VINTAGE RADIO

Building a vintage radio "replica" Have you always wanted a 1920s or 1930s lacquer finish of some sort. Second, a glance at the front panel reveals that "cathedral" style radio. They're as scarce as these sets can receive FM transmis- sions as well as AM. In reality, FM hens' teeth these days - or are they? If you didn't get under way in Australia un- can't get an original, what about one of the til well after the era that the "replica" is supposed to represent. many replicas now coming onto the market? However, it's not until you expect the "insides" of such radios that you From time to time, "replicas" of ites but of course, they're not true realise just how far away they are early radio sets appear in catalog ad- replicas. First, the cabinets are noth- from being a true replica of the era. vertisements from various electronics ing like the those from the 20s, 30s Hidden inside the cabinet will be a and electrical retailers. Consoles and and 40s, usually being made from small transistor radio and that's hardly cathedral sets seem to be the favour- cheap ply or particle board with a something that was around in the 1920s or 1930s! So these sets are in no way an accu- rate copy or replica of any early radio. The fact is, there are very few genu- ine 1920s (and not many more 1930s) sets now available on the market. Many collectors will never own ra- dios of this vintage. But there is noth- ing to stop you from building a replica using the components (either origi- nals or reproductions), wiring layouts and construction techniques of the era. The resulting set will look like a brand new 1920s or 1930s radio (not a restored set), although it still won't be authentic. Building a replica requires a lot of work when it comes to sourcing the parts, selecting a representative cir- cuit and planning the layout. You then have to assemble it and get it to work properly. It may also be necessary to vary the original circuitry or layout somewhat, as some parts may just not be available any more. The Rice Neutrodyne I haven't personally built a replica of an early set but a few members of the Vintage Radio Club, of North East Victoria have. In particular, I was most This view shows the impressed with Jim Birtchnell's rep- completed Rice Neutrodyne lica of a 3-valve Rice Neutrodyne with its loudspeaker. (you've heard of Jim and his vintage radios before in this column). The design for the Rice Neutrodyne was originally published in the 7th October, 1927 issue of "Wireless Weekly". That article was quite de- tailed and covered more than four pages. Jim started from scratch, either sourcing or making the parts himself so that his completed replica very closely resembles the original. In the original article, the author began by discussing the Rice neutralising system as seen in an old publica- tion of 1918, some nine years before! This pub- lication discussed the various neutralised that, were used at that time. The problem is that triode valves, when used in tuned radio frequency amplifiers, are very prone to oscillate at the frequency of one of the two tuned circuits around the stage (ie, the grid circuit or plate circuit). This oscillation is due to the capacitance between the grid and the plate of the valve. The completed Rice Neutrodyne replica closely resembles the original receiver This capacitance is actually quite described in "Wireless Weekly" in 1927. small — only 4pF in the case of a 6SN7-GT. However, this value is more realised when the screen grid is Of course, it is also important to than enough for this triode to oscillate earthed to RF signals. Even so, the isolate the input and output circuitry fiercely if used in a tuned plate and 6BA6 can still oscillate in some cir- of an RF stage, to minimise any cou- tuned grid amplifying circuit. In fact, cuits and so the IF stages in quite a pling between them. this circuit is actually used as an os- few HMV Little Nipper sets are neu- cillator in some ! tralised (we'll explain what neutralis- Neutralisation By contrast, a 6BA6 has a grid-to- ing is shortly). This was not done Radio experimenters of the early plate capacity of only .0035pF, which because the stage was inherently un- 1900s and into the 1920s did not have is over 1000 times less than for a 6SN7- stable but to make sure the stage was and pentode valves to amplify GT. However, this low capacity is only unconditionally stable. radio frequency signals, so other ways

Fi: the +circuit for the receiver. ;The neutralism

iliOt#110::1**00*::Ah0 late the IAF valve tuned_' inding 1. This ca actor effectivel cance ed out — or neutralised the grid-to plate capacitance of the valve, thus making the

JANUARY 2000 75 r: lrt This view shows how the completed receiver fits into the home-made cabinet (it slides in from the front). The large socket in the top panel is for the external power supply connections.

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had to be found to stabilise triode RF ever, that is a story for another article doesn't require enormous skill as the amplifiers. Neutralising, in the vari- in the future. circuitry is quite simple and the arti- ous forms that it took, was not always Another common method of ensur- cle included detailed layout instruc- particularly easy to accomplish al- ing stability was to install a variable tions. However, this is not meant to though the results were quite reason- resistor between the grid of the valve detract from the obvious skill Jim used able. and the tuned circuit. The value of in making this set and getting it to this resistor was then adjusted until operate. Rice Neutrodyne the set was stable (a value of around Obtaining the parts to build such a The Rice Neutrodyne was one such 85052 was commonly used by Atwater set is quite another story. Where does circuit that employed neutralising Kent, for example). the builder get 201A valves, or square (Fig.1). Essentially, this involved in- The big problem with this was that section copper wire, UX valve sock- stalling an additional be the gain of the stage was dramatically ets, 5:1 audio interstage transformers tween the plate of the RF valve and reduced. This meant that extra ampli- or the old style vernier dials? How- one end of tuned winding L1. This fying stages were required to make up ever, as can be seen in the photo- capacitor effectively cancelled out — for the low gain of an "un-neutral- graphs, Jim has succeeded in obtain- or "neutralised" — the grid-to-plate ised" RF . It may seem sur- ing suitable parts. capacitance of the valve, thus making prising that not all manufacturers used Jim told me that he imported most the stage stable. neutralised RF amplifiers, as valves at of the bits from America because of The neutralising capacitor was that time were very expensive. They price and availability. However, if one made adjustable in most cases and is didn't because patents on neutralis- is prepared to hunt around, most if adjusted for optimum stability. It ing were held by Hazeltine and Rice not all of the bits can be found (or worked because the RF signal at the and they weren't going to let anyone made) in Australia. bottom end of coil L1 is 180° out of else use this technique without pay- For example, there are several ad- phase with the signal at the other end ing a considerable royalty. vertisers in SILICON CHIP that cater for (ie, on the grid) and so the grid-plate Having talked about the neutral- the vintage radio buff and contacting capacitance was effectively "elimi- ised RF amplifier stage, it can be seen them should bring results. They have nated". that the rest of the radio is quite con- extensive stocks of all sorts of bits and This system works well but having ventional, with a grid and a pieces that are used to restore or build the tuning gang "floating" above earth transformer-coupled audio output receivers. can cause problems. Because the stage. The Rice Neutrodyne is really Members of the Historical Radio aerial, which is connected to the top only a headphone set but it can do a Society of Australia or the New Zea- of L1 via a 100pF capacitor, can vary credible job on strong stations, as Jim land Vintage Radio Society are also in length, the capacitances around the can attest. often able to assist when it comes to circuit can also vary. For this reason, obtaining that special part. And bits the value of the neutralising capacitor Jim's replica and pieces can turn up in all sorts of sometimes required adjustment which Jim built the set virtually as per the other places — garage sales, second- is why a variable type is used. article in "Wireless Weekly", with hand shops, deceased estates and "for By the way, this circuit can be some slight modifications to suit the sale" columns in local newspapers, to slightly modified to make it much connection of the power supply to the name but a few sources. Placing ad- easier to achieve good results. How- set. Building a replica such as this verts in magazines such as SILICON

76 SILICON CHIP CHIP, in local newspapers and on su- However, an enthusiast is more likely permarket notice boards can also help to build an early wireless set from the track down the bits required to make 1920s rather than a 1960s set. There an authentic replica of a bygone age. are still many of these later radios Each vintage radio buff has his or around and if you can scrounge more her particular area of expertise, so it is than one unit of a particular model, quite reasonable to enlist the aid of it's usually possible to make at least others to help in areas where you are one good working unit using parts no expert. As can be seen from the scrounged from the others. photographs, the cabinet and the For this reason, I always endeavour works of Jim's set reflect the era that to collect several sets of the model I the Rice Neutrodyne came from. Jim want to restore so that I can make one is very good with cabinet work and "perfect" radio. with circuit layout, although he ad- There is of course another type of mits to getting a friend to help with replica. This replica is not a slavish any difficult electronic work on more copy of any particular set but rather a complex sets. copy of the style of set that was used during a particular era. For example I Other replicas know of some enthusiasts who have Replicas can be made of sets from built typical 4-valve mantle sets using any era, from the very first sets made octal valves and a wooden cabinet. (with coherers and the like) up to They look the part, are similar to many transistorised radios of the 1960s. commercially made sets of the era depicted and work much the same. Summary As we've seen, replicas can be direct copies of receivers from by- gone times or can simply be repre- sentative of sets of a particular era while not copying any particular make or model. And al- though they are not true vintage radios, they can be inter- esting and valuable sets in their own right. Other projects that have been held in recent times by various groups in- clude building a "Lit- tle General", a "Little Jim", a "Hikers One" or some other radio, as described in popu- lar radio magazines of the time. All of these are examples of repli- cas. Finally, my thanks to Jim for sharing with us the information on his replica Rice Neutro- dyne. The photographs in this article clearly Above: the top of the cabinet is hinged show what can be to allow easy access to internal achieved with attention components. to detail. SC

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