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Antique Radio with Electromagnet on speaker
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tub...@myshop.com
Dec 18, 2018, 6:21:29 PM12/18/18
to
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.
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.
John Robertson
Dec 18, 2018, 8:35:44 PM12/18/18
to
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 :-#)#
--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
(604)872-5757 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."
peterw...@gmail.com
Dec 19, 2018, 8:20:44 AM12/19/18
to
On Tuesday, December 18, 2018 at 6:21:29 PM UTC-5, tub...@myshop.com wrote:
> 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).
>
A few things:
> 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).
>
a) As this is a 1930s radio, be careful not to add too much additional capacitance when replacing the filters as you do not want to raise the B+ beyond 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 could add a bucking transformer on an outside receptacle - you want to be somewhere around 115 V or so - or you could run everything off a Variac, again if your wallplate voltage is over 120 VAC.
b) An outboard choke would be ideal. "Back in the day" those chokes would be 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 than nothing, as long as your replacement choke will handle the current (depends 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 then some, but is readily available:
http://www.classictone.net/40-18003.html 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+ supply - a lethal voltage at lethal current levels - and it is neither secure 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 shame in putting modern oil in a vintage vehicle.
Best of luck with it!
Peter Wieck
Melrose Park, PA
tabb...@gmail.com
Dec 19, 2018, 10:45:45 AM12/19/18
to
I don't know what you do/don't know. Valve rectifiers have tight max capacitance 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
peterw...@gmail.com
Dec 19, 2018, 1:14:43 PM12/19/18
to
On Wednesday, December 19, 2018 at 10:45:45 AM UTC-5, tabb...@gmail.com wrote:
There was good reason in the choke setup rather than just using a bigger cap.
Some historical perspective from this side of the pond, where 300,000,000 "valve" radios were manufactured from about 1913 to about 1963.
There was good reason in the choke setup rather than just using a bigger cap.
a) Low-cost, permanent-magnet speakers did not become common or reliable 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 speakers such as were on large consoles, even less so.
b) Making the choke a (costly) necessity to provide a magnet for the speaker. 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 limitations on capacitance - which protects both the choke (limiting B+) and the transformer (current). Some AC/DC radios used half-wave rectifiers for their voltage-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 manufacturers such as Atwater-Kent were quite specific in their service literature that capacitance values were NOT to be exceeded, again referring to B+ voltages, 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 back 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 replace compromised elements of/within the original item. The only valid endorsement of what is or is not 'good practice' is whether the radio functions properly, 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 continuing use, to that extent, it is a failure.
There are sources for field-coil speakers. There are those that will rebuild existing field-coil speakers. If that level of restoration is your goal, go for it. Whatever else, enjoy it!
tabb...@gmail.com
Dec 19, 2018, 2:38:41 PM12/19/18
to
On Wednesday, 19 December 2018 18:14:43 UTC, pf...@aol.com wrote:
A couple of points to pick up on there. If you look at valve rectifiers, they come with specs on max C. This should not be exceeded. There is good reason 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, go buy it. Too much historic stuff is ruined & destroyed by people with insufficient clue what they're doing.
NT
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, go buy it. Too much historic stuff is ruined & destroyed by people with insufficient clue what they're doing.
NT
tabb...@gmail.com
Dec 19, 2018, 2:39:35 PM12/19/18
to
NT
Ralph Mowery
Dec 19, 2018, 6:09:37 PM12/19/18
to
In article <c7a86686-2882-4c65...@googlegroups.com>,
tabb...@gmail.com says...
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.
tabb...@gmail.com says...
>
> A couple of points to pick up on there. If you look at valve rectifiers, they come with specs on max C. This should not be exceeded. There is good reason for that.
>
>
>
Yes, the tube rectifiers do have a maximum filter capacitor value.
> A couple of points to pick up on there. If you look at valve rectifiers, they come with specs on max C. This should not be exceeded. There is good reason for that.
>
>
>
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.
tabb...@gmail.com
Dec 19, 2018, 8:12:45 PM12/19/18
to
On Wednesday, 19 December 2018 23:09:37 UTC, Ralph Mowery wrote:
> In article <c7a86686-2882-4c65...@googlegroups.com>,
> tabbypurr says...
NT
> In article <c7a86686-2882-4c65...@googlegroups.com>,
> tabbypurr says...
> >
> > A couple of points to pick up on there. If you look at valve rectifiers, they come with specs on max C. This should not be exceeded. There is good reason for that.
> >
> >
> >
>
> 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.
mean i is the same, but peak i is higher & conduction angle narrower.
> > A couple of points to pick up on there. If you look at valve rectifiers, they come with specs on max C. This should not be exceeded. There is good reason for that.
> >
> >
> >
>
> 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.
NT
John Robertson
Dec 19, 2018, 8:20:57 PM12/19/18
to
on the Input To Filter. The note on this value says:
"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!
tabb...@gmail.com
Dec 19, 2018, 8:31:31 PM12/19/18
to
On Thursday, 20 December 2018 01:20:57 UTC, John Robertson wrote:
> On 2018/12/19 3:09 PM, Ralph Mowery wrote:
> > In article <c7a86686-2882-4c65...@googlegroups.com>,
> > tabbypurr says...
> On 2018/12/19 3:09 PM, Ralph Mowery wrote:
> > In article <c7a86686-2882-4c65...@googlegroups.com>,
> >> A couple of points to pick up on there. If you look at valve rectifiers, they come with specs on max C. This should not be exceeded. There is good reason for that.
> >>
> >>
> >>
> >
> > 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.
> >
>
> An example - the 5U4GB (RCA) is recommended to have no more than 40UFD
> on the Input To Filter. The note on this value says:
>
> "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 :-#)#
NT
peterw...@gmail.com
Dec 20, 2018, 10:38:52 AM12/20/18
to
On Wednesday, December 19, 2018 at 6:09:37 PM UTC-5, Ralph Mowery wrote:
> It is the excessive current that is hard on the rectifier tube as the
> capacitor charges and discharges during each cycle.
Only in direct relationship to the actual load. Assume the cap is sized precisely to the anticipated maximum load - then, theoretically, it should discharge 100% and recharge 100% on each cycle.
> It is the excessive current that is hard on the rectifier tube as the
> capacitor charges and discharges during 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 additional capacitance. If it remains the same, not so much.
The analogy is a number of individual cells in parallel. The amount of current 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 the voltage increases, however. And that is why, by specific reference, the likes of Atwater Kent warned against excessive capacitance in their mains-driven radios - mostly with potted paper caps of 2-or-less uF.
Ralph Mowery
Dec 20, 2018, 11:33:27 AM12/20/18
to
In article <e8bde4d2-1688-4a32...@googlegroups.com>,
peterw...@gmail.com says...
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.
peterw...@gmail.com says...
>
> > It is the excessive current that is hard on the rectifier tube as the
> > capacitor charges and discharges during each cycle.
>
> Only in direct relationship to the actual load. Assume the cap is sized precisely to the anticipated maximum load - then, theoretically, it should discharge 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.
>
>
>
This is one of he dumbest statements I have ever heard about how a
> > It is the excessive current that is hard on the rectifier tube as the
> > capacitor charges and discharges during each cycle.
>
> Only in direct relationship to the actual load. Assume the cap is sized precisely to the anticipated maximum load - then, theoretically, it should discharge 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.
>
>
>
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.
peterw...@gmail.com
Dec 20, 2018, 12:59:59 PM12/20/18
to
On Thursday, December 20, 2018 at 11:33:27 AM UTC-5, Ralph Mowery wrote:
>
> 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 %
>
Let's go back to first principles.
>
> 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 %
>
Back when AK designed their power-supply, they used 2uF paper caps. Which did discharge to nearly zero at each cycle. And, they designed "onward" accordingly. And they warned specifically about increasing capacitance. And they used other means to clean AC out of the line, typically by way of a massive choke.
Not so much time later (about 4 years, in their case), they moved onto almost exclusively electrolytic caps for filtering, typically at about 10uF, whereupon their field coils got much smaller (same size speaker) and they stopped 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 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 mater the size of the capacitor, current through the transformer will not be excessive - 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 much as 200%. The theory behind it is to increase peak response and reduce clipping. That particular unit uses the 5AR4/GZ34 rectifier, nominally rated at 60uF, Power-supplies using this rectifier using 4-section caps totalling 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, and 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-rectifier 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 though 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 radio. But, there it is.
tabb...@gmail.com
Dec 20, 2018, 1:49:25 PM12/20/18
to
NT
peterw...@gmail.com
Dec 20, 2018, 2:08:03 PM12/20/18
to
On Thursday, December 20, 2018 at 1:49:25 PM UTC-5, tabb...@gmail.com wrote:
> so Peter still doesn't understand the issue. What's new.
Tabby:
> so Peter still doesn't understand the issue. What's new.
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-function of a vintage radio. That there are those here chasing after miscellaneous squirrels within the piles is a sideshow.
Dave M
Dec 20, 2018, 2:12:40 PM12/20/18
to
^^^^^^^^^^^^ CORRECT!!!!!!!! ^^^^^^^^^^^^^^^^^^
Dave M
tabb...@gmail.com
Dec 20, 2018, 2:26:11 PM12/20/18
to
On Thursday, 20 December 2018 19:08:03 UTC, pf...@aol.com wrote:
NT
> On Thursday, December 20, 2018 at 1:49:25 PM UTC-5, tabby wrote:
>
> > so Peter still doesn't understand the issue. What's new.
>
>
> 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-function of a vintage radio. That there are those here chasing after miscellaneous squirrels within the piles is a sideshow.
>
> Peter Wieck
> Melrose Park, PA
You should tell all the valve mfrs why their specs don't matter. Why don't I think they'll find your argument convincing.
>
> > so Peter still doesn't understand the issue. What's new.
>
>
> 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-function of a vintage radio. That there are those here chasing after miscellaneous squirrels within the piles is a sideshow.
>
> Peter Wieck
> Melrose Park, PA
NT
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