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.