small valves

[Tony Duell]

> I have an RCA AM broadcast Battery/mains radio that I still use.  
> Works well. The multivoltage battery though bad supplied the 
> authentic looking cover for a box I'd made using uses NiCds 
> to run a switchmode supply to provide the A,B and C voltages 
> required.  Runs for hours on that too.

I'm suprised it needs a separate C supply. UK sets all used self-bias by 
this time, I think. C (grid bias) batteries were not used for radios much 
after the 1930 in the UK.

I have a working Vidor AM/FM valve portable (!). OK, it cheats in that it 
uses a few germanium diodes in the detector stage, but there are no 
transistors. It's battery-only (officially), 1.5V LT, 90V HT. It's not 
much larger than many of the transistorised AM/FM portables that came out 
a few years later either.

The batteriers are long-since unobtainable (at least not easily). I run it
from the mains. The LT is triival to get from my bench aupply. For HT,
I've made a little PSU from common components that gives about 85V (high
enough for the set to work well). It's just a 30V transformer feeding a
voltage doubler circuit. Much easier to get a 30V transformer than
something of a higher voltage now. 

I must try the circuit that was in Elektor a couple of months back to get 
90V from low-voltage batteries. 

-tony