Replacing an electrodynamic speaker with a fixed magnet?

[jimmy74]

Hi folks, I'm repairing an old 50's tube radio which probably needs a new speaker. The original one is most likely to electrodynamic as the OT is fed directly byt the UY41 rectifier tube. I'm a little confused as to exactly what I need to get inorder to fit in a normal ceramic magnet speaker, from what I've understood, I'll have to get a choke and place it after the OT, but what sort of choke should I look for?

The original speaker probably has a burnt coil because the cone is blocked and doesn't move in or out, DC resistence however shows around 4 ohms, and judging from the power tube which is a UL41, the speaker could be 5 or 10 watts. The tubes used in the radio are:

- 1 x UL41
- 1 x UY41
- 1 x UF41
- 1 x UCH42
- 1 x UBC41

given the above information, can anyone help me out in figuring out what choke I need?

thanks

[Nickthedentist]

Hello and welcome,

How many wires are there going to the whole speaker? There should be two going to the voice coil (the bit which you've measured as 4 ohms) plus some others if it's a mains-energised [electrodynamic] one.

Where's the output transformer? Mounted on the speaker , or on the main chassis?

Could you post a picture of the speaker?

Nick.

[Peter.N.]

I think it unlikely that a '50s radio would have a 'mains energised speaker' and the valve line up would seem to indicate that's what it is.

Peter

[Nickthedentist]

Peter's right... but you do get exceptions occasionally. I've certainly got a French "Reela" set from the mid-50s which does.

However, you should be able to tell very easily just by looking, so do post a pic.

Nick.

[Aerodyne]

For clarity: energised loudspeakers derive their magetism from a field winding which is in series with the HT supply (after rectification) and this field serves the dual purpose of a smoothing choke for the HT supply to the valves. If you replace such a loudspeaker with a permanent magnet type you will need to fit a choke or a resistor - say, 2K wirewound, in place of the now missing field winding. If you use a choke you need to find a laminated iron-cored type. These look like an output transformer but with only two terminals.
Take care that the speech coil connections of the PM replacement do NOT contact the HT connections of the field winding - keep things separate.
A number of 1950s receivers used field-energised loudspeakers, some used both at the same time, with twin-speaker set-ups.
It seems to me that if the cone will not move it may be rusted into place and perhaps your field winding is actually intact. It can be measured with an ohmmeter, of course.
-Tony

[kalee20]

I'd be surprised if the loudspeaker was mains energised, as generally the field coil had a rather high resistance with consequent voltage drop. OK if you have a mains transformer to compensate, but not if you have direct off-line rectification as in an AC/DC set - which the valve line-up seems to indicate!

[jimmy74]

thanks for all the replies, I haven't been able to find much info about this radio, btw it is a Radio Ricordi model 352. There's only 1 site which describes it vaguely www.radiomuseum.org and they state it having a magnetic speaker and being made around 1953. There are only 2 wires going to the speaker and yes the OT is mounted on the speaker frame, here are some photos of the radio:
http://www.***********/photos/7383552...7622861186124/

Ok it looks like we're pointing towards a normal ceramic speaker, but my question now is, did radios which mounted magnetic speakers, have the OT wires coming straight off the rectifier tube? All eletrodynamic speakers must have 4 wires coming off, or are there exceptions?

thanks

[Mr Moose]

Hello,
If my memory serves me rightly there are sets where the output from the rectifer (and reservoir capacitor) is taken to an intermediate tap on the primary of the output transformer; the feed to the set and smoothing capacitor is taken from the top of the winding and the feed to the anode of the output valve from the bottom of the winding; so the DC currents in opposite directions cancel out the AC hum. If this is the case, you have a feed directly from the rectifer to the output transformer without a energized speaker. This might be the case in your set.
Yours, Richard

[geofy]

Nothing to loose by making a small hole in the front coil cover and dribbling a small amont of easing oil into the speech coil, it could just be stuck due to corrosion. Make sure the magnet hasn't moved and trapped the coil.

g

[espidog]

Hi there Jimmy! You'll know me as "Vortexion" in another place.

This will be your "Radio Ricordi" Model 352 you were telling me about. I wonder if any of our members here are familiar with that model? You never know: somebody might even have a circuit diagram...

[Aerodyne]

To sum up: you have an AC set with a PM loudspeaker. The primary of the output transformer is in series with the output valve anode - one end of the winding to the anode, the other probably to the smoothing capacitor (unless something a little unusual has been done with any tapping on the transformer). The cone is seized. If you can't free it, a replacement unit will have to be found, of approximately the same speech coil impedance. You could measure the resistance of the existing coil, which for a 3 ohm impedance will read perhaps a couple of ohms or less at DC.
-Tony

[jimmy74]

Thanks to all for the great replies, I forgot to add that the cone is pretty badly eaten up in some places so that's why I was more for changing it out for a new speaker. Being a normal speaker I might just go for the Jensen C8R, what do you think?

If anyone does have a schematic for this model, that would be fantastic!! I will be coming here soon for other problems I might come along during repairs and for other vintage radios the same person will be sending me.

thanks again to all for the fantastic help and tips!!

[Nickthedentist]

For testing, any speaker with an impedance of 3-15 ohms will do.

[jimmy74]

I've added some more photos of the speaker here, just to cancel out any doubts at all:

http://www.***********/photos/7383552...7622861186124/

thanks

[Herald1360]

That's definitely a well rusted permanent magnet speaker. Not a ceramic magnet, just a metal one, but the principle is exactly the same. Later ceramic magnets just give more field from a smaller magnet. For this application, any PM speaker that will fit and has a similar voice coil impedance will be fine.

So far as the HT feed to the O/P TX straight from the rectifier is concerned, this isn't unheard of, particularly on AC/DC sets like this one. It's done for cheapness (saves the cost of a power resistor or choke). It's "gotten away with" because output pentodes anode currents are relatively immune to changes in applied HT such as ripple (hum) voltages because of their high value of Ra, plus the fact that 50Hz ripple is much less audible than 100Hz anyway. The supply to the rest of the set including the screen of the O/P valve will be further filtered by another R-C stage, whose R doesn't have to carry the O/P valve anode current (probably 90% of the total HT draw) so it can be small and cheap!

Chris

[jimmy74]

Well I replaced all the caps (except for the antenna paper cap), tubes and did a temporary set up with an old TV speaker I had lying around which is 8 ohms. I first tried it with the original OT mounted, and with the original tubes but wasn't getting much out of it just a hum from the speaker. So I mounted the new tubes in and put up the NOS OT I bought and that brought it to life.

Just a few things I noticed:
1) There are 2 short wave gamma's and 1 medium wave according to the online descriptions, one of these is working fine as I'm picking up French, Spanish, Arabic, English and eastern europe channels, the other 2 just make classic noise. Could these be the short wave gamma's?

2) I noticed that the rectifier tube UY41 glows up very strongly when I turn the radio on, then dies down after a few seconds.... is this normal?

3)I would like to know if replacing the 50pf mica cap, which goes from the UCH42 tube to the gamma selector, with a 150pf mica would be an acceptable replacement?

4) I replaced the 2.2k 10 watt drain resistor on the 2x32uf filter cap, with a 5 watt one, but I've noticed that it does heat up, I am also wondering if 2.2k is a little on the steep end, and maybe I should replace it with a 1.5k 10 watt resistor, what do you think?

thanks

[geofy]

That speaker looks quite salvageable to me, if the coil can be freed with some Plusgas penetrating oil, the cone is in good condition, any small holes can be repaired. The surface rust can be brushed with a rust neutraliser then painted.

And it will still be the original part of the receiver.

Others will have to advise on the UY41, a cold filament will light up as it has less resistance when cold, a thermistor can be used to offset this but other forum members will have better knowledge.

Geof

[Jimmyhaflinger]

Hi, i'm from Italy too!

Even if you fit a replacement speaker (i'd go for a new Jensen P8R with an Alnico magnet) i'd keep the original one somewhere safely.

the center pole has likely shifted off center due to old failing glue and seized the voice coil in the gap, this can usually be fixed by carefuly manipulating the center pole on top of the magnet until the cone moves freely, then the center pole and magnet can be superglued in place.

The exposed center pole in your speaker does't look rusted at all, so i don't think it's seized by rust.

The cone also looks good, those small pinholes (usually eaten by silverfishes) won't affect its operation at all, i'd leave them alone, glue is only going to look ugly on an otherwise intact vintage cone.

It would be a shame to throw away such a nice vintage speaker, it even has spider centering screws!

[geofy]

Originally Posted by Jimmyhaflinger

The exposed center pole in your speaker does't look rusted at all, so i don't think it's seized by rust.

The cone also looks good, those small pinholes (usually eaten by silverfishes) won't affect its operation at all, i'd leave them alone, glue is only going to look ugly on an otherwise intact vintage cone.

It would be a shame to throw away such a nice vintage speaker, it even has spider centering screws!

Yes I agree with this, and the coil probably is just caught and not rusted in, the pole looks in good condition. Even VERY careful manipulation with finger and thumb on the cone might free it, though this can risk cone damage if done to much and with no experience of speakers, if it shows no sign of moving then it should be left alone. But I would try to repair any weakness around the edge of the cone, a small amount of PVA clear adhesive would not look out of place. But even if the speaker can't be repaired it is worth keeping in good storage as someone might be able to do more with it later. I can't quite make out the picture of the center piece but if it has a central spider screw, even releasing this might recenter the cone.

I can't find any mention of it, but that is an output transformer mounted on the speaker frame which will still be needed regardless of which speaker is fitted.

Geof

[jimmy74]

Well I did spray some contact cleaner inside near the edges of the coil, and it did release, but since the dustcap was missing and probably has been missing for a long time, tiny pieces of dust and metal particles have slid in between the coil and the centre, therefore now the cone moves in and out but it is very scratchy and only moves with force applied then remains where you leave it, I have already ordered the new speaker so the old one will be put aside for a better day. As for the OT, I ordered and received a NOS one which works fine, the only thing is that the new speaker doesn't have that part of the frame where the OT would be mounted so I'll probably have to add a piece of wood into the radio and screw in the OT.

thanks