RL-70H-6 Replacement

[Tube Radio]

Not too sure where the amp cuts off the frequency response. I would advise you connect a small value cap (selected so that frequencies above 16 Khz are attenuated) from the plate of the first audio tube and ground. That should solve any potential feedback issues. Another method would be to insert a variable resistor in series with the cap connected to the plate of the first audio tube and ground. Mount the resistor on the back of the chassis. Now you will have a treble control which will be useful to balance the sound between the 12 inch speaker and the tweeter. As to mounting the tweeter what you could do is find a metal strip that will go between two of the speaker mounting bolts on opposite sides of the speaker and mount the tweeter to that. Tweeter will now be in front of the main speaker cone so the speaker shouldn't be too large from front to back. What would probbably work best is a Magnavox tweeter (6 inches @ 3.2 ohms IIRC) which may be found in almost any late 50's Magnavox console (may also be labeled Columbia as Magnavox made a few models for them). You will have to experiment on the value of cap you put in series with the tweeter determined by what sounds best to you.