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Vintage Radio Baby Beethoven 555 portable regenerative radio The Baby Beethoven model 555 portable radio comes from the UK. It was offered to the public there around 1937-1938. It is a regenerative receiver using the PM2HL valve or its equivalents, such as the HL2K and VT-50. The Reinartz 2 receiver described in the October 2025 issue was also a regenerative set. By Dr Hugo Holden R egeneration results in ‘Q multiplication’ in a resonant circuit. This happens because of energy injection due to positive feedback. The result is a narrower bandwidth, increased gain and improved selectivity. A good regenerative radio can be nearly as good as a superhet radio for gain and selectivity. It needs just the right amount of regeneration, though. This means a regenerative receiver is a ‘Goldilocks circuit’, because everything has to be ‘just right’ for a good result. The designers strive to ensure that the control of the regeneration is smooth, in that the positive feedback is introduced gradually. Otherwise, the user adjusting the control can result in the stage abruptly going into oscillation. If that happens, the highlevel oscillations heterodyne with the received carrier, resulting in a lot of ‘howling’ from the speaker. A ‘super-regenerative’ radio is a little different again as the oscillations are set up to be blocked at a frequency higher than the audio spectrum but much lower than the carrier frequency. 88 Silicon Chip This type of radio can develop astonishingly high gain levels for a single stage, but the zero-signal noise is often very high. Interestingly, this design has persisted much longer that the plain regenerative receiver. It is often used at the receiving end of short-range remote-control systems for home automation. It was used extensively in cheap children’s transistorised ‘walkie-talkies’ because the super-­ regenerative stage readily oscillates and makes for a good transmitter with only a small circuit change needed to switch between receiver and transmitter modes. The ideal amount of regeneration in a regenerative radio is tricky; the required magnitude of positive feedback is difficult to keep uniform on different parts of the tuned band. It can also be affected, to an extent, by the strength of the received station. That is why, in the Reinartz circuit mentioned earlier, the other gang of the tuning capacitor modifies the regenerative energy injection across the tuned band, helping to even that out. Australia's electronics magazine Because of the difficulty of making smooth regeneration controls, manufacturers had some difficulty in marketing regenerative radios to the public. Many circuit variations, such as the one in the Reinartz radio, were devised to help improve the function of the regeneration control. While the regeneration control can be a potentiometer, it was more often a variable capacitor. From the radio user’s perspective, it is a volume control. Thus, most regenerative radios did not have an actual volume control potentiometer in their audio amplifier chains. The regenerative stage can also act as the AM detector as well, as it does in the Reinartz design or the popular Hiker’s 1 radio and in the Beethoven 555. In this case, it is called a regenerative detector or regenerative gridleak detector. Regenerative vs TRF radios Regenerative radios are a subset of the TRF type but behave substantially differently from a standard TRF set. siliconchip.com.au TRF radio receivers were very popular in the 1920s and 1930s era. They were analogous to a crystal set but with active stage amplification; no positive feedback was used. They were simply a chain of tuned bandpass amplifiers, with a detector to recover the audio modulation. The difference with a regenerative radio is that positive feedback has been added at one or more stages. The trick is to add enough positive feedback to improve the gain without sending the set into oscillation. Because the amount of extra gain possible is very large, the number of valves in the set can be dramatically reduced for similar performance (sometimes to just one RF valve!). Multiple regenerative radio construction articles appeared in radio and hobby magazines of the 1930s. A particularly popular one in Australasia was the Hiker’s One. This radio would run from a 6V B+ because it deployed a space-charge valve. That made it a popular choice for children because no high voltages were involved, and their parents did not have to buy them expensive 45V or 90V B+ batteries. Of course, there is a price to pay for the miracle of regeneration; regenerative sets can be difficult to control. As the regenerative stage is pushed further toward oscillation, the bandwidth narrows, and just before oscillations or ‘howling’ begins, the recovered audio modulation becomes muddy and lacking in high-frequency components. This was the challenge for designers and manufacturers of regenerative radios in the 1920s and 1930s: how to make the radio usable for the average member of the public. The user may have limited technical knowledge, making it difficult to manipulate the regeneration control to obtain a good result. The later superhet radio design did not have this problem. Back to Beethoven The Beethoven 555 receiver was moderately advanced for its time, and it addressed the smooth regeneration control concern very well. I found it difficult to acquire the correct circuit diagram, so initially I traced it out by hand in 1987, with the result shown in Fig.1. Later, I found a circuit of a very similar set: the Beethoven P202, shown in Fig.2. The only apparent major siliconchip.com.au The grid leak detector misnomer The ‘leaky grid detector’ terminology is an endless source of confusion, probably because it was not aptly named. The load resistor of the AM detector is responsible for discharging the capacitor between charging peaks of the RF carrier wave; this capacitor is charged via a diode function on RF peaks. The load resistor of this lightly filtered half-wave rectifier system came to be called the ‘grid leak resistor’. This is a function it also served, but was unrelated to the demodulation of the AM signal. In the valve, electrons tend to accumulate at the grid and develop a space charge around it. If there is no DC path for them to ‘leak away’, a negative charge and voltage builds up on the grid, and this can cut off the valve by repelling electrons back to the cathode (or filament if that is the emitter). This can take the anode current to near zero. If very high value grid resistors are used, in the range of 3-10MW or more, the grid current passing via the resistor, in the order of a few hundred nanoamperes or less, can result in a DC bias for many valves in the order of -1V to -3V. This can be helpful in biasing the valve; it is often done when cathode biasing is inconvenient and the cathode is better grounded. For example, when the valve also has an integral anode electrode to act as a diode with the cathode, it is preferable that the cathode is at common/ground potential. Some have wondered why occasional circuits from the 1920s, including in patents, showed valves without grid resistors. There were two reasons. One was that the grid electron current was very low in early valves, and that capacitors and insulating materials of that era were not as perfect as what we have in modern times. Leakage resistances, even in the range of 10-100MW, were enough to dissipate the grid’s electron charge, so the valve never became ‘cut off’ in practice. The second reason was that when these patents were applied for, Mr. Einstein was no longer working at the patent office to detect this oversight; just because something worked in practice did not mean its design was correct or complete. The thing is that this grid-leak biasing function has little to do with its function as an AM detector. This resistance is simply the load resistance that discharges the filter capacitance between charging peaks by the RF carrier. It is a half-wave rectifier circuit with minimal filtering. The circuit of the RF coil, feeding diode, resistor load and filter capacitor is the same for any AM detector. The diode itself, in the grid-leak detector case, is simply the grid-­cathode (or grid-filament) interface of the valve. The discharge time constant of the RC filter sets the upper frequency limit that can be resolved via the detector without distortion. As the modulation frequency gets higher, or the modulation depth gets higher, the capacitor cannot discharge quickly enough to track the modulation envelope present on the RF carrier before the next carrier peak arrives. The highest audio frequency that a diode-RC detector can resolve, when the modulation level is 50%, is 0.275 ÷ RC. In 1920s radio designs, values such as 3MW and 100pF gave a fairly poor result for high-frequency audio recovery. The detector’s distortion and high-­ frequency roll-off began at about 1kHz with 50% modulation; 0.275 ÷ (3MW × 100pF) = 916Hz. By the post-war period, the AM detector situation had improved, and RCA had moved to values such as 100pF and 250kW, allowing audio recovery up to around 11kHz. But in a vintage TRF radio, 250kW would be a somewhat heavy load and would have damped the driving tuned resonant circuit, lowering the gain and selectivity. The trouble was that, in the early days, there was much more of a quest for gain than fidelity, and extra valve stages were expensive. If the detector stage is also regenerative, the losses due to increased loading could be overcome by the additional energy injection from regeneration, meaning the grid resistor could be in the range of about 250kW to 470kW to improve the detector’s high-frequency audio fidelity. The change in the valve’s DC bias conditions would be minimal. In most practical grid-leak detector circuits using directly heated (filament) valves, to encourage the valve to draw grid current on the positive going peaks of the RF carrier, the grid is often made a little positive with respect to the average cathode voltage. The grid-to-filament potential in a directly heated valve is distributed along the length of the filament. The usual configuration is to return the grid resistor or the RF coil to the positive side of the filament connection for that reason. Fig.a shows a common configuration for a ‘grid-leak detector’ using an indirectly heated valve. Fig.a: how a grid-leak detector works. Positive excursions are limited at the grid, and when the RF is filtered out of the inverted and amplified version of this signal at the anode, it leaves behind a reconstruction of the amplitude modulation signal. difference is that it came in a slightly different cabinet and it sported a power lamp. I subsequently added the P202’s component designators to my hand-drawn circuit. In my 555 radio, there were some resistors added in the switching circuit of the MW and LW frame antenna that might not have been original. Capacitors C1, C4 and C12 also had different values, but of course, these may have been altered in the past during servicing. RF amplifier (V1) The valves used in the 555 and P202 are 2V heater types, with the VP2 RF pentode used as an RF amplifier. At this point in history, pentodes were futuristic parts. The design confers properties well in advance of a triode. One major advantage is isolation between its anode and control grid. This means that, unlike a triode with its high Miller (feedback) capacitance, you can connect tuned resonant circuits to its grid and plate operating at the same frequency without them exchanging any significant energy with each other. It will thus not oscillate, as it would if it were an unneutralised triode. It is important to prevent the tuned coil in the VP2’s anode circuit from feeding back to the frame antenna inside the radio’s cabinet, since this coil essentially sits inside the middle of the frame antenna. Thus, the coil is very well-shielded. Grid-leak detector (V2) The job of the regenerative grid-leak detector is allocated to V2, a PM2HL. The grid return voltage of V2 has been set to the centre of the filament voltage with the two 4MW resistors, R2 & R3, and sits at an average of +1V. This is equivalent to a zero-bias condition for V2 because half of the filament’s structure is negative with respect to the grid, and the other half is positive with respect to the grid’s average potential. The grid’s electron current will shift the grid a little in the negative direction, because the Thévenin resistance is 2MW, and a small standing negative grid bias results from that. The PM2HL is a metallised version of the common triode, such as the HL2K or VT-50. The metallisation makes for a helpful shield at the regenerative detector stage. If you see a modulated carrier passed to the grid circuit of a valve and an audio signal is recovered from the anode, there are only two ways that can work. Both require non-linearity in the way the signal is processed or amplified. The common method is the gridleak detector, which is poorly named. In this case, rectification occurs at the grid-filament (or grid-cathode) interface, and the signal at the plate Fig.1: a hand-drawn circuit of my set. The first valve is a pentode that acts as a regenerative RF amplifier. The second valve, a triode, is the ‘grid leak detector’. The second triode is the audio preamplifier while the final valve, another pentode, is the power amplifier that can deliver around 340mW to the speaker. “C2 Gimmick” refers to a low-value capacitor that’s formed by two closely-spaced but separate wires. 90 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.2: the Beethoven P202 portable battery-powered set circuit from the “Trader” service sheet. It’s a very similar set to the Beethoven 555, with the only definite differences being the cabinet style and the pilot lamp in the P202. Source: Radiomuseum – www.radiomuseum.org/r/beethoven_p202p_20.html siliconchip.com.au Australia's electronics magazine August 2026  91 The rear view of the Baby Beethoven 555 with and without the 90V and 2V batteries that I made. Note that the original battery for this set was 80V. resembles an inverted and amplified version of the negative half of the RF carrier wave. The audio modulation is recovered by filtering the carrier out of this signal. However, in some cases, if the detector valve is correctly biased, the positive-­ going halves of the carrier wave are preferentially amplified by the RF voltage applied to the grid. This happens because of the curve, or bend, in the function of anode current versus grid voltage. The signal at the anode of the valve looks like an amplified, inverted version of mainly the positive half of the carrier wave. That is called an anodebend detector. In the boundary between the two forms of AM detection, grid leak versus anode bend, there can be no detection at all. In this instance, a perfectly symmetrical modulated RF carrier would appear at the anode, and filtering that out would reveal no audio signal at all! So, if you see a valve AM detector circuit, how do you know if it is a grid-leak detector or an anode bend detector? In most anode bend detector circuits, the grid-filament or grid-cathode of the valve is fed directly by the output coil of the previous stage. In the grid leak case, there is always what appears as a coupling capacitor that relies on being charged by grid current and discharged by a resistor. It is not really a coupling capacitor though; it is more like the energy storage capacitor in a half-wave rectifier system. Audio amplifier (V3) Triode V3, another PM2HL, is deployed as an audio amplifier, so does not require a shield. In my radio, I used a VT-50 for V3 because of its narrower profile; there is not a lot of room in the battery compartment. The audio power output valve is the PM22A. This has multiple equivalents and is listed as a KT2 for the P202 version. These are capable output valves for a battery radio and can deliver 340mW. One good place to find out equivalent valve types is in the original Mullard valve manuals. The manuals even suggest what possible substitutes could be used with small modifications, which is very helpful if you cannot find an exact equivalent. The speaker in this set is a good size 92 Silicon Chip Australia's electronics magazine siliconchip.com.au A VT-50 triode (left) was used in my Beethoven 555 for V3 as it is smaller than the alternative PM2HL (right). Fig.3: to determine the filter response in the days before SPICE was available, I simplified the circuit as shown here, then derived the gain formula shown on the right. The plot on the left is the result. Besides having loss overall rather than gain, the response is very similar to that of a tuned IF transformer. at close to 5.5 inches (140mm) in diameter, with an Alnico magnet. tapering off at the upper and lower ends. However, if the balance of factors is not correct, there can be a significantly peaked response. Coupling transformers One notable feature is the audio inter-stage transformer. It is capacitively coupled and used as an autotransformer. This arrangement gives higher voltage gain for fewer total turns (less copper and less iron) because the primary voltage is in series with the secondary. As a result, the autotransformer can be made more compact and lightweight. Note that general transformer theories of impedance matching and power transfer do not apply to typical valve inter-stage transformers driving a valve in Class-A. They do in Class-B, where power transfer is required. This is because the grid of the audio output valve, operating in a Class-A condition, never draws any significant current. The equations that do apply are those of the damped tuned coupled resonant circuits. In the interstage transformer case, the damping is normally provided by the anode (plate) resistance of the driving valve, although in some cases it can be added to the primary or secondary windings. When the proportions of inductance, winding self-capacitance, mutual coupling and damping are correct, the interstage transformer can possess an astonishingly flat response in the audio frequency spectrum, siliconchip.com.au Generally, the output from a regenerative or grid-leak detector passes to the next stage via an inductor or an audio inter-stage transformer to assist in filtering the RF out of the signal and leaving just the audio signal. In this radio, no inductor or transformer is present; there is simply an RC network feeding V3, the first audio amplifier stage. This makes sense; the radio was already heavy enough, so there was certainly a motivation to save weight and to get rid of at least one of the normally heavy iron-cored inter-stage audio transformers. I wondered what the bandpass frequency response of this RC network would look like over the audio frequency spectrum. This sort of thing is dead easy to measure with a signal generator and scope, but I wanted a theoretical proof. This was in the days before SPICE, so I derived an equation for it, which took a lot of work (see Fig.3). I ran this equation through a graphing program on my university’s mainframe computer. The assumption was made that the source impedance driving the filter (the Thévenin resistance) would be in the order of 18kW, being V2’s anode (plate) resistance of around 37.5kW in series with R6 (6kW), both in parallel with R5 (30kW). I don’t have the original plot, but I’ve reproduced it from memory in Fig.3. The band-pass response was The badge for this radio is located on the top of the lid. The top of a Mullard PM1HL triode, which has a gold metallising paint. An interesting feature Australia's electronics magazine August 2026  93 Restoring valves when their metallisation fails Sometimes with very old valves, the conductive paint bubbles off and falls away from the glass. Fortunately, there is a method to restore it. Jaycar sells a highly conductive colloidal silver paint, made by Kemo Electronics GmbH, which has excellent adherence to glass. You can paint it on with an artist’s brush after the remainder of the old paint has been removed. Once finished, you can spray metallic silver or gold paint over it to restore the original appearance. This often results in the loss of the original label, so it is a good idea to re-label the valve on its base. The adjacent photo shows a restored PM2HL valve. The rear and front of the leather cabinet. A close-up of the rotating base (‘Lazy Susan’) is shown at lower right, which uses ball bearings to rotate. remarkably similar to that of an interstage transformer, except without the signal gain that the transformer would have provided. Running the circuit through SPICE gave the same result (shown in Fig.4), except it was about a hundred times easier. The network behaves as a bandpass filter over the audio frequency spectrum with an insertion loss of about 8.45dB. This is made up for by the audio pre-amplifier valve (V3) and the PM22A output valve (V4). Sometimes people repair radios where the interstage audio transformers have gone open circuit by replacing them with an RC coupling network. Clearly, this can work, but there will be a substantial drop in gain, more than the typical 1:3 ratio of the transformer. That is assuming that the RC filter is crafted to have a similar bandpass characteristic to the transformer it’s replacing. Antenna and miscellaneous Radios with ferrite rod or frame antennas are very directional. The manufacturers of the 555 put a ‘lazy Susan’ type spinning base on the radio. This runs very smoothly as it is supported by ball bearings. For convenience, the manufacturers used a removable handle with ‘lift the dot’ fitting. These were used in the automotive industry for attaching softtops to convertible cars. The leather handle had perished. The one shown is a reproduction I had made back in the late 1980s. At a glance, the speaker grille on the radio might look like plastic, but it is made from a high-strength woven string (not cord) that appears to have been varnished. It has stood the test of time very well. The adjacent photo shows the radio with the back fitted. The rectangular steel spring-metal clip slides under two screw heads to help retain the back. In this era, the user had many radio stations to select from, if the dial is anything to go by. Batteries Fig.4: now that SPICE is available, we can easily plot the circuit’s response in just a few minutes. You can see that it matches my version very well, including having almost-identical -3dB points. The insertion loss is 8.45dB. 94 Silicon Chip Australia's electronics magazine I had to make batteries to run this set. The 2V battery is composed of two groups of three parallel C-sized NiCad cells in series. A 1W, 10W ceramic dropping resistor is used inside the same battery enclosure to get the loaded voltage close to 2V. siliconchip.com.au Interestingly, the original 2V cell was described as a 14Ah “celluloid jelly acid”. The 90V battery was created from 72 AA-sized NiCad batteries in series. Because of its current-delivering capability, I incorporated a fuse inside that battery. The original battery for the radio was actually an 80V type, although I did not find that out until years later when I found the P202 circuit. I put an Eveready label on the 90V battery for a bit of fun. Summary The Beethoven 555 is a remarkable MW & LW band regenerative radio from the pre-WW2 era. The manufacturers had largely perfected user-controlled regeneration. The radio is well made and has stood the test of time. Its weight was reduced by eliminating one interstage transformer in favour of an RC filter. The resulting gain reduction was made up with a two-stage audio amplifier and a frontend RF pentode, giving up to 380mW drive for the loudspeaker. From a commercial perspective, regenerative radios were destined to become far less common than superhet types. Over time, the cost of valves came down, and superhet portable valve radios appeared with a converter valve. This combined the function of the mixer and oscillator into one valve. With one IF pentode, one diode detector/triode valve and one audio output pentode, the radio was complete. That was the same number of valves for a superhet (four) as a regenerative radio like the Beethoven 555. However, the ease of use and performance of superhet radios out-classed the regenerative designs. History has shown that regenerative designs remained popular with home constructors. This was because of the large amount of signal gain from a onevalve regenerative detector stage; the economy is hard to ignore. In many cases, one valve was enough to drive a set of headphones directly, and even enough to drive a small speaker with only one or two additional valves. It is fair to say, looking back, that the regenerative radios of the early years demonstrated remarkable innovation by the designers. It is fun to restore these radios, experiment with their operation and study their operating SC principles. siliconchip.com.au RP2350B Computer A Fully-assembled general-use computer The RP2350B Computer runs BASIC and is excellent for creating your own programs, games, tinkering with external circuits and more. And we are selling it pre-assembled, with little to no soldering required to have it up and running. It supports a keyboard, mouse or even a SNES controller. Video output: DVI via an HDMI connector <at> 640 × 480, 720 × 400, 800 × 600, 848 × 480, 1280 × 720 or 1024 × 768 pixels Removable file storage: microSD Card, FAT16/FAT32, up to 32GiB Clock Speed: 252-375MHz Non-volatile program memory: 184kiB General usage RAM: 220kiB (expandable to over 6MiB) Internal File Storage: 14MiB Audio formats: single-frequency tones, stereo WAV, FLAC, MP3 & MOD USB ports: four Type-A for peripherals, one Type-C for power/console and one micro Type-B for firmware loading Clock: battery-backed real-time clock & calendar External console: serial over USB <at> 115,200 baud via the USB Type-C socket External I/O connector: 30 pins with 22 GPIOs, including 7 with analog input ability, plus ground, 3.3V and 5V outputs Power supply: 5V <at> 220mA RP2350B Computer Assembled Module [ SC7531 | $90.00 + post ] fully-assembled PCB, except for the optional components (instrument case, mounting screws, 3-pin header for serial wire debugging and APS6404L PSRAM IC [SC7530 | $5]) Front & Rear Panels [ SC7532 | $7.50 + postage ] pre-cut panels, white silkscreen and black solder mask; not included with the kit above For all the details on how to build it, check out the article in the November 2025 issue of Silicon Chip (siliconchip.au/Article/19220). Australia's electronics magazine August 2026  95