Scott 800B - Jensen Coaxial Speaker

[Ron Ramirez]

I just picked up a Jensen coaxial speaker from a Scott 800B.

It looks right...definitely a Jensen...but I was very disappointed to find that instead of being an electrodynamic speaker with the dual field coils I was expecting, it is a PM speaker.

There is a large wirewound resistor mounted to the back of the Alnico V magnet, under the rear dust cover, to mimic the 675 ohm resistance of one part of the field coil of the electrodynamic version (and connected between terminals 2 and 4 of the speaker plug). But there is no 9000 ohm resistor to mimic the other part of the non-existent field.

It has the proper 800B ten-pin speaker plug. There is no connection to terminal 6, nor was there ever a connection judging by the tarnish on the solder end of the terminal.

Oh, the voice coil is open, too, but I knew going in that the speaker would need repair.

It's a Jensen JAP C4646, part no. 85B989.

My question is this: Did later 800B sets use a PM version of the coaxial Jensen speaker, eliminating the 9000 ohm field coil which connected between the 6L6G screen grids and ground?

[David C. Poland]

Yes, the later speaker is PM.  The one I restored for someone (ser# 5968) had the same appearance Jensen, same color blue frame, the same tweeter suspended inside the 15 inch cone as my own low ser # 800B . However, for this later production model, the cover over the magnet was plastic instead of metal.  There was but a single 675 ohm resistor  inside the back cover in place of the 675 ohm field coil. However,  there was no substitute for the 9000 ohm field coil shown in the diagram, and Pin 6 of this speaker's plug was unused - so evidently it's absence isn't an issue.  None the  less, the amp had the 3000 ohm resistor between pin 6 and ground,  I suppose in the event an earlier field coil speaker were to be connected.  Incidentally, once restored, this radio sounded  awsome, but the restoration was a lot of work.

I believe the speaker was set up this way to be backwards compatible. For both version speakers, the AC line to the on/off relay in receiver runs first through the speaker plug pin 9 and 10 as a safeguard lest someone try to power up the radio without the speaker (and onward to pins 1 & 4 of the receiver 's Jones plug).   I suspect the field coil and the PM version speakers would be interchangeable.

The 800-B is complicated. For example, the relay that switches between AM and FM will operate only if certain controls are in particular positions. Up to about Ser# 2000, the motor drive operated off a 12 volt transformer, but subsequent production was then revised to 24 volts.  And the tweeter is switched off when the radio is in AM mode. And so on. If you don't already have one, seek the 1/2 inch thick compilation of the owners manual, operating manual, and factory tech info for the radio which includes extensive info for the Thorens CD-40 Concert 78 changer Scott supplied with the 800-B.

Good luck.  

[Ron Ramirez]

David

Thank you for the confirmation; your explanation confirms my suspicions.

Yes, my speaker has a plastic (not metal) cover in back, with a 675 ohm resistor mounted in back (actually it's gone up in value, to around 750 ohms, so it probably should be replaced).

Thanks also for letting me know the speaker sounds good. I do not own an 800B, just this speaker. I bought it for a different purpose...my intent was to use it in my recently purchased McMurdo Silver Masterpiece VI. The field coil setup of the original Jensen Super-Giant is very similar to the original 800B speaker field.

800B - 675 ohms and 9000 ohms

Super-Giant - 680 ohms and 8000 ohms

I was also aware of the tweeter being switchable in/out, and planned to change the wiring so the tweeter remains on all of the time.

I can rig up a small metal box with an audio output transformer and an 8000 ohm wirewound resistor, connect this between the Silver amp and the speaker, and use this 800B speaker anyway.

Thanks again.

[Carlos Insa]

Pics are welcomed.

Regards ;-)