OT Don't read this (was Re: Altair MBL source)

[Tony Duell]

> >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.

ALmost all UK mains-powered radios did that. We had things like the EBF89 
(2 diodes and an RF/IF amplifier pentode), EBC41 (2 diodes and audio 
triode), EBL31 (2 diodes and a high-slope output pentode), etc.

> 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