Variable Regulator with Current Protection

I can't see a 680 Ω resistor in the circuit? Anyway.....I was thinking earlier today about a portable battery radio which was built in to a smart looking wooden box when they first came out somewhere around 1960 give or take a few years before transistors. The lid lifted up and that had neat varnished winding wires as the aerial. There was a large metal wafer condenser and had two valves (or was it three?). It was powered by two large square looking batteries, the largest one being 90v and the smaller one (which I think was for heating the cathodes??) The large battery could give quite a shock if touched! We took it on holiday with us and many people were amazed to see a radio that could be carried around without mains. I seem to think it was from a well known radio manufacturer like Philips but I just can't remember who exactly.....any ideas on who the manufacturer might have been at that time.....there couldn't have been many. The rich sound of valve radios was somehow different to even expensive modern equipment in my opinion.

Sorry the radio which you describe is a few decades before my time. I have no idea, although i know it was normal for tube amps to use batteries to operate, although with limited capabilities. For instance a 15W tube amp i am about to build uses 300V for the power supply. 90V is just too little. The weight would've been enormous though. The 680ohm resistor is actually a 680mOhm resistor. Sorry type-o.

Thanks anyway....I have just managed to find it on youtube it is 1957 British Pye Model P114BQ "Jewel Case" Attache Portable Radio ....sorry for the large print cut & paste....I was only a small child but it wasn't large or heavy as you can see on youtube there.

The weight and size is reduced because the heater voltage of the tubes needs to be 1.5V, so this allows for smaller cell stacks. The 90V battery can be small because the current draw is quite low (probably less than 50mA)

Interestingly the filament of the tubes is 1.5V at 25mA with the power tube running at 50mA !!! For the 4 tubes the overall draw doesn't exceed 150mA. This means that the battery will last quite long even if it has low capacity.

Cool thanks.....seeing those tubes glow and all the wires sparked my interest in all things electrical...transistors came out shortly after that and I sent away for a two transistor amplifier kit with relay. I had to etch it and drill holes in it etc. I shelved my electronics interest and eventually left school to do a few years electrical apprenticeship....and I took that little amplifier around like a nostalgic teddy bear until I moved a few years back and couldn't find it....so much for part of my meagre life story....sorry about that.

No problem. For me the sparkle was amplitude modulation. Not electrically speaking but just the form of the wave of how the low frequency wave is superimposed upon the high frequency one. It just looked neat to me and it got me going. My latest obsession is vacuum tube. I've already designed a few amps and a wien bridge oscillator, now if i find some time i plan on making an Amplitude Modulator with just 1 tube and see where it leeds me.

It would be hard to miss your enthusiasm here on EC and you are the go-to guy for amps. Having been a teenager through the sixties I dropped out of electrical training and I have never settled to any one thing for more than a few years. It had its advantages though...I have always had an inquisitive mind and it led me through Oxford University in Education as a teacher along with studying Combined Science and Statistics which I used as a University lecturer along the way. In between times I have run a vehicle (transits) hire company and a business mending computers (Windows). Dabbling in electronics and bits of programming have been my interest off and on since the Sinclair Spectrum days. I still view myself as a beginner in electronics, but it's nice to see others here on EC at various levels giving their angle on things and I throw in my bits to help when I think fit. Getting things wrong in not important in my book, it is more important to analyse and declare valid estimates of errors and so learn more from mistakes and failures ....and I am quite an expert in the latter!

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