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 Post subject: How to replace speaker - Phonola
PostPosted: May Tue 17, 2022 5:53 pm 
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Joined: Nov Fri 12, 2021 2:01 pm
Posts: 37
http://www.nostalgiaair.org/PagesByMode ... 030256.pdf

The speaker for this radio, Model 6525-S, is badly damaged and I’ve read that you can replace vintage speakers with a permanent magnet speaker. I would like to do that with this radio but I really don’t have a clue as to where to start. I have already changed the caps and resistors.

Can someone look at the schematic and tell me if it’s possible to do with this radio. The schematic is the only information I’ve been able to track down on this radio and I’m not sure of what options are available.

Any responses will have to be easy to understand because I’m still very new to this hobby.


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Tue 17, 2022 6:55 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 9134
Location: Near Brandon, Iowa
Yes, it's possible. You will need to find a power resistor of approximately the same ohms as the speaker field coil. The resistor will need to be rated for at least 10 watts dissipation. It will need to be mounted on the chassis in a manner that will allow it to be cooled by air convention as it will get hot.

Assuming that the present speaker field coil is intact, you can ohm across it to determine the replacement resistor value. If the field coil is open, that's a bit of a problem as the value isn't specified on the schematic. However, it can be figured out with a little work... but ohming the existing field winding is the easiest way to get this info.

The output transformer is mounted on the present speaker according to the schematic. This will need to be carefully removed and relocated to somewhere on the top of the chassis. This transformer has a primary winding which connects between the output tube plate and B+; its secondary will connect directly to the terminals of the new speakers.

The original speaker had a humbucking coil in series with the voice coil... this won't be needed in a permag speaker.

Most of these sets used low-impedance voice coils (less than 2 ohms). These aren't manufactured anymore so you will need to select a 4 ohm speaker that has a 3 to 5 watt max power rating. Using a high-power-rating speaker (such as those sold for automobile stereo) will not work well as the radio won't have enough audio output power to effectively drive it.


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Wed 18, 2022 12:38 pm 
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Joined: Feb Sun 26, 2012 2:47 am
Posts: 910
Location: Ontario, Canada
Why don't you re-cone the speaker, & keep it original?

John


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Wed 18, 2022 2:01 pm 
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Joined: Nov Fri 12, 2021 2:01 pm
Posts: 37
Thanks John, but I’m very new to this hobby and at this time I have no idea where I would get a speaker reconed. I noticed that you are in Ontario and if you have any suggestions for someone that can do that, please forward them.

Lorenz, I took some readings and in the first photo I got .9 ohms where the clips are and in the second photo I tested all combinations of the 5 pins and the two yellow arrows were the only place I got a reading and it was 1.93 K ohms. I’m not sure if that information would help you provide me with any further advice. Thanks


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Wed 18, 2022 7:22 pm 
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Joined: Feb Sun 26, 2012 2:47 am
Posts: 910
Location: Ontario, Canada
Hi Hamilton 2711
I re-cone speakers. If you want further details, send me a PM.

John


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Wed 18, 2022 8:14 pm 
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Joined: Dec Sat 22, 2007 7:12 am
Posts: 2538
Location: Austin, Texas
Dear Hamilton 2711-PM sent-Gearhead222


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Wed 18, 2022 9:14 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 5232
Location: Lexington, KY USA
Even if you decide to sub-in a different speaker for now, do save the old one.

It might be repaired later, and put back into service. If it goes in the bin, that option is lost.

Ted


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Thu 19, 2022 3:17 am 
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Joined: Apr Sun 08, 2007 6:47 am
Posts: 5784
Location: British Columbia
Hamilton2711 wrote:
http://www.nostalgiaair.org/PagesByModel/256/M0030256.pdf

The speaker for this radio, Model 6525-S, is badly damaged and I’ve read that you can replace vintage speakers with a permanent magnet speaker. I would like to do that with this radio but I really don’t have a clue as to where to start. I have already changed the caps and resistors.

Can someone look at the schematic and tell me if it’s possible to do with this radio. The schematic is the only information I’ve been able to track down on this radio and I’m not sure of what options are available.

Any responses will have to be easy to understand because I’m still very new to this hobby.


I would look into getting the original reconed, or doing it yourself, you may be able to repair the original cone with a rubbery cement, and patches, if most of it is still there. The best sub for a field coil is a filter choke, sometimes you can get away with a resistor, but the voltage drop increases with current load so this is only workable with a smaller radio with five or six tubes.
Regards
Arran


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Thu 19, 2022 7:32 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 5232
Location: Lexington, KY USA
Back to the original question. Yes, a newer permanent magnet speaker can be used to replace the original speaker, along with a resistor to replace the field coil.

When this is done, often one or more B+ filter capacitor values may need to be increased to reduce the AC hum back to the original level. This set uses two 50uF filter caps (C28, C29), which may be large enough already.

It would be best not to make C29 any larger, but for C28, something like 100uF might be used.

Note that the replacements can be the now more common 47uF value, and a 450V rating would give some extra margin. If you get an extra 47uF cap, it could be added in parallel across C28, if hum is a problem.

I tested all combinations of the 5 pins and the two yellow arrows were the only place I got a reading and it was 1.93 K ohms.
Attachment:
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The schematic shows these pins connecting the field coil. 1.93 k ohms is a reasonable value for the coil’s resistance. It is this resistance the added resistor needs to match.

The schematic also shows the primary of the audio output transformer connected to the two pins at the left in this photo. The primary should measure a few hundred ohms resistance. If there is no continuity between these pins, you might check that they are still connected to the transformer. If they are, the old transformer is bad, and you will need a new one.

The schematic has enough information on it to calculate the B+ current, and the field coil resistance. Unfortunately, the schematic information is a bit inconsistent, but we can feel pretty good that the calculated values are approximately correct.

All of the B+ current flows through the bias resistors, R12 & R13. The 14V drop across this 270 ohm total resistance means that we have about 52mA B+ current. This is reasonable for this sort of radio.

If a 2000 ohm resistor replaces the field coil, 52mA through your 2000 ohm resistor gives you 5.4W in the resistor.

At the field coil, the schematic shows a 320V – 225V drop, or 95V. At 52mA, the field coil would be 1827 ohms. This is reasonably close to the measured 1930 ohm value.

As to what parts to get, exactly, there are many options.

You want to locate a speaker that will mount directly in your cabinet. If necessary, a similar but not quite the same unit might require an adapter board. The speaker needs to be a full range, efficient one. Car audio speakers usually require too much power. Sometimes a PA ceiling speaker can work well. Old TV or stereo sets sometimes have suitable speakers.

The resistor value for the field coil connections is not super critical. 2000 ohms is close enough. For old radios, it is generally better to go to the next higher standard value. Things will run cooler and longer at a lower B+ voltage. The resistor will see around 5W of power, so a 15W or 25W part might be used to give ample margin and keep it cooler.

The tube manuals suggest about 7,000 ohms load resistance for the 42 audio output tube. This would be a reasonable primary impedance for the output transformer. The transformer secondary should match your new speaker.
https://frank.pocnet.net/sheets/021/4/42.pdf

Ted


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Sat 21, 2022 3:17 pm 
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Joined: Mar Sun 09, 2008 7:07 pm
Posts: 78
Location: Hay Lakes, Alberta, Canada
If you don't want to recone your speaker, and easier way to not have to do modifications to your set is to disconnect the voice coil on your old speaker, and mount the speaker sideways into the cabinet, so that the field coil is still in place as part of your power supply. Then find an appropriate size PM speaker with a proper voice coil rating and simply run a wire from the output transformer to the new speaker VC.

That way, the original speaker is still there if you (or someone else) decides to recone it in the future. Plus you don't have to modify your chassis.


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Sat 21, 2022 5:48 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 5232
Location: Lexington, KY USA
Measurements appear to have found the original output transformer primary to be open. Likely won't work so well, even with a new speaker.

If there is room to retain the old speaker while installing a new speaker and transformer, this scheme will work. For sure, don't butcher the old speaker to be able to jam it in there somehow.

Ted


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Sat 21, 2022 9:54 pm 
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Joined: Nov Fri 12, 2021 2:01 pm
Posts: 37
Thanks Ted

As you indicated I believe my output transformer is open. I get no measurements from the two pins going to the primary. Do you have a suggestion for what specs are needed for a replacement Hammond transformer?

I am going to recone the speaker myself. Thank goodness for Youtube and this forum for information on how to accomplish that!

Vintageradiojohn offered to recone but with the shipping cost it wouldn’t be worth it for me.


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Sat 21, 2022 10:07 pm 
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Joined: Nov Fri 12, 2021 2:01 pm
Posts: 37
https://www.hammfg.com/electronics/tran ... /125se.pdf

Would this one work for my situation?


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Sun 22, 2022 12:32 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 12029
Location: Omak,WA,usa
Hello Hamilton2711,
Yes, that Hammond transformer will work I have used it as a replacement transformer in the past !


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Sun 22, 2022 3:30 am 
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Joined: Nov Fri 12, 2021 2:01 pm
Posts: 37
Thanks Radiorich. Which model should I order?

I’m assuming the 3 or 5 watt version but I’m not sure. This model line ranges from 3 watt to 25 watts.

https://www.hammfg.com/electronics/tran ... udio/125se


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Sun 22, 2022 4:52 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 5232
Location: Lexington, KY USA
The transformer primary's DC plate current limitation may be the deciding factor in selection.

The 25mA limitation for the smallest of these Hammond transformers will probably be exceeded by a fresh 42 output tube.

The 125BSE, with its 5W power rating, and 45mA primary current limit, will be a better, if pretty expensive choice.

The smaller one will probably actually work.

There are less expensive options, but the Hammond transformers offer a straightforward and reliable solution.

Ted


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Sun 22, 2022 12:14 pm 
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Joined: Nov Fri 12, 2021 2:01 pm
Posts: 37
I’m totally open to other options. I only mentioned Hammond because that was the brand name on the existing transformer.

What transformer would you suggest? This is my first time replacing one so as a novice I will take any and all advice.

I worked on my reconing job last night. New foam surround and I used stock card paper for the cone. Voice coil now moves freely. Pic enclosed.


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Sun 22, 2022 3:55 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 5232
Location: Lexington, KY USA
I only mentioned Hammond because that was the brand name on the existing transformer.

So that's probably a long-ago replacement. Do be sure that the existing transformer is properly connected to the pins on the plug. It would be a shame to buy a new one because of a bad connection or mis-wire.

Ted


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Sun 22, 2022 4:52 pm 
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Joined: Dec Sat 22, 2007 7:12 am
Posts: 2538
Location: Austin, Texas
Nice job, Hamilton2711!:)-Gearhead222


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 Post subject: Re: How to replace speaker - Phonola
PostPosted: May Thu 26, 2022 11:44 pm 
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Joined: Nov Fri 12, 2021 2:01 pm
Posts: 37
Well I got my new output transformer today but will need assistance from the forum as to how to properly connect it. My old transformer had 4 wires, numbered in the photo, while the newHammond 125bse has 7 in total.

It came with no documentation so if someone on the forum can enlighten me as to how and where to connect the new transformer it would be greatly appreciated.


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