24 Feb
2010
24 Feb
'10
10:01 p.m.
On 24 Feb 2010 at 21:18, Tony Duell wrote:
IIRC, there were American valves (perhaps I should say
tubes :-)) that
contained a half-wave rectifier and an audio autput pentode in one
envelope. The number 70L7 springs to mind, but I am not well up in US
valve numbers.
They were desinged for use (in the obvious way) in lo-fi record playes
(a crystal pickup cartridge would give enough signal to drive the
pentode section).
I am suprised your conde practice oscillator didn't use one of those.
Cost, mostly. Even at contemporary prices, the 6SL7 was far cheaper
and more common than a 70L7 or the more usual 117L7/M7/N7. The dual
triodes were part of most TV sets and a fair number of audio
amplifiers.
I remember some "universal" battery-line sets that used a 117Z3
rectifier to drop the line voltage and also to supply the filament
voltage for battery sets when run from the AC line. This was before
selenium rectifiers became common.
Did it have haedphones as the audio output devices? If
so, it only
takes one little insulation breakdown to turn it into a fair version
of the electric chair...
Yes, but it used a resistor as a plate load and a capacitor coupled
to the headphones. An interstage AF transformer would have been
much safer--but again, cost was an object.
If you were any kind of a scavenger, getting on the air meant at most
finding an old AM radio or two and a TV set. You could go to a swap
meet for transmitter crystals or beg one. A 6v 150 ma pilot lamp
would serve as an adquate plate current indicator. Coils could be
easily hand-wound.
--Chuck