Philharmonic Factory Wiring Error?

[Joe Miller]

I am currently restoring a 30 tube pointer dial Philly.  I started with the Amp and found what I think might be a factory wiring error, but not sure.  Here's what I found:

The actual wiring of the C71 500 mfd micas and the R42 (250,000 ohm) resistors were not symmetrical as I expected. When I looked at the bottom view drawing (Riders 14-47) it matched they way the amp was actually wired (not symmetrical). But the schematic (Riders 14-49) shows these components wired "symmetrically" as I expected.

I think the schematic is correct and the bottom view and actual wiring are incorrect, so I changed the wiring to match the schematic.

Has anyone found this same "wiring error" on the amp chassis? Looks like it came that way from the factory, so am I missing something here?

Here's the link to the Rider info. http://www.nostalgiaair.org/Resources/610/M0016610.htm

Thanks,

Joe

[Joe Miller]

Sounds like this speaker won't work with my Philly chassis. Darn. I still plan to reverse engineer the speaker schematic. Once I do I will post it. Maybe then we can figure out what set this speaker was built to fit. Bruce, do you have a Philly speaker you would part with?
I'll also contact Eric to see if he remembers anything abou the speaker. I think he said the set belonged to his Dad.

[Y2K Bruce]

Joe email me

[Joe Miller]

I went through the wiring and found that the speaker is wired consistent with the schematic, if the first large pin on the plug is pin number one. In other words if the pin sizes were correct so the plug and socket mated correctly the wiring would be okay.  That is, with one exception.

The FC for the speaker as shown on the schematic is a split winding. I didn't see this intermediate winding on the field coil and I didn't try to take the speaker completely apart to confirm.  My guess is that it either has none or if it does it has failed.  The high side of the field coil is 375 volts (pin 3) and the low side (pin 4) is 250 volts per the schematic. The purpose of the intermediate FC winding is to provide a voltage between these two values for use by the 6L6 plates.

A voltage divider made up of a 500 ohm resistor in series with a 1500 ohm resistor  (25 watt) was added (connected from pin 3 (375 volts) to ground to drop the voltage by 25% to service the 6L6 plates.  Fortunately the ground was not connected because the power dissipated in that voltage divider would be 375 * 375 / 2000 = 70 watts.

So I have two problems:  1) need to find the correct plug (any ideas where I might find one? and (2) need to come up with a good way to drop the voltage to the 6L6 plates. Or will they operate okay with the higher voltage (375 volts)?  Before I give up I guess I should also disassemble the speaker to confirm the status of the intermediate FC winding.

Also, for information the resistance of the FC is 5000 ohms.  Does that sound about right for this set?

All suggestions are welcome on this.  I really want to get this Philly playing.

Thanks

Joe

[Joe Miller]

Correction:

The voltage I need to drop is the voltage to the 6L6 screens not the plates.  I misread the schematic.  That may be good news since the screens should draw very little current.  Maybe I can go with a higher resistance voltage divider to feed the screens.  Say 5K in series with 15K?  Or maybe even higher to reduce their required wattage. Norm, what do you think about that?

Also, I am going to take the plug apart to see if I can rearrange the pins. That would be a simple fix if it can be done.

Joe

[Norman S Braithwaite]

Joe:

Thanks for your inquiry.  The original Scott field coil was approximately 925 ohms total.  The same speaker with a 5000-ohm field coil would have been used in an organ.  Does the existing incorrect plug have the same pin diameter as the plug from the tuner?  If not, it will be much easier to find the correct sized 7-pin tube socket to match the incorrect plug and swap the socket.  If it is the same pin diameter I would definitely change it to prevent the possibility of mixing the connections.  The correct plug can only be found on speakers for the Phantom and Philharmonic and these seldom turn up as parts units.  Rearranging the pins does not work well (how do you make a small diameter pin fit in the hole left by a removed large diameter pin?  Also the diameter of the pin spacing must be identical to the socket.

I am guessing that the ground end of the voltage divider resistors is not connected because the resistors started to resemble toaster elements when it was connected.  Are the 6L6 screens therefore simply in series with the 500-ohm resistor?  It will help if I can see a schematic of the speaker showing where the resistor divider is located.

The original 925 ohm field coil in the speaker was in series with a voltage divider in the tuner chassis.  The voltage measured at the electrical junction between the two (physically between the speaker and the tuner chassis) should be 250-volts.  How is this voltage derived in the built up speaker?

Norman

[Joe Miller]

Norm,

If this set needs a FC with only 925 ohms I don't think the one I have will work.  B+ is dropped from 375 volts to 250 volts by the FC (per the schematic).  If I use a FC with 5K ohm resistance (rather than 925 ohms), it will drop the voltage to 105 volts (if I did my math correctly).

You are probably correct about the pin rearrangement idea, but that may be moot if the FC is a show-stopper.

I've attached a sketch of the as-found wiring.  The 500 ohm and 1500 ohm resistors were added. I assume this was to be a voltage divider to replace the missing FC intermediate tap (pin2).  The resistors were found NOT grounded or else it may have fried the audio output secondary and the tweeters if it had been connected to the set and powered up.

The actual schematic of the amp chassis and speaker connections can be found at the following link on page 14-49

http://www.nostalgiaair.org/PagesByModel/610/M0016610.pdf

I don't see a solution at this point given the FC resistance.  Do you?

Thanks,

Joe

[Norman S Braithwaite]

Joe:

The 5K field coil will work if a voltage divider having a total resistance of 1.5k is placed in parallel with the 5k field coil.  If you can find a 1.5k 20-watt adjustable tap power resistor, use it and adjust the tap for the correct screen voltage.  The magnetic field of the speaker will be weaker than in an original Scott speaker but it should easily be strong enough to work well.

Norman

[Joe Miller]

Thanks Norm,

Yes, I think that might just work. I didn't think of paralleling the FC like that.

If my math is correct, 1.5K in parallel with the 5K is equivalent to 1153 ohms.  Is that close enough to the 925 ohms of the correct FC?  (that's about 25% higher than spec)

Joe

[Joe Miller]

Norm,

Bruce just measured his Philly pedestal speakers and all have 400 ohm FCs.  Puzzling.  Where did you find the 925 ohm spec?  Any thoughts?

Joe

[Norman S Braithwaite]

Joe:

925-ohms is identified on one or more versions of the schematic.  400-ohms is probably from one end to the tap.  The full field coil resistance should be measured between pins 2 and 3.  Pin 1 is the tap.  It is easy to misread the schematic and believe that the full field coil goes from pin 1 to pin 3 with pin 2 being the centertap but this is not correct.  Also, I have found speakers miswired as a result of the misread.

A 1.2k adjustable tap resistor would be more appropriate (paralleled with the 5k to make a little less than 1k).

Norman

[Joe Miller]

Norm,

I looked (using magnification) at the schematic (Riders Page 14-49 under Scott, link above) and I do see the 925 ohm reference for the field coil.  But, per this schematic the full FC is between pins 3 and 4 with the tap on pin 2.

Is there a different schematic that you are looking at?

Joe

[Norman S Braithwaite]

Joe:

You are correct.  I intended to reference pins 2, 3, and 4, not pins 1, 2, and 3.  The 925-ohm specification is on the Scott schematic in the "Philharmonic XXX" service data accessible by Scott Info Archive link at the top of this page.  The specification is also identified on Page 7 showing resistance measurements of the field coil and tap.

Norman