Teaching Ruby to Purr

My orange tiger cat, Ruby, is a big cat, but she has such a little purr I can barely hear it. 

Not only that, but she hardly ever purrs, only when I slick the top of her head and not every time, even then.

"Ruby," I tell her, "it's not seemly for such a big cat to make such a little noise."

She rolls her eyes at me, but she says nothing.

The veterinarian says Ruby's purr is normal, and so does the humane society, but I disagree. What do they understand about social niceties? 

Obviously, it's up to me to teach Ruby how to purr louder.

First I trained myself how to make a low continuous vibratory purr, like a cat's but much louder. Then every time I stroked Ruby, I purred. Whenever she purred, I purred louder. 

This went on for weeks, but she didn't change an iota, except she laid her ears back.

Maybe cats don't learn from humans. Maybe I need another cat. I think I'll go to the humane society and pet kittens until I find one with an appropriate purr. Then I'll bring him home and shut him in a room with Ruby. Maybe that will work.