Antique Radio with Electromagnet on speaker

I got an old 1930s Crosley AM radio. It needs to be recapped and a few other things, such as a dial string and new power cord. Anyhow, after chopping off the bad power cord, I applied a temporary cord and the radio does get sound and radio stations. (Along with the hum from the weak filter caps).

This radio has a beautiful old wooden cabinet, so I am going to restore it completely.

Anyhow, someone replaced the old electromagnet (Field coil) speaker with a permanent magnet speaker. But what they did is sort of odd. I know that speaker coil served as a filter choke, in the power supply, on those old radios. What they did, was saw off the rear of the old speaker, and mount it to the chassis, with that magnet loosly sitting on top of the sawed off rear portion of the old speaker.

What I now have, is this large magnet sitting next to the audio output tube, and very close to the power transformer as well. That magnet is quite powerful, in fact I magnetized a screwdriver with it.

My question is whether that magnetism is going to affect that tube, as well as interfere with the power transformer's function?

If this is a problem, how to I determine what size filter choke to buy to replace it? (Or isn't that real critical)? If it's not going to cause any problems, I may likely just leave it as it is, except to apply some sort of adhesive to prevent that magnet from falling off the sawed off steel piece. Because as it sits now, if I set the radio on edge, that magnet will fall off and could smash tubes in the process. I have never seen this sort of thing done.... Very strange!

By the way, the audio output transformer appears to be glued on to the speaker frame, but it is tight, so I am going to just leave it.

Reply to
tubeguy
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The field coil will not affect any tubes (in your type of usage) unless it falls on them...so make sure it is secure and that it doesn't get hot enough to heat up the wood cabinet.

John :-#)#

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Reply to
John Robertson

A few things:

a) As this is a 1930s radio, be careful not to add too much additional capa citance when replacing the filters as you do not want to raise the B+ beyon d specified levels. This is hard enough in any case as it is likely that th e line voltage today is higher than it was when the radio was new. You coul d add a bucking transformer on an outside receptacle - you want to be somew here around 115 V or so - or you could run everything off a Variac, again i f your wallplate voltage is over 120 VAC.

b) An outboard choke would be ideal. "Back in the day" those chokes would b e anywhere from about 1 H to about 5 H, and rated from about 20 ma to about 60 ma, at a B+ of about 350 VDC or so. Pretty much as anything is better t han nothing, as long as your replacement choke will handle the current (dep ends on the output tubes), even a lower-value choke will help. One-of-many links is here for one-of-many chokes that will handle what you have, and th en some, but is readily available:

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It may be mounted anywhere that is convenient and safe.

c) Stray magnetism will not affect any part of the radio. Placement of said parts should be a matter of convenience and safety.

d) If it is your intention to retain the old field-coil for its value as a choke, I would strongly suggest that you replace the entire speaker with a (new/different) unit, and remove the stray coil. It carries the entire B+ s upply - a lethal voltage at lethal current levels - and it is neither secur e as you describe it, nor designed for that sort of installation. If you ar e going to retain the PM speaker, use an outboard choke properly mounted an d secured. There is no shame in that, by the way. Just as there is no shame in putting modern oil in a vintage vehicle.

Best of luck with it!

Peter Wieck Melrose Park, PA

Reply to
peterwieck33

e:

pacitance when replacing the filters as you do not want to raise the B+ bey ond specified levels. This is hard enough in any case as it is likely that the line voltage today is higher than it was when the radio was new. You co uld add a bucking transformer on an outside receptacle - you want to be som ewhere around 115 V or so - or you could run everything off a Variac, again if your wallplate voltage is over 120 VAC.

be anywhere from about 1 H to about 5 H, and rated from about 20 ma to abo ut 60 ma, at a B+ of about 350 VDC or so. Pretty much as anything is better than nothing, as long as your replacement choke will handle the current (d epends on the output tubes), even a lower-value choke will help. One-of-man y links is here for one-of-many chokes that will handle what you have, and then some, but is readily available:

at is convenient and safe.

id parts should be a matter of convenience and safety.

a choke, I would strongly suggest that you replace the entire speaker with a (new/different) unit, and remove the stray coil. It carries the entire B+ supply - a lethal voltage at lethal current levels - and it is neither sec ure as you describe it, nor designed for that sort of installation. If you are going to retain the PM speaker, use an outboard choke properly mounted and secured. There is no shame in that, by the way. Just as there is no sha me in putting modern oil in a vintage vehicle.

Secure the choke, there's no reason to remove it. Replacing things without reason on prewar equipment is bad practice.

I don't know what you do/don't know. Valve rectifiers have tight max capaci tance specs, radios put as much C on them as they can. Don't increase C or it will overload the valve. There was good reason in the choke setup rather than just using a bigger cap.

NT

Reply to
tabbypurr

On Wednesday, December 19, 2018 at 10:45:45 AM UTC-5, snipped-for-privacy@gmail.com wro te: There was good reason in the choke setup rather than just using a bigger c ap.

Some historical perspective from this side of the pond, where 300,000,000 " valve" radios were manufactured from about 1913 to about 1963.

a) Low-cost, permanent-magnet speakers did not become common or reliable un til well into the 1940s, and much of that was driven by war-time research. Sure, small speakers for portable radios were common, but larger speakers d esigned for severals-of-watts of power, not so much. And big, honking speak ers such as were on large consoles, even less so.

b) Making the choke a (costly) necessity to provide a magnet for the speake r. Not a convenience. And, with that in mind, many manufacturers then cut b ack on the size of the transformers to save costs. Leading to the limitatio ns on capacitance - which protects both the choke (limiting B+) and the tra nsformer (current). Some AC/DC radios used half-wave rectifiers for their v oltage-dropping properties.

c) One may, pretty much, used as much capacitance as one wishes _AS LONG AS _ the B+ remains withing spec. And this can be a close-run thing. Note that in the days of potted paper caps, some of the earlier consumer radio manuf acturers such as Atwater-Kent were quite specific in their service literatu re that capacitance values were NOT to be exceeded, again referring to B+ v oltages, not rectifier or transformer overloads. And, as it happened, these radios tended to have very large chokes, weighing several pounds in copper alone.

c) When repairing (vs. a museum-quality restoration), you have options not available to the OEM, as well as knowledge and materials unknown to them ba ck when all this stuff was "New". What this means is that you may, without sin, make upgrades or make repairs based on present-day technology to repla ce compromised elements of/within the original item. The only valid endorse ment of what is or is not 'good practice' is whether the radio functions pr operly, safely and without danger to its users into the foreseeable future. That it uses as many as is safe and practical of the original elements is a good thing. But, if it does that by means of compromising its safe and co ntinuing use, to that extent, it is a failure.

There are sources for field-coil speakers. There are those that will rebuil d existing field-coil speakers. If that level of restoration is your goal, go for it. Whatever else, enjoy it!

Peter Wieck Melrose Park, PA

Reply to
peterwieck33

cap.

"valve" radios were manufactured from about 1913 to about 1963.

until well into the 1940s, and much of that was driven by war-time research . Sure, small speakers for portable radios were common, but larger speakers designed for severals-of-watts of power, not so much. And big, honking spe akers such as were on large consoles, even less so.

ker. Not a convenience. And, with that in mind, many manufacturers then cut back on the size of the transformers to save costs. Leading to the limitat ions on capacitance - which protects both the choke (limiting B+) and the t ransformer (current). Some AC/DC radios used half-wave rectifiers for their voltage-dropping properties.

AS_ the B+ remains withing spec. And this can be a close-run thing. Note th at in the days of potted paper caps, some of the earlier consumer radio man ufacturers such as Atwater-Kent were quite specific in their service litera ture that capacitance values were NOT to be exceeded, again referring to B+ voltages, not rectifier or transformer overloads. And, as it happened, the se radios tended to have very large chokes, weighing several pounds in copp er alone.

t available to the OEM, as well as knowledge and materials unknown to them back when all this stuff was "New". What this means is that you may, withou t sin, make upgrades or make repairs based on present-day technology to rep lace compromised elements of/within the original item. The only valid endor sement of what is or is not 'good practice' is whether the radio functions properly, safely and without danger to its users into the foreseeable futur e. That it uses as many as is safe and practical of the original elements i s a good thing. But, if it does that by means of compromising its safe and continuing use, to that extent, it is a failure.

ild existing field-coil speakers. If that level of restoration is your goal , go for it. Whatever else, enjoy it!

A couple of points to pick up on there. If you look at valve rectifiers, th ey come with specs on max C. This should not be exceeded. There is good rea son for that.

And no, it is not good practice to re-engineer antique radios without even having a reason. Not a bit. If you want modern junk there's plenty about, g o buy it. Too much historic stuff is ruined & destroyed by people with insu fficient clue what they're doing.

NT

Reply to
tabbypurr

er cap.

00 "valve" radios were manufactured from about 1913 to about 1963.

e until well into the 1940s, and much of that was driven by war-time resear ch. Sure, small speakers for portable radios were common, but larger speake rs designed for severals-of-watts of power, not so much. And big, honking s peakers such as were on large consoles, even less so.

eaker. Not a convenience. And, with that in mind, many manufacturers then c ut back on the size of the transformers to save costs. Leading to the limit ations on capacitance - which protects both the choke (limiting B+) and the transformer (current). Some AC/DC radios used half-wave rectifiers for the ir voltage-dropping properties.

G AS_ the B+ remains withing spec. And this can be a close-run thing. Note that in the days of potted paper caps, some of the earlier consumer radio m anufacturers such as Atwater-Kent were quite specific in their service lite rature that capacitance values were NOT to be exceeded, again referring to B+ voltages, not rectifier or transformer overloads. And, as it happened, t hese radios tended to have very large chokes, weighing several pounds in co pper alone.

not available to the OEM, as well as knowledge and materials unknown to the m back when all this stuff was "New". What this means is that you may, with out sin, make upgrades or make repairs based on present-day technology to r eplace compromised elements of/within the original item. The only valid end orsement of what is or is not 'good practice' is whether the radio function s properly, safely and without danger to its users into the foreseeable fut ure. That it uses as many as is safe and practical of the original elements is a good thing. But, if it does that by means of compromising its safe an d continuing use, to that extent, it is a failure.

build existing field-coil speakers. If that level of restoration is your go al, go for it. Whatever else, enjoy it!

they come with specs on max C. This should not be exceeded. There is good r eason for that.

n having a reason. Not a bit. If you want modern junk there's plenty about, go buy it. Too much historic stuff is ruined & destroyed by people with in sufficient clue what they're doing.

I've just realised it's you again. You'll never see sense so I won't bother continuing.

NT

Reply to
tabbypurr

Yes, the tube rectifiers do have a maximum filter capacitor value. Adding much more capacitance will not raise the voltage in most cases. It is the excessive current that is hard on the rectifier tube as the capacitor charges and discharges during each cycle.

Reply to
Ralph Mowery

mean i is the same, but peak i is higher & conduction angle narrower.

NT

Reply to
tabbypurr

An example - the 5U4GB (RCA) is recommended to have no more than 40UFD "Higher values of capacitance than indicated may be used, but the effective plate-supply impedance may have to be increased to prevent exceeding the maximum value for peak plate current."

From the RCA Receiving Tube Manual - RC-30 - 1975.

I have a nice assortment of tech books on tubes as we fix tube amps regularly for jukeboxes.

Hmm, tube amps...I wonder if we should install them in pinball games...that would be a hoot!

John :-#)#

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Reply to
John Robertson

My 1924 set has all of a few 2uF paper caps on the supply line. They cheated though - it's for dc mains. I've seen rather less than 40uF on some ac mains sets.

NT

Reply to
tabbypurr

Only in direct relationship to the actual load. Assume the cap is sized pre cisely to the anticipated maximum load - then, theoretically, it should dis charge 100% and recharge 100% on each cycle.

Assume that the cap is sized to 100% more than the anticipated load - then, theoretically, it will discharge to 50% and recharge to 100% on each cycle .

Also assuming that the power-supply is capable of delivering the full load

  • some additional amount of current.

Overloading the transformer will occur if the load increases due to the add itional capacitance. If it remains the same, not so much.

The analogy is a number of individual cells in parallel. The amount of curr ent they can deliver increases. But a 5 watt lamp will remain a 5 watt lamp , no matter what *could be* delivered. And that DC motor will not spin any faster if it is fed its design voltage and amps. What is a capacitor other than an ultra-fast battery? All sorts of (usually bad) things happen when t he voltage increases, however. And that is why, by specific reference, the likes of Atwater Kent warned against excessive capacitance in their mains-d riven radios - mostly with potted paper caps of 2-or-less uF.

Peter Wieck Melrose Park, PA

Reply to
peterwieck33

This is one of he dumbest statements I have ever heard about how a capacitor works in a power supply. In the normal tube or simple diode supply the capacitor never discharges anywhere near 100 %

As a capacitor discharges to any percentage and then recharges, the ammount of capacitance determins how much peak current will be drawn from the transformer and through the rectifier. That is what is hard on the rectifier tubes and why they sp;ecify a certain maximum capacitance.

Reply to
Ralph Mowery

Let's go back to first principles.

Back when AK designed their power-supply, they used 2uF paper caps. Which d id discharge to nearly zero at each cycle. And, they designed "onward" acco rdingly. And they warned specifically about increasing capacitance. And the y used other means to clean AC out of the line, typically by way of a massi ve choke.

Not so much time later (about 4 years, in their case), they moved onto almo st exclusively electrolytic caps for filtering, typically at about 10uF, wh ereupon their field coils got much smaller (same size speaker) and they sto pped warning about increased capacitance. In most cases, they still used th e 4-pin 80 tube as their rectifier. Rated at about 10uF. Note that in the H andbook, the 80 will accept much higher capacitance values "if voltage and current are controlled" - which is entirely the point.

Now, taking what you state - a larger capacitor will draw a larger charge. True *upon first charge*. After which, it will cycle just as with any other cap based on downline draw. If the downline draw is not excessive, no mate r the size of the capacitor, current through the transformer will not be ex cessive - assuming voltage and current are stable.

Now, moving into semi-modern times - one of the most popular hacks for the venerable Dynaco ST70 is to greatly increase capacitance - some times as mu ch as 200%. The theory behind it is to increase peak response and reduce cl ipping. That particular unit uses the 5AR4/GZ34 rectifier, nominally rated at 60uF, Power-supplies using this rectifier using 4-section caps totallin g as much as 320uF are common. The downline draw is not changed on-average, the output transformers prevent any stray DC from reaching the speakers, a nd there is some momentary additional peak-response capacity.

A note here from a discussion on chokes vs. caps vs. input size vs. output side and so forth:

Probably because the d.c. output voltage from the same transformer-rectifie r system will be higher when a capacitor is added in front of a choke-input filter. This neglects transformer heating, which is higher for the same d. c. power output with capacitor input than it is with choke input, even thou gh the output voltage is lower with choke input.

Leading to the point on voltages.

This is straying away from the original discussion on modifying a vintage r adio. But, there it is.

Reply to
peterwieck33

did discharge to nearly zero at each cycle. And, they designed "onward" ac cordingly. And they warned specifically about increasing capacitance. And t hey used other means to clean AC out of the line, typically by way of a mas sive choke.

most exclusively electrolytic caps for filtering, typically at about 10uF, whereupon their field coils got much smaller (same size speaker) and they s topped warning about increased capacitance. In most cases, they still used the 4-pin 80 tube as their rectifier. Rated at about 10uF. Note that in the Handbook, the 80 will accept much higher capacitance values "if voltage an d current are controlled" - which is entirely the point.

. True *upon first charge*. After which, it will cycle just as with any oth er cap based on downline draw. If the downline draw is not excessive, no ma ter the size of the capacitor, current through the transformer will not be excessive - assuming voltage and current are stable.

e venerable Dynaco ST70 is to greatly increase capacitance - some times as much as 200%. The theory behind it is to increase peak response and reduce clipping. That particular unit uses the 5AR4/GZ34 rectifier, nominally rate d at 60uF, Power-supplies using this rectifier using 4-section caps totall ing as much as 320uF are common. The downline draw is not changed on-averag e, the output transformers prevent any stray DC from reaching the speakers, and there is some momentary additional peak-response capacity.

t side and so forth:

ier system will be higher when a capacitor is added in front of a choke-inp ut filter. This neglects transformer heating, which is higher for the same d.c. power output with capacitor input than it is with choke input, even th ough the output voltage is lower with choke input.

radio. But, there it is.

so Peter still doesn't understand the issue. What's new.

NT

Reply to
tabbypurr

On Thursday, December 20, 2018 at 1:49:25 PM UTC-5, snipped-for-privacy@gmail.com wrote :

Tabby:

What I understand is that there is no issue. That which is being discussed here is on the separation of flyshit from pepper, not the restoration-to-fu nction of a vintage radio. That there are those here chasing after miscella neous squirrels within the piles is a sideshow.

Peter Wieck Melrose Park, PA

Reply to
peterwieck33

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^ CORRECT!!!!!!!! ^^^^^^^^^^^^^^^^^^

Dave M

Reply to
Dave M

d here is on the separation of flyshit from pepper, not the restoration-to- function of a vintage radio. That there are those here chasing after miscel laneous squirrels within the piles is a sideshow.

You should tell all the valve mfrs why their specs don't matter. Why don't I think they'll find your argument convincing.

NT

Reply to
tabbypurr

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