I've used filters between the light source and condenser as well on my Leitz; heat resistant thick Oriental VC filters I cut to size. Not as convenient as under the lens.
The link works for me That's what I used for many years with my 5x4 enlarger, I still use them occasionally with other enlargers very easy and reliable.
You need a below-helens filter attachment and don't worry about a quality loss because of it; there is none as long as the filters are clean.The one from Alford is most convenient.
Can you remove the thing that screws onto the red filter? You don't need it. The kit Ian linked to comes with a red filter, if you like using one. Honestly, I haven't used the red filter in decades.
Can you remove the thing that screws onto the red filter? You don't need it. The kit Ian linked to comes with a red filter, if you like using one. Honestly, I haven't used the red filter in decades.
You just get a longer bolt and use three nuts: one nut to fix the bolt's end into the opening. And the other two nuts you position lower with in between them a filter holder. Look at a metal Beseler 23 filter holder or any other brand. All it will take is to drill a hole in it's arm