My Second Radialva Groom Restore

[pwdrive]

If I’ve done the sums right, with a 3,000 Ohm field coil, if 45 mA was to be drawn, then the voltage dropped across the coil would be .045 A x 3000 Ohms = 135 Volts. That Wattage dissipated in the coil would be .045 x 135 = 6.075 Watts.

With a 450 Ohm field coil, if 45 mA was being drawn, then the voltage dropped would be .045 x 450 = 20.25 Volts. The Wattage dissipated in the coil would be .045 x 20.25 = 0.91 Watts. (Call it one Watt).

Presumably a resistor in series with the 450 Ohm field coil to increase the total resistance to 3,000 Ohms would be in order? IE, 2550 Ohms. No such thing, so a 2K2 resistor would I assume be OK. The voltage dropped across that resistor would be 135V – 20.25V = 115 Volts, so the Wattage of that series resistor would be 135 x 0.045 mA = 5.15 Watts.

I appreciate that the field coil is of course an inductor – not a resistor, which has the dual role of energising the speaker and acting as a smoothing choke, but it terms of voltage dropped, 3000 Ohms is 3,000 Ohms. If for example - as often happens - someone substituted the speaker for a modern permanent magnet one, they'd dispense with the field coil (the usual reason being that it was OC anyway), and substitute a resistor of a similar value to the field coil across the smoothing/reservoir caps in the power supply.

Don’t know if that makes sense – any thoughts anyone?

Is the radio a 110v or 240v?

Lawrence.

[Geaney]

Thanks David, that's very helpful. I was thinking along those lines but I don't trust my theory once inductors become involved!

Cheers,

John

[Geaney]

Is the radio a 110v or 240v?

Lawrence.

Hi Lawrence, officially it's for 120v supply which was apparently the supply voltage in France during the war years.

Cheers, John

[Geaney]

Output transformer now mounted on the speaker assembly - almost done with mechanical stuff!

Cheers, John

[pwdrive]

Is the radio a 110v or 240v?

Lawrence.

Hi Lawrence, officially it's for 120v supply which was apparently the supply voltage in France during the war years.

Cheers, John

Hi John, I have no real experience with field coil speakers or replacing them with a different one, I'm guessing the field coil is part of the filter circuit ? Any chance of posting the circuit diagram on the forum? I have looked in the past at various European diagrams, but a lot of them leave me confused, maybe it's the way they are drawn.

Lawrence.

[Yorkie]

If it's for 110 Volts, and the field coil was originally 3000 Ohms, and required to pass 45 mA HT, the sums don't add up - there'd surely be no HT?

V=I/R , so I x R = V. Hence, 0.045A x 3,000 Ohms = 135V dropped. Huh? It can't be so.

Over to learned colleagues!

Mumbled Yorkie,
AKA 'Confused of Cottingham'

[tiv48]

the field coil of a speaker is use instead of a choke in the power suppy.connected between the smothing and reseroir cap buget circuits use a resistor.the effect of the currant though the field coil turns the centre pole into a powerfull magnet.
rob t

[Geaney]

Any chance of posting the circuit diagram on the forum?

Hi Lawrence, here's the only diagram I've been able to find: The layout is not exactly they way we would do it.

Cheers, John

[pwdrive]

Any chance of posting the circuit diagram on the forum?

Hi Lawrence, here's the only diagram I've been able to find: The layout is not exactly they way we would do it.

Cheers, John

Thanks John, certainly is interesting, I can see how the bias rail is derived, the field coil connection is a new one on me though.

Lawrence.

[Yorkie]

the field coil of a speaker is use instead of a choke in the power suppy.connected between the smoothing and reservoir cap buget circuits use a resistor.the effect of the currant though the field coil turns the centre pole into a powerfull magnet.
rob t

Yes, I understand that Rob, but surely in terms of voltage drop, as the 3,000R field coil (acting in its dual role as a smoothing choke) carries the HT to the radio, such a high resistance will drop too much voltage when fed from 110V mains, as would a 3K resistor in the same position between smoother/reservoir caps. But now that John has the circuit, it's apparent that the field coil doesn't in fact carry the full HT. The DC comes off the cathodes of the rectifier and there is 105V at that point, which the field coil has no influence over as all of the HT doesn't pass through it. I suppose the issue then becomes whether or not the 450 Ohm field coil will draw excess current as compared to the specified 3,000. Maybe the only was of knowing would be to check the voltage drop across the coil to find out from Ohms law what current is flowing in the field coil. The usual reason for field coils failing is due to excess current being drawn when DC blocking caps become leaky, given that the coils consist of many thousands of fine wire - 48SWG I'd guess.