I have an admiral portable that i picked up on ebay. It uses series string filaments, and when i got it, the 12c5 audio output tube was broken. Would it be possible to use a 6aq5 if I rewired the socket and put a resistor in series with the filament string?

TDSL indicates that a 12CU5 might be a possible sub in some situations
http://tdsl.duncanamps.com/show.php?des=12C5

You can buy a brand new 12C5 from www.tubesandmore.com for $1.80 so why go with the jury-rig repair?

I'm anxious to get it going as usual. I just ordered one, so now i will wait.

What matters in a series string is that all the tube filaments draw the same current. a six volt difference in the string can more or less be absorbed, but in order to drop a 450ma-heatered 6AQ5 into a 600ma filament string you'd have to put a resistor across its heater to lower the resistance between pins 3 and 4 to 10.5 ohms. Across--as in parallel, not series. Then the string thinks its seeing a 6-volt, 600ma tube. Easy as scrambling the rest of the connections, but obviously doesn't even speak to the match of the tube to the OPT etc. A really, really squirrely quick solution to the problem when the only problem is the wait.