Hi Eino,

Yes it is ok to earth the bulbs through the chassis with the digital upgrade. The only downside is you will get the flickering effect in the lights.

Cheers….

Mike

My understanding, which is admittedly limited. is that an incandescent bulb is essentially a resistor and the filament glows because of the resistance. So as long as the bulb is rated for the voltage you should be fine. If not it will either glow brightly for a very shot time or dully for ever.

The older models had the lights which went bright when you initiated a reverse pulse, so those bulbs were designed to withstand 24 or 25VAC surges. Since the 1980s, the new reverse units did not make the lights flash, so many models started to have lights with a rating of 18VAC.
When the digital system was introduced, it involved constant current which meant that lights were on all the time (when on) and did not dim at low speeds (low voltages). This meant that the lights had to withstand constant voltage and constant use, so once again, higher tolerances became an issue. For example, Hag recommended that all models that were digital or digitized should have the lights replaced.
One issue for Maerklin was that many analog models used the frame for ground return, so the lights were connected by one live lead to the circuit board and the return was via the frame. The second generation digital models started having the lights connected to the circuit board/decoder with live and return cables. The first generation models had lights that were not active in analog use and flickered in digital operation, which led to the search for a better system.
Regular bulbs can be used if they are appropriate to the model. As long as it fits in the space and has the proper connections (screw, pins, etc) and can withstand constant use at 19V, you should be ok.
Some models may have warnings with risk of heat damage from constant or prolonged use, such as the first model of the Hag SBB Re 450 where the plastic destination sign holder would melt from heat.

Regards

Mike C

No.
Same story for light bulbs and smoke generators:
If connected that way, they will only get about 75% of the power, so the "analogue" type of light bulbs (19 V) and smoke generators (up to 16 V) can be used.
If both poles are connected to the decoder, use "digital" light bulbs (22 V) and smoke generators (16 V to 22 V).

Lights will flicker, but this cannot be noticed with smoke generators.

One solution to remove the flicker in these locomotives is to use an optocoupler
eg: https://digitaltog.dk/wordpress/flimmerfrit-lys-med-optokobler/ (In danish)

and the lights remain dimable. eg you dim the aux out in the decoder and you will also dim the light.

more on it on this site:https://moba-hgh.de/modellbahn/tipps-tricks-infos/1718-tuning-der-rauchsatzansteuerung-jetzt-raucht-s-richtig.html