Seeking help – connecting a single Scott field coil speaker to a two speaker AW12D chassis

[Stuart Smolkin]

Our museum (avlradiomuseum.org) has a later Scott AW12D chassis (AVC with Wunderlich tube) which we believe originally had two speakers (because pin 6 on the Jones jack is used, with a capacitor connected to it).

We recently acquired a Scott field coil speaker (the kind that sits attached to its own metal stand - see photo). The field coil is connected to the two fat pins on its plug and its resistance measurement is around 760 ohms.  Scott%20speaker%20photo%20from%20Estes%20auction.jpg

QUESTION:  Is it possible to connect this early AW12D field coil speaker to our later model AW12D chassis?  If so, what (if anything) needs to be done to accomplish this?

For reference, we briefly connected this speaker and it appeared to be working well (we measured current draw while doing so and it seemed OK).  But, after powering down the radio we tried again, only to discover that the 80 rectifier tube had failed.  We replaced the 80, and the replacement started glowing cherry red (so we shut down immediately). After discovering and replacing a shorted electrolytic, testing showed no apparent damage to the radio or power pack.  But, we wonder if our “early” speaker’s field coil is not the correct resistance for this chassis and is causing the problem.

Thanks in advance for any reply, we appreciate the assist.

[David C. Poland]

Not a problem  if the correct 12 inch pedestal.  The speaker casting detail appears correct.

FYI - the 1932-1934  Allwave Deluxe 12 tube with Wunderlich tube  was the December 1932 upgrade to add AVC.  This upgraded AVC model continued to use the twin speaker flange mount 7 inch and 9 inch field coil Scott badged ROLA pair. Aproximately Spring 1933 Scott began instead supplying the 12 inch pedestal Scott badged Jensen auditorium speaker with part number 3569 ink stamped on the side of the pedestal. So, backwards compatible.

The 12 inch pedestal is electrically equivalent to the earlier twin speaker set. Both had the 6 pin speaker cable plug. (The earliest Deluxe version amp had a 5 pin plug and continued to use the earlier single speaker of the 2 Dial model for a couple months only) The pedestal speaker has 2 field

coils, thus preserving the use of the separate field coils like the twin speaker set, - one in the B+ circuit and the other in the bias circuit of the output tubes. 

(The Scott Model Fifteen also used the dual field coil Jensen pedestal which had a different output transformer, different basket casting and different part number - and should not be considered equivalent.)

The diagram in Riders is a decent representation of the Deluxe model amp circuit with speaker showing the field coils. We have additional circuit diagrams  to cover the evolutionary changes to the Allwave Deluxe.

[David C. Poland]

For most of the Allwave Deluxe production run with the set cable 6 pin Jones plug, pin 6 was unused. But late production examples have an audio path change for the 2nd AF plate circuit that made use of pin 6. We have a diagram showing that revision for the receiver and amp.. 

[Stuart Smolkin]

Hi David, and thanks for the quick reply.  When I go back to the museum I will check to see if there is a 3569 mark on the side of our speaker (although it may have been covered over when the previous owner decided to repaint the speaker, apparently for cosmetic reasons).  It was sold as being an AW12D speaker.

I will also recheck the ohms readings.  As mentioned, I recall the fat pins showed around 670 ohms (and perhaps you can advise whether this is the coil for the B+ circuit or the coil for the output bias circuit)?  Also, do you happen to know what the other coil should measure?

Changing topics - the electronics engineer volunteer who initially worked on this radio for us indicated that there was apparently, at some time in the past, a problem in the audio output transformer, leading it to be replaced with a non-original part that apparently is not correct (i. e., audio volume is weak... only acceptable if we turn up the volume control almost all the way).  He advises that he has been unable to find the spec on the Scott transformer, and that it was custom made for them.  Would you happen to know the spec, or have any suggestions as to where we might find an original transformer to purchase?

Regarding the schematic showing the audio circuitry (6th pin in use) - can you tell me where to find it?  I looked in the AW12D set folder, but didn't find it there.

Thanks again for your guidance, it is extremely helpful and we are very grateful.

Cheers -

Stuart

PS you may receive a reply query from a gent named Bill Ryno.  Bill is another electronics engineer volunteer who is currently working on our Scott radio.

[Kent King]

Stuart - I just approved Bill's membership here on the forum. Weak output on the AW12D is also caused by a failed choke in the audio stages. There is a modern Hammond equivalent for that, I'll have to dig up the number. They sound pretty weak if that choke is open.

Kent

[William Ryno]

Kent, thank you for approving my membership in this forum.

Question is the choke you are referring to the 50 Henry (or 30 Henry I can’t tell image quality of Riders is poor) choke above the 45 tubes on Riders schematic? Also we believe the interstage transformer connected to pins 3 and 4 on the Jones plug is not original and was replaced in the 1980’s with an unknown replacement transformer. This transformer originally was a proprietary Scott unit and we can’t find specs on it. A mismatch of impedances of this replacement transformer could cause low volume output. 

Bill

Kent King said:

Stuart - I just approved Bill's membership here on the forum. Weak output on the AW12D is also caused by a failed choke in the audio stages. There is a modern Hammond equivalent for that, I'll have to dig up the number. They sound pretty weak if that choke is open.

Kent

[David C. Poland]

The 2nd field coil measurers about 2500 ohms. 

Follow the Riders diagram circuit diagram for the field coils. Colored pencils helps to follow..

The 675 ohm field coil is in the bias circuit - off the 2.5 filament secondary center tap through the Speaker plug fat pins #1 and #6  to this field coil and on to ground  -  provides a negative bias to the grids of the output 45 tubes.

The 2500 ohm field coil is the B+ circuit  - after the amps 30 Henry choke to pin 3 of the speaker plug, and then off the center tap of the output transformer to this field coil and back via pin 2 to the amp and then on to the set plug pin 4 for receiver B+.

All 3 of the amp 8 MFD filters are in the B+ circuit. I replace with 10 MFD caps.

[David C. Poland]

I do not understand where the "D" come from. You say is for the AW-12 D , unless the D stands for Deluxe

Scott used the term a ALLWAVE DELUXE  to denote the first model with single dial tuning.

The Previous ALLWAVE model ( we call the 2 -DIAL) involved 2 tuning dials and also a set of plug in coils to insert for short wave reception.

If your receiver and amp are wired to use pin 6 of the set plug, then you have very late production from early 1934. if so we have another diagram for receiver and amp.

Otherwise, the Rider diagram is sufficient and is available in the Info Archive above:  SET FOLDERS, then  ALLWAVE DELUXE, then various files. one of the manual should be close to your example.

This version amp diagram should help you. see attachment.

IMG_0438.jpeg

[William Ryno]

The D refers to Deluxe nothing else.

There is a wire factory connected from one of the 10mfd (8mfd originally) capacitors positive side to pin 6 of the Jones plug. I measured voltage at that pin and measured 0 volts there with respect to ground. Is that capacitor supposed to have its negative side connected to pin 6 of the Jones plug? We have been unable to find a schematic for the radio where all 6 pins of the Jones plug is used.

David C. Poland said:

I do not understand where the "D" come from. You say is for the AW-12 D , unless the D stands for Deluxe

Scott used the term a ALLWAVE DELUXE  to denote the first model with single dial tuning.

The Previous ALLWAVE model ( we call the 2 -DIAL) involved 2 tuning dials and also a set of plug in coils to insert for short wave reception.

If your receiver and amp are wired to use pin 6 of the set plug, then you have very late production from early 1934. if so we have another diagram for receiver and amp.

Otherwise, the Rider diagram is sufficient and is available in the Info Archive above:  SET FOLDERS, then  ALLWAVE DELUXE, then various files. one of the manual should be close to your example.

This version amp diagram should help you. see attachment.

IMG_0438.jpeg

[David C. Poland]

William, ...  as for the D issue, it had been awhile since I  browsed the set folder for the 12 tube Deluxe. I see Kent added the D suffix more recently to the several files. Now I understand your model reference AW-12D.

So -  you do have the really late version of the AW-12. I redrew the amp diagram some years ago to include the actual example I had -to show the use of pin 6 of my set's Jones plug, and the several other changes.

 -- So here: my revised amp diagram using set plug pin 6  IMG_0439.jpeg

---And for the receiver, standard diagram to which I added the revised audio path using pin 6.

Having file size trouble - hope this works for lower right portion of diagram showing new audio path using pin 6.

 AW-12%20AVC%20very%20late%20receiver.jpeg

The added choke in the new path to pin 6 may act as a low frequency resonator to improve bass response - unsure about that. Scott was starting to pursue improved tonal response which became very evident in later models. 

Most amps do have the earphone socket, which is omitted on the published amp diagrams. It is 2 position as described in the owners manual - earphone plug partly inserted both earphone and the speaker plays - fully inserted, the speaker is silent. Clean the jack contacts.. 

Hope this helps. Good luck