Calling Philly Speaker Experts

[Joe Miller]

I am working on a Philly speaker Model #1535016, Part 920 (the last digits on both these numbers may be wrong as they are barely visible.)  (Note to Bill Liers--this is the speaker that may have caused the damage to Bruce's Philly).

Anyway, Field Coil checks good, audio output transformer checks good (primary DC ohms are 102 ohms P1 to B+ and 325 ohms B+ to P2; secondary appears to be okay at 2 ohms), VC is good at 27 ohms, BUT I found the primary windings are not isolated from ground and read, P1 to ground is 142 ohms, B+ to ground is 43 ohms, and P2 to ground is 360 ohms. These readings were taken with the VC disconnected from the output transformer, suggesting a short (~40 ohms) between primary and secondary on the output transformer.

I would expect to see a dead short rather than the 40 ohms noted above if the output transformer has failed.  A couple of questions:

1.  Is this the wrong speaker for a Philly with a different schematic than shown on the Philly schematic?

2.  Anyone seen anything like this on the 15 inch pedestal speaker?

3.  Is there a source for a replacement audio output transformer?

Thanks for your help,

Joe

[Norman S Braithwaite]

It does appear that the primary of the output transformer is shorted.  The two halves of the primary winding should have much closer DC resistance to each other (but not necessarily equal) and there should be no measurable resistance to the frame or chassis.  Why not a direct short?  Because you are measuring carbon deposits after sparking.  The carbon path has reduced the resistance of one half of the primary as well as providing a current path to the frame/chassis.

A transformer having a primary impedance of 3300-ohms, a secondary impedance of 32-ohms, and capable of handling 60-watts will be sufficient to replace the original.  Connect the 39-ohm voice coil to the 32-ohm tap.  They are close enough.  Alternatively, an output transformer having a primary impedance of 1500-ohms and secondary impedance of 16-ohms and having adequate power rating will also work.  Connect the 39-ohm voice coil to the 16-ohm tap.  As such the reflected impedance seen by the 6L6 plates will be approximately 3600-ohms.

Norman

[Joe Miller]

Hi Norm,

Thanks for the info.  I will look around for a replacement.  Anyone out there have a junk speaker with a good output transformer?  I'll also check with Gary at Play Things of Past.

Also, I just noticed that the speaker is missing a cover in the center of the cone (see picture).  What am I missing here (is it just a dust cover or does it provide some function?)  Will it play without this piece (after I find a replacement output transformer?)

Joe

[Bill Liers]

Good luck Joe,

There's always a risk when you don't test the speaker with the rest of the set.

regards, Bill

[David C. Poland]

The 15 inch pedestal speaker would have a half sphere hard paper cover over the center, with a smaller felt like center.