The identity of the two tubes is a big deal. Good chance that one of them is a 6AQ5. Its only job is to sense the radio's carrier and activate the relay.
The other tube will be a slightly-larger 6BQ5 or 7189 (same thing) if it's a JB-12. Won't see much more than 30 or at best 40 Watt peaks. Only lasts very long with the carrier control on the rear turned down to 3 Watts or less.
The other possibility is the JB-150, using a larger tube like 8417. It's bigger, tends to get you 100 Watt peaks or more. Can tolerate up to about 5 Watts of carrier.
They're popular as drivers, since they allow you to reduce the carrier level while maintaining high modulated peak power. Typical nickname for one of these is a "modulator", even though the textbook meaning of that word is something altogether different.
And if it contains 40 year-old electrolytic caps, they should get changed before they release any magic smoke from the other parts inside.
The fuse found in these is usually sufficient to prevent it from burning down your house. But the size used by the factory won't protect it from being run too hard for too long.
Enjoy it while it lasts.
73
