[mg34.42]

ok just wondering dose any one have a drawing of converting a electromagnet speaker to a pm speaker. such as for a philco 90 with one output tube or a 70 and would the same process be done for a 91 whit one speaker. just wondering how this is done 
benny

[morzh]

You pretty much insert a resistor equal in ohms to the field coil ohms instead of the field coil.
And connect the voice coil same way it is originally connected.
Tsit.

[mg34.42]

what I would like to see is how a choke is used 
benny

[morzh]

Choke? If it is the original choke, it stays put.
If it is a choke that you want to replace the field coil with (instead of using the resistor), it goes instead of the field coil. It needs to have the same or close resistance to the field coil. And a smilar inductance. If the resistance is much smaller, an extra resistor is put in series with it.

[Arran]

I would be inclined to go the filter choke route, unless this is just a temporary substitution or for testing purposes, what you need to be concerned with is how many Henries the choke has, or the DC inductance, but I don't know if the Philco service info of the time gives this, probably just DC resistance. I'm not sure why you bring up whether the set has a single ended output or push pull output, it doesn't matter as both sets used the same speaker with the same field coil, the difference being in the output transformer that was used. If you want to go cheap on a filter choke, or can't locate one, you can use the primary of a small transformer, like a door bell one, maybe it isn't 100% correct but it does work.
Regards
Arran

[jcassity]

or a primary off an old PCP from something that was going in the trash.

[Arran]

It has to be a transformer designed for 60 cycles, if it's out of a switch mode supply it will not work, the frequency is too high. Since model 70s have a full wave rectifier you will have a 120 cycle ripple, so a 50/60 cycle transformer primary will be safe, you can use a lower frequency device at a higher frequency but you can't go the other way or the core of the choke/transformer will over saturate and burn up. In terms of filter chokes I think that a 50 Henry one would work fine, as long as it's 125 ma at least, it would be large enough to filter out the ripple and offer a similar drop, if not you can add a resistor in series with the setup as Mike suggested.
Regards
Arran

[Radioroslyn]

As I peruse the 70 and 90 diagrams I see that the hv current is split. Most of it is handled by the filter choke and only a small amount is being drawn thru the field coil. It's feeding the plate of the 1st audio and the sg's of the rf if stages and the osc plate. The output stage, rf and IF plate current comes from the filter choke before the field coil so the current rating of the fc is going to be about out 30ma or so.