I've found that with the 70 volt transformer, I could never get a suitable rf output. Although the audio sounds great I couldn't get any more than about 2 to 3 watts peak. I was bummed out, this was the only thing that I hadn't been able to completely figure out. The original "grenade" had a Radio Shack hash choke that had been modified into a modulation transformer by adding a secondary and more wire. I couldn't have figured it out without ripping into the transformer, this was something the original owner did NOT want me to do.
I tried a few different modulation techniques with no success. In kind of a desperation move I went down to my local audio store and got a Speco 25volt/10watt line matching transformer as opposed to a 70volt/10 watt transformer. Hooked it into the circuit and... 10.5 watt peaks! holy crap! I don't know why this one works better, could it be internal resistances?
The only problem I ran into was kinda tinny audio, but it doesn't sound bad at all. With my head phones it was hardly noticeable, so it could just be my receiver.
The Speco 25 volt/10 watt line matching transformer (part# T2510) can be found at:
Stark Electronics - $3.95