I have an open primary interstage transformer in a 3 tube battery regen radio, after the detector. To bridge this, what resisitance would you recommend?

To do so, disconnect the transformer and use from 100,000 ohms to 1 meg on the primary side (from plate to B+), and then use a condenser from .005 to .02 MFD, chosing a value that yields the best audio response, and then use a resistor from 1 to 5 meg on the grid on the next tube. If you use a resistor around 1 meg on the primary side, and a resistor around 5 meg on the secondary side, you will get more gain (to a degree...some tubes won't work well with these high resistances used in more modern radios). There will be more bass response, though, so a .005 MFD condenser or slightly higher should be used. Start with these figures and work the other way (lower resistances, higher capacitances). Experiment with each component separately if you wish.

Audio interstage transformers are available through www.tubesandmore.com, though, and you can find some old ones on eBay.

T.D.