Replacing Field Coil speaker

[Valvehead]

Something from a thread in vintage amps -- Hope some more guys can see this -

Im working on a 1938 National amp and the Field Coil speaker is toast .

Id like to throw in a regular 10" . So I would need either a choke or big resistor ?

the scem shows the coil and just says 1K ohms.



http://www.valcopages.com/Schematics...al-dobro_c.pdf


Can i get any detailed info on what resistor / where / etc.. to make this safe with a regular 10" speaker ?

Im assuming we are trying to match the impedance or resistance of the field coil ?

[Prairie Dawg]

Here's some info. Hope this helps.

Antique Electronic Supply - Speakers

[Enzo]

The coil resistance needs to be there. You could mount a choke, but I'd probably just mount a 1k resistor of a few watts. I have no idea what the current draw through those old tube might be without doing some research. And I'd then update those 12uf filter caps to 22uf.

That is an old schematic drawing, so just a note for you, those resistors with an M? The M means thousand, as in Roman Numerals. Those are not 100 meg or 20 meg resistors.

[Gtr_tech]

You'd be better off with a choke if there's room to mount one. Better stability/noise rejection on the screens. Something intended for a Deluxe type amp would be plenty. You could try it both ways (resistor vs choke) and see which way you like it.

[Valvehead]

thanks guys ! im glad you chimed in with that M , enzo !

Now , Looking at the ol girl , looks like i can clip the 2 wires and install at that point , correct ?

Heres a pic

[Valvehead]

Fired it up today with a 1k/10w resistor . It does work , but it sounds like its got about 1 watt! i have it driving 8 ohm speaker



also- looking at the scem--what should the bias be in this ol thing?

6V6 the plates are at 270v

[Prairie Dawg]

Before you get any farther you should probably check all the resistors for drift. I've found resistors in stuff of that vintage wildly out of spec.

[olddawg]

Before you buy a new speaker, you might want to check out the price of getting the old one re-coned.

[soundguruman]

Something from a thread in vintage amps -- Hope some more guys can see this -

Im working on a 1938 National amp and the Field Coil speaker is toast .

Id like to throw in a regular 10" . So I would need either a choke or big resistor ?

the scem shows the coil and just says 1K ohms.



http://www.valcopages.com/Schematics...al-dobro_c.pdf


Can i get any detailed info on what resistor / where / etc.. to make this safe with a regular 10" speaker ?

Im assuming we are trying to match the impedance or resistance of the field coil ?

The sad thing is that field coil speakers sound much better than modern speakers.

[Prairie Dawg]

That's right.

[J M Fahey]

The sad thing is that field coil speakers sound much better than modern speakers.

Agree.
They have the best of both worlds: featherlight cones, coils and adhesives, coupled to scary high (even by today´s standards) field strength.
Those field coil Leslie speakers were real wall shakers with very low (for us) power input.
Maybe it´s not cost effective, but I´ve been toying for ages with the idea to build a couple, just for kicks.
Oh well, so many interesting projects, such a short Life.

[gcmillgc]

Orange County Speaker Repair will re-cone and coil the speaker for moderate bucks. They've always come thru for me with the right stuff. G