Do not use a red filter with orthochromatic film unless you want unexposed negatives.
A red filter will only allow "red" light to reach your orthochromatic film which does NOT see red.
"Colorblind" film i.e. unsensitized silver emulsions are only sensitive to UV and far blue light, the addition of the orthochromatic sensitizing dyes allows the film to see UV, Blue and Green light all of which a red filter will block.
If you want to shoot by natural light and have your orthochromatic (or panochromatic film respond to the same spectrum that the "colorblind" film see, you need to only let those colors through, UV and Blue light is what you are after, so use a dark Blue (that Wratten #47B for example). If shooting indoors or under artificial light, just use a Blue light source.