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  #1  
Old 10-11-2018, 09:49 PM
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Philco 77

I've got a Philco 77 in which someone replaced the speaker. It appears as if it's missing the field coil and is missing the center tap on the input of the output transformer. They somehow connected the wires weirdly, they hooked the outputs of the output transformer and the voice coil in series and then hooked that up to where the Field coil should be. Would this have worked?

The resistances of the speaker are as follows
~Voice Coil 5 ohms
~Output of transformer 0.5 ohms
~Input of transformer 855 ohms

Here's a picture if you're as confused as I am
20181011_212205.jpg
The blue and green go to the input of the trans
The yellow and black should go to the field coil
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Old 10-11-2018, 10:38 PM
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In case of a non-field coil speaker being used...I would replace the coil with a generic choke...and try to get a voice coil whose impedance closely matches the amp's output trannie.
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Old 10-12-2018, 12:46 AM
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Are you sure that speaker does not have a field coil (it looks like there is cardboard in the magnet and some wires going to it)? Some field coil speakers had an extra winding on the field coil that was placed in series with the speaker...This extra winding picked up any hum from the field coil and fed it to the voice coil with reverse polarity to cancel the hum from the field coil.

If you need to sub a perm magnet speaker for a field coil type you can place a power resistor of comparable resistance to the old field coil in place of the field coil too...A choke will reduce hum more, but a resistor will work too if on a budget or in a pinch.
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Old 10-12-2018, 08:27 AM
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Well, I mixed up my terminology. I think I've confused the output transformer and the field coil. (Sorry about that still new to the hobby) What can I do about a missing output transformer?


This appears to be the speaker that I have.
Philco 36-1450
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Old 10-12-2018, 11:32 AM
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Look for a transformer with the correct primary topology (push pull or single ended depending on how many audio output tubes there are), Primary impedance (should roughly match plate resistance in output tube data sheet), secondary impedance (should match speaker voice coil impedance which is usually the same as voice coil resistance), and should be rated for the max audio power the radio is rated for (or the tubes are rated for if you can't find an audio power spec).

Note: not all radios regardless of speaker type (PM or field) mounted the audio output transformer on the speaker, some mounted it on the chassis instead. If the speaker was subbed and they were smart enough to wire the field coil correctly then they probably made sure an output transformer exists within the set.
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Old 10-12-2018, 01:17 PM
old_coot88 old_coot88 is offline
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Here's the schematic..

http://www.nostalgiaair.org/Resource...=\M0013838.pdf
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Old 10-12-2018, 01:30 PM
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Quote:
Originally Posted by old_coot88 View Post
That's the one I've been using as well as this one, which is nearly identical and has most of the values. http://www.philcoradio.com/images/phorum/schem/76.jpg

No output transformer that I can see, the 45's plates go directly to the speaker. Not sure if it ever worked, either way, can someone help me figure out the maximum wattage I'll need? The datasheet calls for like 18 watts max watts in a push pull configuration.

http://www.hammondmfg.com/125H&J.htm
The Hammond 125 series seems to suit the resistance needs fairly well

This is the datasheet that I've been using
https://frank.pocnet.net/sheets/021/4/45.pdf
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Old 10-12-2018, 02:54 PM
old_coot88 old_coot88 is offline
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Quote:
Originally Posted by Zsuttle View Post
No output transformer that I can see, the 45's plates go directly to the speaker.
Your schematic definitely shows the output xfmr, designated #24. The field coil is #26, and the power supply choke #35. Can you post pics of the top and underside of the chassis? The output xfmr has gotta be there somewhere unless it's been physically removed.

(This design uses a dedicated field coil, whereas later designs made the field coil double as the choke).

(Edit.) In your photo of the speaker, it looks like the speaker has been replaced with the later type that uses the field coil as choke. Notice the two black wires coming out of the field frame. Those go to a 'hum bucking' coil which is in series with the voice coil. It serves to null out the field's ripple. Your radio doesn't need hum bucking, since the field coil does not double as choke. So just take the HB coil out of circuit, and run the voice coil direct.

Last edited by old_coot88; 10-12-2018 at 03:27 PM.
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Old 10-12-2018, 03:33 PM
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Heres pictures of my chassis.
20181012_151551.jpg
20181012_151628.jpg
The plates of the 45's go directly to the speaker plug. My understanding was that the output transformer was mounted on the speaker frame however, when they replaced the speaker, they bypassed any sort of output transformer and hooked it directly to the speaker. If you look at the schematic for the 77, you can see the plugs off of the 45 tubes and off of the field coil
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Old 10-12-2018, 06:17 PM
Titan1a Titan1a is offline
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Push-pull amplifier. Requires tapped output transformer to speaker and field coil.
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  #11  
Old 10-12-2018, 07:59 PM
old_coot88 old_coot88 is offline
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Quote:
Originally Posted by Zsuttle View Post
Heres pictures of my chassis.

The plates of the 45's go directly to the speaker plug. My understanding was that the output transformer was mounted on the speaker frame however, when they replaced the speaker, they bypassed any sort of output transformer and hooked it directly to the speaker. If you look at the schematic for the 77, you can see the plugs off of the 45 tubes and off of the field coil
Yep you're right. Originally the p-p output xfmr was mounted on the speaker.
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Old 10-12-2018, 10:44 PM
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My only other question is how do I determine the wattage I'll need?

This line seems to suit the demands given
http://www.hammondmfg.com/125a.htm
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Old 10-14-2018, 07:13 PM
EdKozk2 EdKozk2 is offline
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A 10 watt p-p should be just fine for your Philco.
Ed
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Old 10-18-2018, 10:40 PM
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Well, I'm happy to report that I received the transformer today and promptly installed it. I'm also happy to report that I'm now receiving stations, though slightly garbled and with a high pitched whine. I'm assuming that aligning each section should fix the whine, though I'm not sure about the garble. (Weak output tubes maybe?)
Thanks for everyone's help,
Zach
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Old 10-19-2018, 12:01 AM
7"estatdef 7"estatdef is offline
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You seem to be missing the large tube shield. This very well might be the cause of your issues. Also be advised that the input filter cap (inside the box from connections 2 to 6) should be a non polarized mylar 2mfd/630v. If you use an electrolytic cap it will short after a while do to the high ripple current at that point in the p/s.
Depending the style of the cabinet the original speaker would have looked like this one. https://www.ebay.com/itm/vintage-PHI...gji:rk:22:pf:0
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Terry
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File Type: jpg Philco 77 rear.jpg (44.9 KB, 16 views)

Last edited by 7"estatdef; 10-19-2018 at 12:16 AM.
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