These days Victorian jewel boxes are hidden away in a cupboards, they also were used for deeds and birth certificates, never to be see. I decided to use Grandmothers box to good use by building a 1 valve radio receiver with 1920's components that are easily available still on line.
I start by drawing a well published from the day 1 valve receiver diagram, using only 10 components plus 2 battery packs, headphones (2k) or a period speaker. The terminals are not counted, just 4 as the battery's are fitted inside. Back in the day they would have been bigger than the box but these days battery size has reduced. The LT needs to be 2 volts for a period 2 volt heater valve, I used 2 torch battery's (3v) in series with a 10 ohm to reduce the volts to 2. The HT only needs to be around 45v, so 5 x pp3's in series will be sufficient as the valve will only draw around 15 mA HT and the heater around 300mA.
The box was unfinished inside so I painted it black and cut a piece of wood to fit where the old lock had been broken away. This was wood dyed to mach the colour of the box and varnished. In fact over the 130 years it had a few bumps and bruises to be seen to also. A plaque on the lid had gone green so I polished that back to copper and lacquered over it.
Not forgetting that the panel needs some internal supports, 4 small pieces were cut to to hold it in place, best to Gorilla glue them in after the build as they might get in the way of components underside. Originally the 20s radios used Ebonite, a form a hard rubber used before Bakelite became popular. Today it's not made anymore so a modern black plastic can be found on-line for jest a few pounds cash.
I was quite pleased with the fit of the panel until I realized that there wasn't enough room for the dials and knobs inside the closed lid, so that another reason to fit supports after you assembled the radio. I used about 5 coats of fast dying varnish, if you use the slow stuff it would take weeks to do 5 coats and wait for them to dry. Fast drying type is ready after about 20 minutes.
Next I fitted the tuning dial and capacitor, making sure enough room was left underneath for the HT battery pack that's only 3/4 inch deep. Make sure the control is dead center otherwise it will look home made afterwards and annoy you, lol
Next I found an old aerial coil, made from 60 turns of tinned copper wire and center tapped, easy to make your own if you haven't got a former a spice tube is around the right diameter. This I fitted to the un-opening side of the tuning capacitor, for Medium Wave it will need to be 0-500pf. The thick copper wire will support the coil and hold it mid air.
Next to fit is the valve, it''s holder and the signal coupling capacitor for the grid of the valve. Triodes were common in the period, PM2HF I used here but they do stand quite high so the trick is to mount it sideways using a plastic 'L' shaped bracket, or even a metal one, I held it in place using the bolt holding the aerial terminal, this saves you drilling unsightly holes on the top. A 200pf period grid capacitor is also held in place by one of the tube base bolts. The 2 mega ohm grid bias resistor can be fitted across that too.
This picture shows the finished underside, top left is the RF choke fixed to a bracket I made for the tickler coil, it's round and is held in place by a position adjusting spindle, close to the aerial in order to feedback some of the anode signal of the valve back to the grid. Unwanted extra feedback (whistles) are controlled by the tuning capacitor underneath. Make sure there is enough room for it to operate without binding on other parts, or box brackets and sides. On the right you can see the battery box for the heaters with the on-off switch above it. The old style switches are still made for cars and bikes (pull for on) just a few pounds cash. The 45v battery pack lies under this layout with two white wires connecting it, HT doesn't need a on/off switch as when the heaters are off no current can be drawn.
Last picture shows the top, I've yet to find some terminal labels on line, but when I do it will make it look better. 90% of the parts are period and gives good reception with just a 10 foot length of wire thrown out of my upstairs window, for earth I use a radiator or heating pipe, it won't work without the earth. Good luck with your project, this is a crystal set really the valve does the work of the crystal when it's biased at it's grid and amplifies at the same time with extra help from the feedback loop.
No comments:
Post a Comment