small valves

[Tony Duell]

>  The direct heated cathodes turned on fast enough that
> one didn't have a noticeable delay. It wasn't until the
> indirect heated cathodes that things changed. That was
> because it was too hard to build an AC radio with the
> directed heated tubes. There was both a hum problem
> and it was more difficult to get each tube biased correctly.
> Once they had selenium rectifiers, direct heated became
> practical for low power applications ( like the Zenith TO's ).
> They still had to do some tricks to get the biasing
> working right for each tube. Some clever engineering.

I have a UK battery/mains radio that uses valves throughout (even the PSU 
rectifier) and directly heated valves in the signal stages. The filaments 
are in series (carefulyl ordered to get suitable cathode bias voltages), 
for a total of 7.5V. The batteries were a 7.5V LT and 90V Ht pair. On 
mains, there is a mains transformer, the output of which is full-wave 
rectified (EZ40 IIRC). That provides the HT (B+) directly, and the 
filament supply (A+?) via dropping resistor.

One think to be wary about. There is a capacitor across the filament 
string, after the capacitor. If the filament string is open-circuit for 
any reason, then that capacitor will charge to about 90V. It normally 
withstands that (although it's not rated to do so), but if you then 
replair the filament strign, there's enough energy in that capacitor to 
burn out one of the filaments. I always make sure that capacitor is 
discharged before fiddling with the valves.

-tony