Replacing field coil speakers with permanent magnets - Output transformer placement

[Resonator Guy]

Field coil speakers usually have the output transformer mounted right on the basket. When replacing the speaker with a newer permanent magnet speaker (only when the FC speaker is broken) I have used older PM speakers with a mount already made for an output transformer,
Any newer speakers, other than a Weber I bought, don't have spots for the OT.
Has anyone tried carefully drilling holes to mount an OT on a newer speaker? Other than possibly poking holes in the cone, does anyone see a problem in doing this?
I have "borrowed" several speakers from other Gibson amps to replace FC speakers, but now I would like to put them back.

[km6xz]

Another consideration is hum. The power supplies in old electromagnetic speakers were serving double duty, as field coil and power supply smoothing choke. You will have to install a choke if you want the maintain the same power supply output voltage. If you do not mind a lot more sag, a resistor can be used. Old gear that would be using a field coil was designed for lower voltage AC mains, 110volts so with choke installed make sure the caps are not exceeded in max voltage rating.

[Resonator Guy]

Yep, I have replaced several, and now I wish to re-replace them. Just concerned about drilling holes in newer speaker baskets. I have used power resistors in some and chokes in others where there was excessive hum. They all sound good but I want to put the Gibson PM speakers back in the original amps.

[km6xz]

I would be hesitant to drill baskets, that dd not already have a mounting plate and screw holes, for fear of distorting the shape when screwing the transformer between two arms of the basket, in addition to the possibility of getting filings in the gap. Mounting on an L bracket attached to the surround mounting bolts or into wooden cleats or the case wood would work without damaging any vintage originality if you want to go back to stock. Most amps that used field coils were pretty low power so the transformer is light and small so mounting options are aplenty.
Good luck

[Resonator Guy]

The L bracket idea sounds interesting. I will look into that. Most of my older amps have such thin cabinets, I wouldn't try mounting to them, but something on the speaker mounting bolts might do.

[Resonator Guy]

KM, your suggestion of a bracket on the speaker mounting bolt worked perfectly on my Gibsonette. Now it has a new Jensen and my 64 Skylark is back to stock. Thanks for the idea. Two more pairs to go!