My latest addition - Radialva Groom

[Geaney]

Based on all the comments about the value of the electrolytics I'm tempted to leave them as 50uF. I found an interesting french doc online - a list of different circuits which were the ref designs for various configs of receiver up to the late forties. The ref design that looks very similar to mine in fact uses 2 x 50uF in the power supply. That's another vote for keeping away from the larger value stated in the circuit diagram.

I won't place any bets on when this little beast will jump into life - but jump it will!

Cheers, John

[pwdrive]

Just a thought for the unweary, using a capacitive dropper is not as simple as Xc = Line Chord resistance, it's a complex impedance. DC rated capacitors should not be used.

Lawrence.

[Geaney]

Pretty close - http://mysite.du.edu/~jcalvert/phys/caps.htm
Alan.

Thanks Alan,
Whew, that was lucky - I'd started re-cappng on that assumption.

Cheers, John

[Geaney]

While I'm waiting for the replacement valves to arrive I thought i need to tackle the question of the dropper for the heater chain. I'd be very grateful if those with more experience than me could check out my logic for the calculations.

The two bulbs in series seem to have about the same resistance as the bypass resistor so I've assumed this splits the current about half each via the resistor and the bulbs thus the total voltage drop is less than two bulbs in series without the bypass resistor.

The dropper circuit is taken from the great article on the UK vintage-radio forum and the calculation are taken from the related spreadsheet tool. I notice that if I reduce the voltage drop for the surge resistor it will generally reduce the value of the cap and the surge resistor so I can play ground with that. I guess the challenge will be finding a non-electrolytic cap of the required value.

Let me know if this is anywhere close.......

Cheers, John

[Robert Darwent]

Hi John,

From a quick look at your calculations you seem to not have missed anything, although I haven't actually used the spreadsheet to confirm your resistance and capacitance values they seem reasonable..

I had assumed however, that you would be calculating a capacitive dropper for a 240v supply not 110v?

Regards

[tiv48]

if you are right john you are going to have to look at motor start caps or microwave hv caps but they may be too big.
useing 230 volt would reduse the cap value but then you would need a dropper for the ht.if you are going to use 110 volts i would maybe reconsider a resistive dropper ie one of the metal clad types.
rob t

[AlanBeckett]

... you are going to have to look at motor start caps

No Sir, Start Caps are specifically designed to take a very large current for a short time and then get switched out. You need Run caps, which are designed, as the name implies, to stay in circuit. It's fair to say that for low values - less than about 20u - it tends to be academic these days as they're not Electrolytics. However, they can be physically large.

Alan

[tiv48]

sorry alan motor run cap is what i ment to say.
rob t

[pwdrive]

If all else fails maybe a half wave dropper + R, I don't have the diagram for your radio so you would have to check first.
Thorn used a capacitive dropper in the 60's in there "portable" 14" TV, I think that was around 4 uf, we never had one fail at that time.

Lawrence.

[Yorkie]

if you are going to use 110 volts i would maybe reconsider a resistive dropper ie one of the metal clad types.
rob t

Good point Rob - with only 34V to drop the wattage will be quite low. I recently used metal clad resistors on the Portadyne Princess and look-alike 'Noble' TRF sets on which the mains droppers had burnt out. Metal clad resistors must be de-rated if not mounted on a heat-sink as they will not dissipate their stated wattage if not heat-sinked. I just used a piece of 5mm thick alloy angle as a heat sink, which worked out fine. Although I calculated the total dissipation to be about 23 Watts, I used 50Watt resistors rather than 25 Watt ones, as 50W ones are only about 2.5” (6cms) long.

See: http://golbornevintageradio.org/forum/sh...p?tid=2154&highlight=Portadyne+Princess