Philharmonic 15" Pedestal Speaker Output Transformer

[Jeremy S]

Hey everyone! Long time lurker, first time poster here. Currently restoring a Philharmonic with the 15 Magnavox pedestal speaker. Unfortunately, the output transformer in my speaker is shorted. 

I am looking for find a suitable replacement transformer. Before I scour the internet for an option, I was curious if anyone else had recently replaced one of these and what part that you used? Open to recommendations!

Jeremy

[Thomas Day]

I would check out Hammond transformer company. They make several models that would work with t.he tubes used in the amplifier. Another option would be Heyboer transformer in Grand Haven Mi. They made a new power transformer for my Allwave 23. Not cheap, but worth it to restore the radio. .

[Jeremy S]

Thanks Tom! I have been looking at the Hammond 1608A-1650A Series. One issue I see is that my VC is 38 ohms, the Hammond options are only rated for up to 16.

Per my RCA manual, with a PP 6L6 class A, cathode bias, I need an impedance of around 5000 ohms. However, I have never sized an output transformer for a parallel push-pull circuit. With this being a parallel PP circuit, how does that affect the load resistance? I know from past experience that impedance has some wiggle room as long as you are in the ballpark, at least on most consumer grade radios. Per the manual, undistorted output is 40W.

I sent a message to Heyboer to see fi they can rebuild my transformer, or can sell me a custom replacement. I also reached out to Hammond for their recommendations.

[Thomas Day]

For me, since I didn’t have the original speaker, the transformer I used allows 8 or 16 ohm speakers. I plan on using a regular 3 way hi fi speaker. You could do the same, and save the original speaker til you can find a good transformer, and just use the built in resistor pack and field coil. Check the actual dc resistance of the voice coil. 16 ohms may work ok. 

[David C. Poland]

Does the construction of a new Hammond transformer have room inside the laminations to add windings around transformer coil? Is what I did for a Scott Masterpiece speaker to match a new Hammond xmfr to speaker’s 38 ohm VC. If half your old output transformer is still good, you can measure the ratio of secondary to primary to get a decent approximation and add windings to the new Hammond 16 ohm secondary lugs for a 7th lug to further decrease the ratio. In my case, about 25 additional turns worked well on a 15 watt p-p output transformer. I posted a write up some years ago here on my effort.

[Jeremy S]

Unfortunately the primary is shorted to the secondary in my transformer. Don't think that I can get a reliable measurement from the exiting windings.

I was chatting with Norman on ARF and he had some good advice:

No on the custom replacement or rewinding. The Philharmonic employs four 6L6 tubes in fixed bias push-pull parallel. The transformer primary impedance should be 2500- to 3300-ohms and secondary impedance should be 32-ohms which is completely satisfactory for 38-ohm voice coil. However, what is really required is a turns ratio of 8.8 to 10.2 (square root of impedance ratio). Hence an output transformer with a 16-ohm secondary impedance and 1250- to 1650-ohm primary will be equally satisfactory. The minimum power handling capacity of the transformer should be 50-watts.

I still have the original problem of trying to source a replacement, so I am still looking.

David C. Poland said:

Does the construction of a new Hammond transformer have room inside the laminations to add windings around transformer coil? Is what I did for a Scott Masterpiece speaker to match a new Hammond xmfr to speaker’s 38 ohm VC. If half your old output transformer is still good, you can measure the ratio of secondary to primary to get a decent approximation and add windings to the new Hammond 16 ohm secondary lugs for a 7th lug to further decrease the ratio. In my case, about 25 additional turns worked well on a 15 watt p-p output transformer. I posted a write up some years ago here on my effort.

[Jeremy S]

An update: Phil at Heyboar is going to build me a transformer. Posting here so others have it as a reference.

3,300 primary, CT

32 secondary, with 16 ohm tap

10.1 winding ratio

50W

Standard M27 laminations (Phil didn't see a need for the more expensive "hi-fi" steel)

My cost was $140 + shipping. Will post an update when I get this installed an working.

[Thomas Day]

Thats great. They came through for me on the power transformer. It did take about 3 months. I was able to pick it up in October on our way to Muskegon.