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Valved radio

Dear Reader,

Yesterday while in out looking for a child seat I visited a second hand shop, I saw a pair of old valved radios. Sadly I did not have space in my car for both and also I could not buy both with a clean conscience. I had a choice between a working West German made radio and a broken Swedish radio. The German radio was a two band (or maybe one band) radio which the shop thought is working. The Swedish radio was a multiband radio which included shortwave bands.

It is a Elektriska Industri AB (EIA) radio type (Typ) V 099.

I took the back off the radio and I found two 20 mm fuses, I checked both of them and they were intact. OK so far so good.

I plugged it in on the lawn for fire safety reasons as I thought that if the radio is going to catch fire it is better it happens in the garden away from the house rather than in the house. Nothing happened.

I then examined it with a voltmeter, the mains was reaching the power switch but no matter what I did to the power switch the output terminals of the switch still had zero volts. I disconnected the mains from the radio and reached inside with a pair of wire cutters, I removed the wires from the switch terminal and then bypassed the switch.

I reconnected the radio to the mains, suddenly the two dial lights came on and one of the valves started to glow dull orange. Well at that point I knew that the heater supply worked.

Then a few seconds later the radio started to make a static noise, even better !

I tuned the radio to about 10 MHz (31 meter band) and I then switched on my dip meter. I set my meter to have AM modulation and I then tuned it to about 10 MHz. I brought the dip meter close to the tuned circuit for the front end and as I tuned the meter backwards and forwards I heard a whistle from the radio.

While it might be just a sine wave output it was music to my ears, the radio works. I now need to connect an aerial to it and to try to tune into some broadcast stations. The best bit is I only paid 195 SEK for the radio in the second hand shop !

I plan to do this and compare the valved radio with my FT450 which is a modern communications radio with three IFs and digital signal processing. The FT450 has the first IF at 400 MHz which should put a stop once and for all to the image rejection problem. I have noticed that the valved radio (single super het with IF at 450 KHz) does show a second response to my dip meter.

When I have more time I will try to map out the responses of the radio in more detail. But at least the radio does work. I want to do some work on the radio to improve its electrical safety, I think that the mains lead is a nasty old rubber which is not safe when compared with modern PVC. I will change the lead and I may change some of the safety critical wiring for PVC insulated wire.

I also have noticed that the radio has a metal chassis and it is not earthed, I will look at the radio and work out if I can add a protective (safety earth) to the radio to make it safer in terms of electric shock. I will also check to see if any live (high voltage DC) exposed parts are present above the surface of the chassis, if there are any these will be covered and also marked with a warning.

Since I was about 21 I have been doing high voltage electronics, so valved electronics is well within my comfort zone. For my readers who have never done high voltage electronics I would like to ask you a question

“Do you feel up to it ?”

I would also like to suggest you think long and hard before you have a go with valved electronics. Many harmless looking valved devices hide a range of mortal threats within them, while it might be almost 100 years old it can still give you an “access all areas backstage pass for the afterlife” ! Or put it another way it can make you experience “sleep eternal” or a trip to “the other side“.

Some quite comment valved systems can deliver bigger electric shocks than the american electric chair ! For example the power pack for a pair of 4CX250B valves (400 W VHF amp) is typically 2000 volts and is normally limited to about 80 amps, this is a bigger threat than “old sparky”. While a typical 1 kW HF amp typically uses a bigger supply.

I can accept no responsibility for either fires, electric shock accidents or nightmares you might now have about high voltage accidents (which can include non electric shock phenomena such as arc flash, I doubt if a twin triode (813) amplifier supply can cause an arc blast which will dismember you but I have no plan to put it to the test. I am sure that the arc flash from such a power pack could cause serious eye injury or at least give you flash blindness which then leads to some other accident.

If you do choose to have a go at high voltage electronics then I would like to commend to you the life saving mnemonic which I heard came from the UK’s atomic energy authority (I think it has its origins in their early plasma fusion experiments). They had the slogan of “stay on the safe SIDE”.

Switch off

Isolate the system

Dump any charge / energy stored in the system

Earth the objects you are going to work on

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