A couple of basic questions

[Jim R.]

First question is for PJ -- just curious why you chose to run the filaments at 1.5v instead of 3?  Is this a center tapped filament as on many of the 12v noval signal tubes?

And second question is can somebody give me the pinout for the 3s4?

I'm contemplating my build and this information would be useful to have.

Thanks,

Jim

[earwaxxer]

Hey Jim - I saved this white paper on the 3s4 - I'll attach it here. Not sure if this will help.

[Jim R.]

Hey Eric,

Thanks for trying, but it appears to be a scanned image file so my screen reader can't do anything with it.

--- Jim

[earwaxxer]

It should download a pdf if you have the pdf reader installed.

[Grainger49]

Jim,

The 3S4 is a single pentode.  Pins 1 and 7 are the heaters with a center tap at pin 5.  That is the key to a 1.5V heater. 

It looks like the top grid (suppressor?) is internally connected to pin 5, CT of the heater.

The control grid is pin 3 and pin 4 is connected to the plate, externally.

I hope this isn't too much of Bottlehead's design.

[Jim R.]

Eric,

Yes, it is a pdf, but the pdf is an image file, not electronic text.  From a screen reader's POV, not all pdfs are created equal.

Thanks Grainger, that should get me going.

-- Jim

[Paul Joppa]

The filament is as Grainger said but there are some notes in the data sheet:

If the filaments are paralleled, pin 5 should be the negative and pins 1-7 the positive. If they are in series, then pin 7 should be negative, pin 1 is negative, pin 7 positive, and there should be a shunting resistor between pins 1 and 5 "to limit cathode current to the value specified" - not very clear but I suppose some of the plate current will also heat the cathode, and the most negative end of the filament will carry the highest current.

Pin 2 and pin 6 are the plate.
Pin 3 is the control grid (grid 1)
Pin 4 is the screen grid (grid 2)
Pin 5 is the suppressor grid (grid 3) as well as the filament center tap.

I chose parallel filaments mostly so I could use two D-cells instead of four - remember the filament power has to float to allow independent bias of the two tubes. But at the very low plate voltage used in Quickie, the uneven current referred to above becomes much more of an issue, so parallel filaments will last nearly twice as long as the active end of a series arrangement - that's serendipity though, not engineering!

[Jim R.]

Paul,

Thanks -- I thought there might be more to it than meets the eye.  Gotta love that engineering serendipity!

I've actually got another potential tweak, but I'll keep it quiet until I get mine built and can try it first.

Thanks,

Jim