The AM Forum

I bought this "mystery" board at a local hamfest one year because of its vintage. I shelved it and I took a closer look at it today. Two 01A's and One 76 tube with a what I assume to be a 5 volt filament transformer. The 01A's have 5 volt filaments and the 76 has a 6.3 volt filament. Odd ? The fahnestock clips note headphone connections and B-. And the variable resistor mounted on the board. Is this a Code practice oscillator from the early 40's ? I havent had time to diagram out the tube yet.

maybe it is a small amplifier. CPO would have to have some resistors, capacitors  or transformers. Actually, where are the passive components?

It could be an amplifier, most likely for audio work, since I have read about basic amps that use nothing but tubes.

The variable resistor is the only passive component. I would imagine perhaps if this is an oscillator of some sort, they leverage the interelectrode capacitance in the 01A's somehow for the feedback.  ??? The lack of an input connection leads me to believe it is an oscillator of some sort.

It could also provide some adjustment for the plate voltage to the 01A, which may had varied the tone if it was an oscillator.

It does not look like either a code practice oscillator or a receiver.   

Rather easy to trace out the circuit, just from the photo, assuming 01A triodes in the left and center sockets, and a 76 in the right socket.

All filaments are tied directly to the transformer secondary.  One side of the filament circuit is tied to the left fahnestock clip in the front row.  There is a variable resistor across the filament supply, with the wiper going to the grid of the middle 01A, and that lead is carried to the second fahnestock clip in the front row.

The grid of the left 01A is connected to the filament.  Does not make a lot of sense, a diode connected triode usually has the grid tied to the plate.   Plates of all tubes are tied together, and to the far right fahnestock clip.

The 76 has both screen and control grids tied to the third fahnestock clip.

After looking at the circuit, my guess is this might be a test jig for testing tubes, or possibly a breadboard to support tubes where other circuitry might be attached externally.  It does not appear to have any direct application in it's present configuration.

I think the 76 has its grid and cathode connected together. The 76 is a triode. No screen grid.......Very odd circuit. Of course one of the headphone connections is not connected to anything.....Such a mystery !  ???

You are correct, my mistake.  I was looking at the wrong page in my 1937 RCA Receiving Tube Manual.  It is puzzling why they would connect grid and cathode together????

In old breadboard rigs, it was customary to connect the headphone between the plate of the audio output and the B+ battery connection.  Perhaps the double fahnestock clips at the upper right in the photo were intended for the other headphone connection and the battery + ???  But what is the rest of the circuit doing?  'Tis a puzzlement! 

Yes, I just noticed the B+ written on the opposite edge of the breadboard for the other headphone connection.  ;D

The pot is most likely for filament balance to eliminate the hum. The first socket looks like it is wired up to run the 01A as a diode?  None of the wiring makes any sense if you follow it out.

Pete

It may be the beginnings of a tube tester or rejuvenator.

Naw, I think it is a prototype for a fuse tester... ;D

 :) This is an interesting thought

Stephanie

It looks like a lteaching tool (student lab experiment) to demonstrate the difference between:

a) A triode whose filament is also the cathode: B- attached to one end of the filament... and to the grid (hum).

b) A triode whose filament is also the cathode: B- attached to weighted combination of both sides of the filament (via the pot)... and to the grid (hum can be balanced out)

c) A triode whose filament is separate from the cathode: B- attached to cahtode... and to the grid (no hum)

Procedure: 

i. All three tubes plugged in (or one at a time if you wish)

ii. Attach headphones to plate bus and to B+

iii. Attach B- to only one of the the three clips on the long side of the board. Left 1 gives you case a) [hum heard in headphones].  Middle clip gives you case b) [hum can be balanced out]. Right clip gives you case c) No hum.

Best regards
Stu

Ah ! This makes sense now  :)

Stephanie,

Looks as if "Professor Stu" hit the nail squarely on the head!  Last night I stumbled across the exact schematic for your device in an old (1943) text, Elements of Radio..  I will attempt to attach a scan of the schematic pages from the "demonstrations" sections of the book.

73,
Rick

No kidding  :)

clarivoyance.