Hello,
Situation:
Radio set DeForest Crosley 405D (Canada)
Missing Speaker.
We are talking about an approx 8" unit with 2100 ohms field coil.
Seems that this resistance is less common than say a 1200 or 1500 ohm unit.



The field coil is actualy in the B+ return in order for a bias to be generated for the grid of the output tube. The divider is R6/R7 accross the field coil.
What I propose is to use a field coil with coil of say 1500 ohms and make up the difference with a 600 ohms power resistor and connect the bias divider across the two, coil and resistor that is. Does anyone see aproblem with this solution?
Regards.
David.

That will be fine. Speaker field strength will be somewhat low, but it's only a 47 output tube, so not much power output to worry about.

I wouldn't add a resistor, but do check to see that the overall B+ rail is managable. Too high add a bucking transformer. Bring up little by little on variac and measure B+ You will get an idea of what you need, 12, 24 volt buckers are common soloutions. Field coils aren't that picky, but some of the old speakers are just plain crappy no matter what you do. Another option is to use the old field coil as a choke if it is still patent, and retain the pole piece, then replace the speaker with a PM type using the original output transformer. Works does, but way out of bounds for a purist restorer. Up to you.

It's the cathode bias resistor for the 47. Well actually it carries the cathode current for all the tubes. But it needs to be about the right value, unless you wanted to diddle with the ratio of the divider across the field coil. I think it's preferable to substitute the field with the right value, and leave other components alone. That's easier for the next owner to understand.

Thanks Alan and Codefox,
If I can't find a suitible 2k ohms speaker. I will go with the resistor method plus a 1500 ohms coil and see what happens.

Codefox. I am more inclined to use a bucking transformer if I was experiencing Hi line voltage. If I use one in this case I would be reducing all voltages which I don't think would work out. Maybe you were thinking of bucking another way or for another purpose?

Regards.
David.

David

Most important to have bias voltage on the 47 correct after replacing the field. Grid #1 should be negative around 15-17 volts when measured from filament on the 47.

As mentioned there are several ways to get this voltage. Use correct field resistance, add a wire wound resistor to make up for a lower coil or change voltage divider resistors shown between two electrolytic filter caps on the schematic. I would still check this voltage once components are replaced. Over time divider resistors may have gone up in value?

Norm,
Thanks for the input. I had gleaned that bias info off a Cunningham tube data sheet. I am going to start looking for a speaker soon.
David.