How to add dirt to a glass shader? I am rendering an image of a grotty old looking light-bulb. I can make a decent clear glass material, but, not a perfect one.

I simply cannot get the dirt textures to overlay properly.

How can I add dirt to my glass shader without ruining the glass material underneath it.

There is also an emission shader on the filament inside the bulb.

I tried looking for tutorial but they are all based on non transparent materials.
You can try adding a scratches map to your glass material. A quick Google search for "scratch bump map" can lead to some good results.

For the nodes, I would suggest two Glass BSDF shaders, one with 0 roughness and one with 0.4 roughness, run into a Mix Shader (0.4 Roughness one in top shader input). Use the image as a fac in the Mix Shader. You can add a Bright/Contrast node between the Image Texture and the Mix Shader. Adjust the Bright: value to affect the roughness.

Unfortunately, I can't post screenshots right now.