Horn works, except if Lights are on???

[Diastole]

Good mourning? Before I tear into the switch, steering column, etc., I thought I would pose the question to all you experts and see if ya'll could offer a noob to Model As (but, full of late 50's GM knowledge and experience, amongst other useless things) any advice?

All thoughts are helpful. My guess is contact plate problems.

1) The horn works and sounds well when lights are off. This is a new problem. Worked well three months ago, just finished replacing headgasket, manifold gasket, muffler, rebuilt water pump, checked rear mains, replaced oil drain tube, changed timing gear, and had to rotate steering column to align upper plate arm to distributor housing.
2) whether the engine running or off is same phenomenon with horn
3) car is stock, 6V positive ground. 1930 early Standard Coupe, oval speedo.
4) the voltage at the horn is 6.39 volts and at the battery it is 6.47 volts.
5) incandescent 2 bulb headlights. No cowl lights.
6) the lights do not dim and the ammeter does not deflect more when the horn button is depressed and the lights are on. Suggesting NO Connection
7) new condensor. The modern one, new from parts supplier failed after 3 mos. Measurment showed .36 nanofarads not the 242 nanofarads needed

Thanks in advance, us newbees always appreciate your help and assistance.

Diastole in the Smokies