[MZip]

Hey MrFixr, thanks for the kind words and also for all the great info!

I’m with you on the Burgess and RCA covers. Definitely my favorites too.

When I started on my friend’s radio a few things were really obvious and easy but the battery box thing has been more of an adventure and at the start of it I didn’t appreciate or know all the nuances. I found information online but come from an op amp and digital electronics background.

I found the tube data sheets and they call for 1.4V and I found another article talking about care and feeding and how low voltage can be bad through poisoning. So naturally I started with a DC-DC converter to properly feed the tube filaments. Directly-heated cathodes don’t really like that switching noise it turns out so did more digging and that’s why sensitivity went way south.

I was trying to keep filament voltage pegged at 1.4 and the DC-DC converter I used had a neat feature of dropping out instead of letting the voltage sag like a battery or LM317. Since it’s my friend’s radio, he isn’t especially handy with electrical stuff and I was looking to have it work or not with no in-between.

I tried some neat little pi filters used to trap out drone motor controller noise in FPV drones and they make a big difference in knocking down the harmonics on the converter input and output but the cutoff is too close to the fundamental so it’s still there - just smaller and rounded off.

I get the radio back Tuesday and want to try the DC-DC converter still to see if knocking down the harmonics is “good enough”. I put a cap on the output as is and it really helped so hopefully the pi filters are better enough. But the fallback is maybe the LM317 or just straight 1.5V battery and a resistor.

I know what the article said about care and feeding but in practice, how tolerant are the 1.4V direct-heated tubes to filament voltage? I know the originals were battery powered but tubes were plentiful. With them getting harder to find, I want to be as easy on them as is necessary but I don’t have much feel for what range they are ok with.

All that data is very handy. Thanks! Still sorting out the details how the filament supply will ultimately go. I don’t want to appreciably change sensitivity but also, my friend may use an AM transmitter and Bluetooth receiver to make it where they can play music from phones and tablets so sensitivity may not be so important to him.

On the cover designs, I used battery images from the web to draw them up. I saw where one person had printed the Philco wave version and used it to wrap a box and that looked cool. I 3D print a lot so making the box and covers was really easy.

I’ll know more soon if I can get that DC-DC converter quiet enough. I know just the cap made a big difference in the noise but not so keen on what frequencies are actually limiting sensitivity and if the pi filters will help enough. Will post what I find. I do like the fast dropout of the converter vs a long slow ride down the battery curve. If it was mine, no problem. But it needs to stand up to use and demos without monitoring filament voltage.

Cheers!