[fields 100]

What are ramifications of replacing the speaker with 1100 (appr.)ohm field coil with 450 ohm field coil? Same voice coil resistance.

[morzh]

Higher B+ voltage. Depending on your load the degree may vary.
Tubes will run hotter, agai, how much depends.
Might be still within specs, might be not.

However there are radios where the coil is only a load and not a filter. This will increase your rectifier current but will not increase B+. Those are mostly small AC-DC radios.

[Steve Davis]

Also, being there would be less voltage drop across the coil, the field would be weaker. I'm not sure how much difference it would make.

Steve

[morzh]

Steve

Thing is, a particuar coil is tailored to its speaker so whatever field it produces is adequate PROVIDED the current through it, which is dictated amongst other things by load and applied voltage is what it was designed for.
Also less rsistance does not necessarily mean less field as there might simply be a thicker gauge wire used, and again, the magnetic field, or magnetomotive force is plainly speaking Amp-turns, which means that if you have half the turns and twice the current, your field remains the same.

[Arran]

So what was wrong with the original speaker? Cone torn up? If there is that much difference in DC resistance it sounds like the 450 ohm job might be for an AC/DC set or some oddity like a DC mains or 6 volt vibrator radio. The lower resistance could result in higher B+ voltage but more importantly it will not filter as well. If you absolutely must use this speaker with the 450 ohm filed coil you should add a filter choke in series with it to make up for the difference.
Regards
Arran

[fields 100]

Sounds like very iffy thing.

[fields 100]

Arran,

If I install the filter choke to compensate for the lesser resistance, won't the max. output suffer due to the lower current going through the field coil?

[Arran]

It won't if the field coil and the filter choke equal the combined DC resistance of the old speaker, the current draw is the same either way. But again what happened to the original speaker? If it can be repaired you should give it a try.
Regards
Arran

[morzh]

The lower resistance itself does not mean worse filtering, it CORELATES with worse filtering as it corelates (not by law by by practice) with smaller inductance, and the inductance provides the main filtering in concert with a cap after it.
It is possible to have a larger inductance with smaller resistance and vice versa.

The typical resistance being 1K and cap 10uF give RC=10ms which is not effective filtering for AC Mains with its T=16ms, this is why resistance alone matched to coil's does not filter as well.
However when subbing a smaller coil with extra choke, ir is dsirable to match DC resistance reasonably close to original as the resistance determines the B+ serving as part of the divider.

So, if you subbing a Perm magnet speaker, choose a choke with a reasonably close inductance and current ampacity, and if the DC resistance is smaler, add a resistor to bring it clise and with proper dissipation ratin.

If using a field coil speaker, in practice unless the coil itself is way smaller, I would not add a choke but simply would add a resistor, again not as much to help filtering but rather to kep DC voltages the same.

Not an exact science, there will be compromises to make.

[fields 100]

Morzh,

You correctly mentioned earlier that the amp/turns determine inductance. Won't,by adding either the choke or resistor for that matter, reduce the current, cutting down the max. available speaker output and if so, how bad this reduction roughly will be ?

[morzh]

It cannot be told off hand, you need to know

- what current the speaker's field coil is designed for
- what ACTIVE resistance you are adding in series (either choke's DC resistance or resistor, or oth if you add both)
- field's own DC resistance
- consumption of the radio.

Now, for the most part, from my understanding of tubes, average current consumption does not strongly depend on voltage so adding active resistance will not affect it as much (to a point of course). So, if you stick a field coil that was designed to work with 50mA into a radio that work with 10mA, it will be weak, and vice versa, it will be too much and no amount of added resistance will help.

So I would look for a speaker that was designed for a similar radio, and if a resistance is less I would add some to compensate.

Like I said before, it will be a search for a compromise.

In practice though most speakers will work in wider range of parameters scattered around of what they were designed for and you have to look to not bring it into some extreme incompatibility mode, where it will be too weak or too overloaded, otherwise it will probably be fine.

And an extreme case is easy to see.
In other words, best is enemy of good enough

[codefox1]

Field coil is just a choke. If you are subbing a PM speaker, jsut make sure the B+ is manged well. If another field magnet speaker is subbed, as mentioned might be a little lower in volume if turns are fewer.