My Second Radialva Groom Restore

[Radio Fixer]

As Lawrence says the 3000 Ohm coil is typical for a speaker with a shunt field. I rewound one once that was 4000 and it took 4.5 miles of wire and in excess of 40,000 turns!

These were not used as smoothing chokes and as you can see they got away without this by large electrolytics.

The bias res. has 7.8V across it which is 31mA so the drop across the field is 93V giving roughly 100V at the rectifier cathodes.

Not sure where the 300 field comes in but if you have that then the speaker has been replaced and it would be correct to wire it in series with the HT feed. It would be wired between the two +ve ends of C24 if they are brought out separately. This way it would contribute a small amount of extra HT smoothing. It would drop maybe 15V but the HT will rise a little anyway without the shunt field load. However the bias for the output valve would now be incorrect and more changes would be needed. Two ways to do that which we can go into if this is how things are.

Gary

[pwdrive]

It might need to be wired in series with the rectifier output, if wiring it between the smoothers then the field coil will only be passing the current flowing through the mixer, IF, 1st AF stages which might not be enough to give a decent flux density.

You could also connect it from the HT to line neutral via a suitable dropper, if the rectifier is up to it, not sure what the HT current consumption is in a typical 4 valve (excluding rectifier) radio is maybe 40 ma..ish? (in relation to your 300 ohm coil which was probably series fed) If so then use that as a basis for working out a ballpark dropper resistor value.

Lawrence. EDIT: Didn't write the original very clear, have now made a few amendments to what I wrote originally.

[Radio Fixer]

That's true Lawrence but a better way would be to put the field between the +ve ends of C24 as I said and move the O/P Xformer primary to the far end of the field, which I didn't say !

This would make it a 'conventional' arrangement as used in US radios.

Gary

Edit: suggested bias arrangement for the O/P valve was incorrect. Will offer solutions if needed.

Edit: wooley thinking !! see below.

[pwdrive]

Yes, I agree with that Gary as the output valve will be passing a good chunk of the receivers current, also not much chance of over stressing anything if experimenting in the series mode.

Lawrence.

[Radio Fixer]

OK! got it wrong. What you need to do is to replace the 250 Ohm R12 with the the 300 Ohm field and then IMHO its a correct configuration. The negative bias will be higher but a potential divider across the field coil can be used to get this back to -7.8V where R11 would return to. I would guess two resistors of say 47K Ohms would be a reasonable start.

Gary

[pwdrive]

I saw a diagram somewhere that had the field coil in the -ve rail, it was tapped part way down to supply the bias.

Lawrence.

[pwdrive]

These old circuit techniques fascinate me, so why the two main types of field coil speakers, high impedance field coil and the lower impedance coil, would it be fair to say that the higher impedance field coil would make a more sensitive speaker? If so then would it be fair to say that the high Z was used in lower gain receivers and that the low Z was used in higher gain receivers or is there no particular rule, anyone know?

Lawrence.

[Radio Fixer]

Its strange that the Groom in 1941 is still using a field coil speaker when Philips was using PM in the early 30's. By post war you would have thought everyone would be.

In the early days when electrolytics were difficult to make, in large values then the series field coil was useful as it acted as a choke as well. But later then caps of 100 mF, as the Groom, were around and shunt field could be used. The shunt field has the advantage that the rectifier has load across it all the time making high rises of voltage across the electros less likely before the O/P valve warms up or if someone pulls it. But why would you want to wind thousands of turns of wire? I suppose for some it was still the cheapest way as access to high grade material for the magnets was in short supply.

Gary

[Refugee]

The strength of the magnet depends on currant multiplied by the number of turns.
If these two figures are the same the magnetic strength will also be the same making the sensitivity the same too.

[Radio Fixer]

Yes! I tried a field coil on a variable power supply once. As the volts went down the volume, by ear, didn't change that much at first. Perhaps because the magnetic material was saturated. Then it reached a critical point and the volume fell noticeably. There was no sign of distortion though.

I see if I can find the figures ...

Gary