22 Apr
2005
22 Apr
'05
4:11 p.m.
The most
complex common multi-section valve in the UK (i.e. with the most
sections), is the triple diode triode. It came with a variety of heater
ratings -- the EABC80 (6.3V heater), UABC80 (100mA series string heater,
by far the most common version) and the oddball PABC80 (300mA series
string heater). It turned up in numerous AM/FM radios in the 1960s, where
it was used for the AM detector (1 diode), FM detector (the other 2
diodes) and audio ampifier (triode).
Hi
Higher quality AM receivers used the two diode tubes. One diode was for the detector while the other was
used for the AGC. That way the loading could be optimized
for each function.
The main reason was to provide a 'delay' (in voltage, not time) for the AGC.
But most AM/FM sets didn't bother. They had a total of 3 diodes, 2 for
the FM detector, one for the AM detector. I've never seen a quad diode
valve (well, OK, I have, it was a double full-wave HT rectifier, but
that's not what we want here). And I've never seen a set that switched a
diode between the AM AGC and FM detector stages as approriate.
Whether there existed sets with, say, an EB91 (double diode, I think you
call it a 6AL5) for the FM detector and an EBC91 (say) for the AM dector,
AGC, and 1st audio, I don't know, and I am not looking for such a
schematic tonight!. When germanium diodes became common in domestic
radios, it was, of course, quite common to fit 4 them (or fit 2 for the
FM detector, and keep the double-diode triode valve for AM detector, AGC,
and 1st audio functions).
-tony