Heater voltage.

[PerdioPal]

In reality, does it matter much, if at all, valve life is fairly long to start with and as Tommy Flowers said vales are verry reliable if you don't turn them off(and on). A collection radio would more than likely be turned on and off a lot, that will limit the valve life a lot more than heater voltage.

There is a lovely huge motorised three phase Variac on the Colossus at Bletchley Park to warm up the valves slowly, something like that may be a better solution. I think it was custom made and donated by Claud Lyons.

PP

[catch29]

I glad it's caused a discussion. I got a schematic and it shows lamp voltage at 7volt .3amp. I put a mm on the lamp socket,(no valves or lamps) mains is 243v, mm reads 7.6v. I found a couple of 6.5v lamps put the valves back and the mm settled down to 7v. I then connected my lamp limiter with 100w bulb and the mm reads 6v. (lamp runs dark). I'm favoring a beefy resistor in line with the pos' mains, I can knock up a heatsink for it, thing is what value and wattage?, I'm lost in the calculations. The radio is rated at 50w. I would like a cheap solution if possible. I've a few old radio's waiting work and I'm sure to come across it again. This Loewe Opta Toccata 1745w is a beaut' and needs lots of work, after 60/70 years the plastics are just crumbling in my hands, no solution for that though!.

[Mike Watterson]

Valve life is about halved at 7V.
7V lamps doubles lamp life approx vs a 6.3V part. Torch bulbs are different to panel lamps as the life isn't an issue, so a 6V battery torch might use a 4.5V lamp, which is run at higher efficiency, bluer colour temp, but about 1/10th running hours of a panel lamp.

It's why real panel lamps look much more orange than real torch bulbs. Car bulbs are actually 14V., so a 6V (really 7V) motor cycle or old scooter lamp lasts much longer than a torch bulb in a radio as vehicle lamps are lower colour temperature than hand torch bulbs and designed to last much longer.

[Mike Watterson]

50W at 250V = 5000/25 mA = 200 mA
A 30V drop at 200 mA = 150 Ohms
That's 6W.

There are 5W, 7W and 10W wirewound resistors. Two 68 Ohm 5W in series or two 5W 270 Ohm in parallel is fine. No need for a heatsink,

[ppppenguin]

If you have an assortment of wirewound resistors in your junkbox there's a fair chance you can cobble together something that will work. Prove it by measuring the heater voltage when the radio is working. If the result is too untidy then buy something that gives a neat result. Your 100W lamp looks like it will work perfectly well as the dropper.

You could steampunk the set and put a lampholder on the top.

[This last comment is a joke. The mere thought of doing that to a nice radio should get me lynched]

[Refugee]

You can work out the exact resistor value by measuring the AC current with the radio playing a station and then the lamp limiter voltage. I have a lamp limiter with a pair of sockets for measuring current and lamp voltage.

[catch29]

Great advice guy's, I was thinking of screwing a batten holder inside the case and trying a range of pygmy bulbs untill I got the correct voltage - maybe not!. Thank's for giving me your 'know how'.

[ppppenguin]

I suspect a pygmy bulb will have too much resistance. You'll almost certainly need something bigger.

[PerdioPal]

A bonus with a series resistor is that the valves have less surge current.

[catch29]

Great, got myself a 150ohm 25w wirewound. I have noticed something on the chassis I don't understand, a 4uf cap with positive to earth?, It looks original, it's been like that for 70 years?. Am I missing something?.