small valves and RE: OT

Tony Duell ard at
Sun Apr 24 16:02:54 CDT 2005

> >
> >Subject: Re: small valves and RE: OT
> >   From: ard at (Tony Duell)
> >   Date: Sat, 23 Apr 2005 23:45:16 +0100 (BST)
> >It would be interesting to know what's going on in this set. Can you get 
> >a schematic? Being an American set, it's not in Poole and Molloy :-(
> It's an RCA 8BX5  lineup is 1R5, 1U5, 1T4, 3V5 and 117Z4 if memory serves.

1R5 = DK91 == Pentagrid changer
1U5 = DAF92 == Detector and 1st audio
1T4 = DF91 == IF amplifier
3V5 isn't in the equivalents book, but 3V4 = DL94 == audio output

And of course the 117Z4 is a rectifier with a 117V filament.

The signal valves have the filaments as I described in my last message 
(i.e. 1.5V each apart from the output which is 3V centre-tapped). A 
battery-mains set would nrarly always have them in series (on mains, 
they're fed via a dropping resistor from the B+ line, putting the 
filaments in series reduces the dissipation in that resistor). That 
explains the 7.5V part of the battery. The 1.5V part is still a mystery. 
It could well be bias for the audio output valve, but what the heck does 
it do on mains? self-bais on mains only. I could _just_ believe that.

I am sure I don't need to warn you about this, but it's almost certainly 
a live chassis set when run off the mains. 

> I don't have the schematic for the set.  I expent I could find it if needed.
> Being functional I'm reluctant to tear into it and hand draw the schematic.

I know the feeling, although doing it while the set is still working 
would allow you to note down valve electrode voltages, etc.

> If it breaks I will.  Believe it or not I use it to listen to local station
> for ball games.

No why on eart hwould oyu want to do that???


> I know the magazine, rarely see it.

Oddly we can get Nuts and Volts and more usefully Circuit Cellar in shops 
in the UK... IMHO Circuit Cellar has really gone down over the last year 
or so, it seems to be just yet-another-microcontroller-circuit now, and 
quite often the microcontroller picked doesn't seem to be the most 
appropriate device .Oh well...


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