part 1 and Part 2 are reproduced here, from August 1993 DIY Supplement No4

and October 1993 DIY Supplement No5.


See 'Developing 300B' for the updated final design.


 


 

This is an unaltered copy of the original 1993 article, re-published for the information it contains, but with better pictures We do not now supply kits, or parts, and cannot help with supply or repair. You can however still buy parts to build one and they are in build today. We sold hundreds of these amplifiers around the world and they are still difficult to better as a platform for the 300B and a pure triode sound transistor amplifiers are unable to match.


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 – and are supplying – 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

300B Valve Amplifier, Part 1, DIY Supplement No4, August 1993 issue.

 
The 300B is one of audio's most linear valves.

Tim de Paravicini designs; Noel Keywood provides an introduction. 


This amplifier is our first step into original and specialised design with valves. It uses the highly regarded 300B valve, a low distortion triode designed for high power audio work, now being re-manufactured by Golden Dragon in China. But the circuit is original, designed by Tim de Paravicini to meet various modem criteria. So this is no re-run of an old design. It is a brand new valve amplifier in all respects and has been specified to satisfy current thinking on the subject of audio reproduction. And before I go any further, let me tell you the most important thing of all - it sounds absolutely gorgeous! 


What were our criteria? Not to produce a replica of any sort, or even an acknowledgement of the past (although to some extent it is that), but to produce an amplifier that can offer the very best sound quality by modem standards. The notions of simplicity and purity were paramount. It is a minimalist design in some respects, with as few amplifying devices as possible, of basic but well engineered circuits, with superb component quality and possessing the best measured performance possible within these criteria. For those not 'au fait' with valve amps and audio arguments (!) the triode valve is the most linear of all and this is why we chose to use it, come hell or high voltage -  just look at that mains' transformer!). 


Yes, we do fret about distortion, frequency response and all those things, but we also measure and listen in-house on the magazine, in order to be able to assess their impact. Our view is that not all measured distortions are so bad, or even audible, but that there are insidious, but significant problems that blight modem solid state amplifiers. Trouble is, at present there are few really good valve amplifiers around; it's are-emergent technology and too many circuits in use today are old, and often poor. 


When engineers were struggling to design valve amplifiers in the thirties, they looked towards achieving certain basic measured goals, the ones we use today, like low distortion. The 'perfect' amplifier did not then exist, as it does today, so the validity of these goals couldn't be questioned. Naturally enough, engineers tacitly assumed that perfect measured specs gave a perfect amplifier with, of course, perfect sound quality. 


I first measured and heard a 'perfect' amplifier somewhere around 1974 and I've been measuring and listening to them ever since. A 'perfect' solid state amplifier sounds little different from an imperfect one; sometimes it can sound worse. For example, I pitied Hitachi with their HMA7600 MOSFET amplifier; it had the most astonishing measured performance, yet nobody liked it much. The Japanese design engineers must have cried! 


These days, we have the benefit of such experience. We can compare a 'perfect' solid state amplifier - in fact a range of them - against a modem valve amplifier that is, for all intents and purposes, inconsequentially imperfect and listen to differences. Many people now acknowledge that the latter is audibly superior, even if the reasons remain obscure and open to speculation. The coarseness, apparent modulation noise and distortion (harshness) of solid state may well be related to the fundamental non-linearity of a transistors p-n junction; if we can 'hear' cables then we can also hear this underlying phenomenon I suspect. 


SIMPLICITY

Whatever, our 300B amplifier offers a converse design philosophy to that of transistor amps. It uses thermionic amplification; it uses very few active devices; it uses innately simple yet linear amplifying devices - triodes - and it uses just a little feedback. What feedback it does use can be varied or removed altogether - it makes little difference. I'm not anti-feedback, nor is Tim de Paravicini who designed this amplifier, but we both feel that ideally it shouldn't be needed and that an amplifier for music reproduction must be acceptably linear without it. 


If say, an amplifier produced 10% distortion at ordinary music levels, we wouldn't add an amplifying stage to get more gain then apply feedback around the whole caboodle to get the distortion down - we'd junk the design and start again! It is because the 300B is one of the few high power triodes designed for low distortion audio amplification that it is so favoured for purist amplifiers like this one - it can be used without heavy feedback. There is a small amount of feedback used in this design, mainly to keep the high voltage drive stage very linear, but the 300B output stage has none.

 
There is one other factor I should mention that distinguishes modem valve amplifiers from old ones: the use of better grade components. In particular, the transformers we have used are of a very high standard, designed by Tim of course. They have the best thin-lamination, grain orientated silicon steel cores, without comer fixing holes which produce flux concentrations. The windings are specially laid down for optimum audio performance. Even the mains transformer (our design) is efficient and runs cool because of its generous core. 


There are some limitations to a purist design like this, ones common to triode valve amps without feedback. Firstly, this is a self-bias pure Class A amplifier, so power output is limited, in this case to 28watts; reasonably efficient eight ohm speakers need to be used. All the same, we found the amplifier went loud without difficulty; in use it didn't seem limited in the way that 7watts (say) designs do.

 
What we did suffer initially though was a dull sound caused by loudspeaker mismatching (silly us - we were too eager to listen to the prototype!). Unlike solid state amplifiers, a triode amp. without feedback has a high-ish output impedance of many ohms. Connect it up to a loudspeaker that possesses strongly falling impedance at high frequencies and it may sound dull. Our own three-way loudspeaker, also featured in this supplement, worked wonderfully with it, and so did Heybrook Quartets. In tnuth, triodes are less sensitive to load matching than pentodes, but the latter generally have feedback applied, which alleviates the difficulty. Pondering on this, we decided that overall feedback should be applicable to reduce output impedance in order to overcome this problem, should it arise. You'll see an optional overall-feedback connection in the circuit diagram, from output back to the top of R3 in the cathode circuit. More about this in Part II. 


There was, long ago, a debate on the sound of triodes versus pentodes where such differences were at the time noted. They still exist today, but the solution zisn't to raise gain and apply feedback by using pentodes, it is to build a better loudspeaker! Triodes are intrinsically more linear than pentodes (they possess more degeneration) and our choice for this amplifier was firmly for the best triode available, the 3008. It is a directly heated valve (no cathode) designed around 1932. Although difficult to make, it has a great reputation. Western Electric of the U.S.A. are most commonly associated with it and undoubtedly made some of the finest versions, although it seems that S.T.C. (Standard Telephones and Cables) of the U.K. may have designed the original around 1932, naming it the 4300A.


Many are the supposed problems of zero-feedback (overall feedback) triode amps, but like the impedance difficulty I've just talked about, they are either illusory or soluble. The other hoary old chestnut, that of electrical damping and its affect upon bass quality, happens to be in the former category. I have spent weeks listening to this amplifier, swapping between it and various top quality solid state units and in all cases the triode amplifier had tighter and deeper bass. In reality, the series bass inductor within the crossover of a loudspeaker has a resistance of many ohms, high enough to cancel out any electrical damping effect. Add to this the predominance of magnetic and acoustic damping on a speaker cone and you realise that in truth amplifiers do not control loudspeakers to the degree commonly supposed. So the high output impedance of a zero-feedback triode amplifier is not a problem in theory or practice, as experience proves. 


This amplifier was designed to be easy to build, possessing very few parts. That's why Tim decided to use high performance phase splitter/driver transformers. They eliminate the need for an entire valve stage, plus the imbalance problems intrinsic to such stages. Also, 300Bs are directly heated (there's no cathode) and need 5volts a.c., so a dedicated mains transformer is required. The circuit is pure Class A and self-bias ensures that if anything fails, the valves don't burn themselves out. Tim feels the potential safety and reliability of self-bias more than compensates for its limiting effect on power output. 

 

psu

 

In the power supply two GZ34 fullwave valve rectifiers have been used, for slow thump-free turn-on, to extend valve life by applying H.T. only after the filaments have lit, and to make experimenting easier and safer. Removing a valve kills one channel, effectively disconnecting it from the power supply. Tim also insists, like others, that valve rectifiers don't switch like diodes and therefore give a cleaner supply and better sound. 

Similarly, for reasons of quality, we have used a smoothing choke in each of the independent power supplies. It gives less hum and noise, and better regulation. It also allows high performance polypropylene power supply capacitors (C II & 12) to be used - a very unusual feature in any amplifier. 


So this amplifier, because it makes no concession to convenience and parts availability, as It could not, being designed for very best sound quality, is unlike modern pentode based amplifiers, but it is not difficult to build. Tim has designed the transformers, since these days such specialised items aren't available off the shelf. But if the 300B valve is to be used, this is a necessity. The transformers will be made available separately, for all those who feel they can do the rest themselves. All other parts are available from Electromail and valve suppliers like P.M. Components, Billingtons, Chelmer, Collomore, Cricklewood Electronics, G.T. Audio. (See Kit Suppliers on Page 8) 


Finally, I must warn you that lethal voltages exist in the circuits and that you should not consider building this amplifier unless you are fluent with handling valve circuits and know what precautions to take. This circuit is offered for those who know what they are doing. 


A kit will be offered in Part II of the project (October issue), which makes life easier. It will be based on a professional 16 gauge steel chassis fully punched and finished. All parts, including transformers will be included. You may be able to get someone like an electrician to build it, but tell him first that in addition to the lethal mains voltages present, the H.T. runs at 550volts!

300b-circuit-no1small

THEORY OF OPERATION
One half of an ECC83 and ECC82 triode act as input stage and driver for the transformer stage on each channel, being shared by the two channels. 
The input ECC83 acts as a low noise voltage amplifier which feeds the ECC82 as a Class A 0.2 watt anode dissipation driver with the driver transfonmer as its load. 
A small amount of feedback is taken from a tertiary winding back to the input stage to provide low distortion (0.1 %). High voltage push-pull drives the two output valves (about 400V peak to peak). 
A pair of 300Bs are used in a push-pull output stage with self bias by means of a cathode (filament) resistor bypassed by a parallel capacitor. A potentiometer is used to balance out hum in each valve (a problem with directly heated triodes). The output transformer matches the output to an eight ohm loudspeaker load from a 3,000 0hm plate-to-plate (anode) resistance. The amplifier is run on 550volts d.c. which is about 450volts across the power valves at 80mA (approx 40watt plate dissipation). 


The power is derived from a reasonably smoothed H.T. using two 25µF polypropylene capacitors and a 5H choke acting as a pi section filter fed by GZ34 full wave rectifier valves from the 450 volt windings on the mains transformers. The mains transformer also has six 5 volt windings (one for each directly heated 300B and GZ34 rectifiers). Finally, a 6.3volt winding is for the input valves. 
Total power consumption is about 220 watts. 
Tim de Paravicini 


SAFETYWARNING 
Lethal voltages exist in this amplifier. We do not suggest you attempt to build it unless you are conversant with valve circuits and safety precautions. 
For safety, never hold earthed metalwork when testing. Make sure your body is isolated by rubber soled shoes. You should posses a voltmeter capable of reading up to 1000volts. 
The final power amplifier should have a protective underplate and a wire mesh top cover, since the 300B valves run very hot and will burn. 


SOUND QUALITY
Valve amps have always had the affection of audiophiles, the 300B is no exception. My first acquaintance with the 300B came from listening through Heybrook Quartet speakers and a Sugden CD player. An Audiolab 8000 pre-amp and monoblocks were at hand to represent the higher end of transistor amps. 


What struck me first was a sense of delicate spaciousness, and superb articulation. With Paul Simon's The Boy in the Bubble', bass lines progressed with fluency and speed, totally unhampered by bass boom. Imaging was excellent with great depth as well as a sound stage which truly disguised the speakers positioning and height. With 'Homeless', voices seemed to come from beyond the usual sound stage between the speakers. 
Mary Black provided the female vocals. 


On 'Katie' a natural subtlety prevailed that allowed her voice to breathe freely. There was a slight reediness to the upper midrange vocals, however, as I later discovered this was due more to the speakers sonic footprint than that of the amplifier. 'Columbus' went on to show further qualities of the 300B; beguiling, smooth, and a lack of sibilance that is denied to lesser transistor amps. Separation was accomplished with the music being presented rather than hurled at the listener. One was able to clearly distinguish between the divergent guitars of Mike Oldfield. It was possible to pick out a particular guitar and follow its rhythm, yet the piece remained coherent throughout.

 

MEASURED PERFORMANCE 

Power output measures 28watts per channel before clipping (1.5% distortion). That's enough to make all but the most insensitive loudspeakers go loud, including our own three-way DIY loudspeaker featured in this issue. We found the two went together surprisingly well. 


Frequency response extends right down to 5Hz, and up to 36kHz. The output transformers have very good cores, allowing them to deliver full power from 20Hz up to 22kHz (-I dB), an excellent full power bandwidth figure for any valve amplifier, let alone a specialised no-feedback design like this. 
Sensitivity is high at 250mV, meaning the amplifier can be used with a passive pre-amp and will match all sources, even those with a low output. These days the lowest outputs from tuner or cassette are usually no less than 300mV or so. 


Hiss was very low at -I 00dB. However, direct heated triodes have a little hum, although if well suppressed it will be just about inaudible, as it is on this amplifier. Measured hum level was around -72dB on our prototype, but this low level can be reduced still further by careful balancing of the hum bucking potentiometers on the 300Bs, so hum is not a problem. 


Distortion rises Iinearly with output level on a no-feedback amplifier such as this. Below I volt output distortion is lower than 0.1 %. At 3V (I watt approx.) it measures 0.2% at I kHz and 6kHz, and 0.4% at 60Hz; the analysis (I kHz) shows second harmonic dominates. At I 0 volts output (12watts) levels hover around 0.6% and at full output of 15volts, or 28watts, as defined by visual clipping on a 'scope, it measures 1.5%. There's no great increase in distortion at low or high frequencies, such are the abilities of the output transformers. NK 

 
Power                               28watts 
Frequency response      5Hz-36kHz
Separation                        54dB 
Hiss                                  -IOOdB  

Hum                                   -72dB 

Distortion                          0.2%

Sensitivity                         250mV

 


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. 


layout


BUILDING THE 300B 
The 300B amplifier is available as a kit, but we will be offering a transformer set for those intrepid souls who would like to do the rest themselves. It comprises a mains transformer, two output transformers, two chokes (mounted underneath the chassis), and two inter-stage phase-splitter transformers, which come encapsulated in a metal case to protect their secrets; Tim says they are very special. 


The set of seven transformers weighs no less than 16kgs (35lbs), so be aware of carriage charges, as well as the need for a strong chassis. In addition to a good knowledge of electronics, you'll need civil engineering skills to build this monster. No wonder the feckless fled to transistors all those years ago! 


Ideally, a strong steel chassis should be used, with a protective bottom plate, to prevent accidental contact with lethal voltages (550V H.T.). The valves can cause burns, so a well ventilated top cover is sensible too. These items are part of the kit, but a cover is an optional extra.

 
Anyone attempting to build this amplifier from the transformer set I imagine will have a good electronics knowledge, plus some practical experience with valves. If you don't have this, then for safety's sake, I suggest you take up knitting instead. This is a highly specialised amplifier and although it is easy to build, having few components (not including transformers!) it has a lethal 550V H.T. line inside, which makes testing or fault finding potentially hazardous. A skilled electrician or radio/TV repair man should be able to handle the job though. 


PARTS 
There are some unusual parts in this new design. The power supply uses high quality 'motor run' 25µF polypropylene smoothing and reservoir capacitors, instead of polarised electrolytics. They are rated at 630V d.c., do not retain a charge after switch off and are neat. The slightly cheaper alternative is a pair of 47µF electrolytics wired in series, with d.c. balancing resistors across them, and a I µF or so bypass polypropylene to keep their impedance low at high frequencies. That amounts to quite a few components just to do one job, so the 'motor run' caps., from Electromail, seem an elegant and sensible solution. They keep high frequency noise off the power lines too, contributing to sound quality. 

psu-2


The chassis layout is very important; spacings and orientations are chosen to minimise hum. We suggest the mains transformer is best placed in the centre of the chassis, either at front or rear. For DIY, at the front makes most sense, since the mains runs across the chassis only once and the input valves can be placed close to the input sockets, whilst also staying away from the mains transformer, which should of course be kept away from all low level circuits and transformers to avoid hum induction. 


If a valve starts to fail and you'd prefer to run away than towards the mains switch, then the properties of the fuse need to be relied upon. The amplifier draws 0.9A from the mains (240V), but switch on/off surges go way above 10A, so a slow blow 1.6A fuse is recommended in the transformer primary circuit. The transformer and rectifier valves handle current well enough to make a secondary (H.T.) fuse unnecessary. 


Put the mains transformer at the rear, unless you don't mind the unusual appearance of a centre-front mounted unit. Put the output transformers either side, spaced away from the mains transformer by at least 50mm to lessen hum induction. The cores must be pointing in different directions too. Similarly, the chokes go under-chassis and away from the mains transformer; put the power supply components in-between. 


The transformer cores should be electrically isolated from the chassis, that's why the screws have fibre washers underneath their heads. Ideally, all transfonmers should be spaced off the chassis by an insulating gasket. The mains transformer is best spaced up above the chassis by 3-5mm, using spacing nuts on each of the comer bolts. This lessens hum induction and allows convective cooling currents to pass freely up past the transformer. It runs very cool, as is to be expected with a well designed amplifier with high quality parts. 


On the subject of cooling, large vent holes should be put into the chassis to ensure heat from the cathode resistors is vented. It is good policy generally to heavily vent an amplifier such as this, for its heat output is substantial. Our own amps have vents around the valves and a spaced-off mains transformer, plus a vented bottom plate and cover of course, to ensure the unit runs cool. Even though the heating effect of the amplifier is noticeable in a room, it is only equivalent to two 100W light bulbs. 


WIRING 
When building the amp, my advice is to wire in the 'services' first. By this I mean power supply, earths, heater leads and what have you. After that I wire the stages in succession, from input through to output, using red cable for all dangerous H.T. lines, black for earth and yellow for signal. Remember to use heavy cable for the heaters, since they draw quite a lot of current. Twist them tight and keep them close to the chassis. 


It is very important to split the earths and run them back to one earthing point on the chassis, by the input sockets; this avoids instability. In particular, make certain the earthy end of the output valves (R 15,16) runs back to the single (star) earth point using heavy cable and without any other connection being made to it. Having suffered instability more times than I prefer to remember and it's a brute to clear - avoiding it should be a priority. In my experience, good earthing and keeping outputs away from inputs results in a very stable unit, one where even grid stopper resistors are often unnecessary. 


To lessen hum pickup from induction loops, run signals alongside their earth return lines. They can often be lightly entwined, but if you do this watch out for the effects of distributed capacitance, or 'stray coupling'. 


Having wired the amp, check and re-check all connections, looking for connection errors and dry joints in the soldering and electrolytic capacitors the wrong way around. Ideally, the amp should be powered up in stages. First run up the power supply alone, disconnected from the H.T. line at C I 1/ 
L 4 junction, using a variac. If the variac hums furiously, you're drawing too much current, so shut down and find out why. Otherwise, power up and watch the H.T. rise with a d.c. voltmeter, making sure it settles at around 575V with no load. 

Then with all power disconnected and the H.T. line disconnected from C I I /L 4, measure its resistance to ground. If this is low or a dead short, there's a fault that must be cleared. Expect to see I MOhm here, but wait for the capacitors to charge. 


If all is well, connect H.T. line to power supply and switch on. Although this is for most people the nail biting bit, very little can in fact go seriously wrong, since the fuse will blow immediately if there's a fault, so there won't be much of a flash or a bang! 


I suggest that the feedback connections are left off until proper working has been verified. If the amp goes unstable when feedback is connected, then the feedback polarity is wrong. Before connecting up the feedback, check all d.c. levels; they should be within 10% of our published values assuming 550V H.T. Be aware that mains voltages fluctuate from place to place and over the length of a day and that this will affect all internal voltages. 


APPLYING OVERALL FEEDBACK 
As I explained in the introduction, the design rationale behind this amplifier was to produce a unit that was basically linear (distortion free) without feedback The 300B valve is prime contender for this role, because it was designed in 1928 to be an inherently linear audio amplifying device. Feedback wasn't used then, so valve manufacturers were obliged to consider this requirement. 


The introduction of feedback freed valve designers from such constraints, allowing them to optimise other properties, like gain. So the innately linear amplifier became a thing of the past, by force of events. That's why the 300B is so popular today. 


But feedback has many attractions, reduction of output impedance being one of them. Without feedback an amplifier such as this has a high output impedance; this one measures 50hms. It would be no problem if loudspeakers had a flat impedance, but most don't. Impedance usually varies wildly and this affects frequency response, the amplifier develops an actual frequency response that mimics the loudspeaker's impedance curve, although far less pronounced. So if, as in most two-way speakers, there's high impedance across the midband, then an amplifier such as this will have a lift in its response here as well. 


All the same, I've used the 300B with Heybrook Quartets and our own large KLS-I and have enjoyed superb results. At the end of the day, the response changes aren't so great as to be obstructive; the properties of the 300B, which I'd describe as a beautifully open and clear sound, sweetly transparent across the midband and treble regions, yet with strong deep bass, come across all the same. 


Overall feedback can be applied to reduce output impedance and lessen 'speaker sensitivity though (see circuit). The amp won't take more than around 
I OdB of overall feedback, with R8 increased to 470 0hms to reduce tertiary feedback, before going unstable. Experiment showed that around 6dB of overall feedback is ideal, set by making R8b 3.6kQ. If more overall feedback is to be applied, R8b set at 2.7k being about maximum, tertiary feedback will need to be kept low by setting R8 high. With no overall feedback, R8 can be reduced to 180 Ohms. 


For best square wave response (minimum ringing) when R8b is set to 3.6k, 2000pF (C6b) needs to be shunted across it for phase compensation. With 6dB of feedback applied, output impedance fell to 2 0hms and distortion halved from 0.2% to 0.09% at I W / I kHz. Frequency response and sensitivity don't alter significantly. 


Readers might like to try balancing tertiary feedback against overall feedback. Note that if the amp goes unstable whenever overall feedback is applied, then the feedback is positive, not negative. Reverse either the primary or secondary connections on the output transformer.

 

FEEDBACK SETTINGS 

 
R8               R8b

Tertiary only                180            none

Overall & tertiary        270            3.6k 

Max. overall+t               470          2.7k

 

OBTAINING THE PARTS 
The 300B valves are commonly available from the various valve suppliers around Britain, most of whom can be found advertising their surprisingly wide and esoteric stocks in this Supplement. This amplifier needs four, which new will cost around £300. Valve bases are available from the same suppliers. 
The transformers are available as a set (7 off) from us, price £430 inclusive of VAT and carriage in the U.K. These are available now. 
The other components are widely available from suppliers around Britain. 
We will be offering a kit of all parts, including a high quality welded steel chassis with top cover and bottom plate, transformers and all parts of best quality, except valves, for £750 inclusive of VAT and carriage. 

300b-parts

SAFETY WARNING 

 Lethal voltages exist in this amplifier. We do not suggest you attempt to build it unless you are conversant with valve circuits and safety precautions. 
For safety, never hold earthed metal· work when testing. Make sure your body is isolated by rubber soled shoes. You should posses a voltmeter capable of reading up to 1000volts. 
The final power amplifier should have a protective underplate and a wire mesh top cover, since the 300B valves run very hot and will bum.

 

END

Go to Developing 300B for the updated final design.

 

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