possible to mine whit an 7990radeon 6gb ??
Basically ever since ZEC was released in November of 2016, most Tahitis like your 7990 went and mined it. The Tahiti's due to the DAG thrashing bug were performing poorly with ETH so most of them did the switch to ZEC. Same with the Pitcairn GPUs like the R9 270X.
Now however most Tahiti's moved onto Cryptonight-V7 and are mining XMR. Since the XMR algo is not very intensive most do a max undervolt to like 0.95V and run at 950/1500 (core/memory) and hash at 500-600h/s (depending on memory type) and it only uses around 125 watts or so. Basically like an RX 470 mining ETH.
Not much profit these days but if you want to put them to work, you can try that algo. I think next month it forks again, so the first week or 2 might be even more profitable as the difficulties adjust.