[Ron Ramirez]

I did use lockwashers. But I was going to also use threadlock, and I forgot!

[codefox1]

Take your time. Had a few pieces in and out over the decades. Nothing remains. Parts were good, QC marginal.. Check all solder joints recarb and recap of course.

[Ron Ramirez]

I found my Masterpiece VI schematic, and it tells me that the voltage going in to the 680 ohm section of the speaker field is 430 VDC, and 332 VDC on the other end, so the 680 ohm field drops 98 volts. Using Ohm's law, I = E / R, the MP VI will have just over 144 mA of current flowing here - more than Leigh's solid-state filter choke can handle, which is as I expected.

I'll have to either go ahead and use a resistor, or possibly explore an alternative design for a high-voltage, high current regulator. (Any and all ideas on this subject are most welcome.)

But there's no hurry at this point.

My next move is to see if I have enough cloth-covered wire to make a speaker wiring harness to run from the coaxial speaker to the extra box I am going to have, which will contain the 2-way crossover, the audio output transformer, and either a power resistor or voltage regulator to replace the 680 ohm field. I also need to disassemble that nice new coaxial speaker and add some threadlock, then reassemble.

[Arran]

What about using two filter chokes in parallel if one can't find a single one that can handle enough current? Or is the problem finding a choke, with a 680 ohm DC resistance, that can handle 144 ma?
Regards
Arran

[Ron Ramirez]

Hammond sells a choke that can handle 150 mA, but its DC resistance is only 100 ohms.

In the MP VI, the B+ goes through two chokes (and four electrolytic capacitors, the first two in series) before it hits the 680 ohm section of the speaker field. There is yet one more electrolytic following the other side of the field.

Since there are already two chokes in the power supply, this is why I'm not really worried about the replacement for the 680 ohm field not being another choke. I just need to drop 98 volts so the B+ hitting the screen grids of the 6L6G tubes, and the voltage divider resistor network inside the tuner, is not excessive.

[codefox1]

I think I would just as soon use 6L6GC outputs and tame the bias if need be. Add a little cap up front, but you know that. Easy enough to get the rest of the voltage divider in line.

[Ron Ramirez]

I actually have a pair of Sovtek 5881 tubes...

but no, I think I'll get a pair of old ST-style 6L6G tubes for the McMurdo to keep that authentic "vintage" look. It's already going to have a modern speaker inside...no use in going overboard on modern stuff, I think.

Speaking of the speaker (heh heh, I made a funny - cymbal shot)...

Very productive day on the coaxial speaker project.

I disassembled it and reassembled, using threadlock this time on all of the screws.

I found that two of the four arms that attach to the woofer had the holes drilled in the wrong places - they were just a bit off, which was keeping the tweeter from being suspended flat as it should. A little work with my Ryobi "Dremel" tool and a special bit designed to enlarge holes in metal, elongating the two mounting holes, cured the trouble and the tweeter then attached to the woofer just fine, no problems.

I attached cloth-covered wire to both speakers for that "authentic" look, and a 4-pin Amphenol plug and cover on the end of the speaker cable.

I also used some large shrink tubing to hold the tweeter wires to one of the support arms:

[Image: http://www.philcoradio.com/images/phorum...0c5ab4.jpg]

[Image: http://www.philcoradio.com/images/phorum...e49b42.jpg]

Another view of the coaxial speaker, showing the speaker cable and Amphenol plug:

[Image: http://www.philcoradio.com/images/phorum...a174c5.jpg]

Next came mounting the speaker in the Clifton cabinet. I used two layers of stick-on speaker gasket material to ensure a good seal between speaker and cabinet. My cabinet had holes drilled around the speaker opening, but they were not spaced properly for this speaker. So I drilled four new holes and attached the speaker to the sounding board with four #8 wood screws.

[Image: http://www.philcoradio.com/images/phorum...da9d78.jpg]

No pedestal on this speaker, but it will have to do.

The speaker cable will plug into a project box I have yet to buy - this box will hold the 2-way crossover and the power resistor to take the place of the missing field coil. It will have an audio output transformer mounted on top. This box will have a cable running from it with a 5-pin plug; the 5-pin plug will plug into the speaker socket on the McMurdo amp.

[Image: http://www.philcoradio.com/images/phorum...295f94.jpg]

Slowly, it's coming together. Yes, it needs grille cloth; that is a problem for another day. When I begin electronic work on the set, I'll start a new thread.

Oh, in case you're wondering what happened to the Scott Philharmonic speaker that was sitting in this cabinet, it now resides inside my Allwave 23 Tasman which has a Philharmonic amp.

[GarySP]

Looking great so far, Ron. Please keep the threads on this set coming. If I can't have one, at least I can enjoy one through the eyes of another, and learn a little along the way. Take care. Gary

[codefox1]

Just a thought, I have used large filament transformers as chokes in the past. l just use the primary and leave the secondary open.

[y2kbruce]

I wonder if anyone makes a pedistal stand for that size speaker ?

[Ron Ramirez]

In case you're wondering about the field coils in the Masterpiece VI and the Scott 800B, here is a partial schematic showing the Masterpiece VI amp/power supply:

[Image: http://www.philcoradio.com/images/phorum...91f816.jpg]

And here is a small version of the Scott 800B amp/power supply:

[Image: http://www.philcoradio.com/images/phorum...aa7775.jpg]

Larger version may be seen here.

[Doug Houston]

Real quickie here. I'm trying to ride about 5 horses at once here, so the Jensen speaker project is going slow.

Got another theory about the dual fields. You never hear of them on lower ticket sets, so I just wonder about this.

Flux level in dynamic speakers was hard to get in high levels back before some of the super magnets that came to be in the WW II days and up to now. It appears that the series field winding did an accptable job in must sets, but a stronger field was desirable for the ultra-fine sets like the Scotts and McMurdos. Those power amps were running class AB. The shunt section of the field brought more flux level, and the resulting efficiency that they wanted I find that on some juke boxes, there was a shunt field used. I have an old Wurlitzer amplifier that has P-P 45's in the output, but with a 45 driver. That would have meant that Wurlitzer was pushing the 45's into class AB, and a shunt field would do the job better. It was a 1936 model. Today, a good PM speaker would do the job perfectly.

I have a pair of 18 inch speakers on my living room systems, and I can barely lift one of them. The magnets are the heaviest part of them.

[Ron Ramirez]

Doug

Thanks very much for sharing that info. It makes perfect sense to me now, and I now understand why Scott did not make any effort to replace the 9000 ohm shunt field in the 800B with any resistor when they switched to the Jensen PM speaker - only using a 675 ohm resistor to replace the 675 ohm series field of the older electrodynamic speaker.

[kirwin]

I recently purchased a Mcmurdo Masterpiece with the chassis and clifton cabinet but no speaker or power supply. After reading some of these postings I feel somewhat foolish because it is clear that the missing components will not be easily acquired. Is there any point in keeping the parts that I have. The chassis is complete and the cabinet is in good condition.

[Arran]

If you have the tuner chassis and cabinet those are two of the most important and harder to find parts. Many of these types of sets, Scotts at least, were sold as bare chassis without a cabinet so there are considerably fewer cabinets then there are chassis to go in them, it's likely a similar story with a McMurdo Silver set. The main issue with this set is the 18 inch speaker which had a concentric mount for the tweeter, I don't know if all McMurdo Masterpiece sets used these or not. It may take some time but I think you could probably find a power supply/amplifier chassis eventually. If push came to shove you could always make one from new parts.
Regards
Arran