original schematic

I'm trying to figure out what's going on with these capacitors, so I can replace them in an old tube radio, but why are they connected in this way?


simulate this circuit – Schematic created using CircuitLab

  • 2
    \$\begingroup\$ L1 should be 24H. If you read data sheet, they form a filter block to eliminate 60Hz. They also discuss changes between 25Hz and 60Hz. \$\endgroup\$ May 24, 2015 at 22:43
  • \$\begingroup\$ This is a pi-section constant-k or m-derived filter. \$\endgroup\$
    – user207421
    May 24, 2015 at 23:32
  • \$\begingroup\$ Actually, being a filter for a full wave rectifier, it eliminates 120Hz. \$\endgroup\$
    – user16324
    May 25, 2015 at 8:42

1 Answer 1


From the schematics it seems some sort of smoothing filter for a full-wave rectifier. At 60Hz the fully-rectified voltage will have a 120Hz frequency and that filter has a notch about at that frequency, as you can see from this LTspice simulation:

enter image description here

enter image description here

On the schematics there is also what seems "RECT-80" (to the left of the input of the filter), which may hint at a rectifier (the symbols they use are unknown to me!). It might be a double vacuum diode (wild guess), i.e. a valve with two anodes and one catode which was used to rectify current. The symbol may indicate that the cathode is one with the heating filament of the valve.


It turns out that there exists an old rectifier vacuum tube named simply "80".

In that page there is a link to its datasheet, in which you can see this:

enter image description here

So effectively this confirms my initial guess.

  • \$\begingroup\$ They forgot to put envelopes on the tubes (no circles around the tube elements). The Type 80 was a very common full-wave rectifier, with a 5 volt filament which also served as the cathode. \$\endgroup\$ May 24, 2015 at 23:22
  • \$\begingroup\$ These old tubes have two digit part numbers. 80 is a full wave rectifier. The angled part is the filament (heater) and also the cathode. The two adjacent rectangles are the anodes (plates). It would be a little clearer if they showed the glass envelope (a circle) around the elements (as they did later). \$\endgroup\$
    – gbarry
    May 24, 2015 at 23:23
  • \$\begingroup\$ I was checking out the tubes on this beast the 240 series came out circa 1929 \$\endgroup\$
    – Old_Fossil
    May 18, 2016 at 17:58

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