Conversation so far:
how did Lee de Forest amplify the audio frequency?
by feeding it into a second tube for additional amplification.
did he have any problems?
It was found that de Forest's "gassy" version of the Audion could not handle even the relatively low voltages used by telephone lines.

Question: how did he fix it?
Self-contained rewrite: how did Lee de Forest fix an issue with the voltages used in telephone lines ?