hi everyone im a bit new to making my own bow having only ever shot them and given taster sessions to kids before so please forgive me for being the newbie  
Im looking at making a two laminated 70 inch bow about 50draw weight and was wondering if an ash backed oak with a maple power core would be the right combo.
any advice would be welcome even on what to read
thanks

I've heard of many doing hickory as the backing like 1/8 inch thick.
Someone on here should know if the ash,would work,as well.
Probably.

Steven,
I'd say if those are the wood materials you have, I would use the maple for the backing and the ash for the power lam.  I have never used ash for a backing.  It may be ok.  I don't know about it.  I do know maple makes a good backing.  If you had access to hickory it would also be a good wood for backing.  IMHO

The only woods that I have used for backings are hickory and bamboo so I would not be much help.

I wouldn't use ash at all if it was me.  Find some Hickory if at all possible.

thanks guys will go out and search for hickory or bamboo for backing. i have some oak which came down last summer drying which should be ok thanks again all

If the hard maple is straight grained it doesn't need a backing unless you are trying to glue in some reflex in which case you could use a strip of maple to back the maple with. For no reflex, but for extra protection you could use rawhide or linen. I have one ash backed elm bow that was gifted to me which has held up well, but never used ash as a backer myself. I have made a number of bows with hickory, white oak, and bamboo as backers with good success.