Early AC-powered sets, c. late '20s and early '30s, have extremely heavy power xfmrs. Sure, xfmr technology, particularly core material, has improved, but we're talking about maybe a factor of 10 heavier compared to more recent models.

What were they doing (or not doing) that caused the xfmrs to be so heavy?

Early sets of the 1920s often used tubes employing high current heaters or filaments. These require a hefty filament winding. Take a look at the Grigsby Grunow 180 (I own this set). Look at the 8-P-6 power supply. It has two transformers, one for A and one for B current. The A transformer has two windings, one for each #50 tube, and then one or two more windings (I forgot) for the rest of the 2.5 volt tubes. #24 and #27 tubes draw amazing amounts of current. 2A3, #50, #45, #47, and similar tubes draw amazing amounts of current as well.

If you have a radio with a 25 cycle transformer, this is going to be HUGE. The lower frequency requires more iron.

Transformer technology did improve slightly by the 1930s, but not really. Most of the transformers of the 1920s are either built for the high current consumption or are overbuilt. You'll still find the high current tubes, mentioned above, in 1930s sets, but unless they're expensive sets, the transformer will be underrated, and will get very warm.

Take a look at cabinet construction before and after the Depression start, too. A lot of cabinets had 3/4 inch plywood prior to the Depression. Afterward you find a lot of 1/4 inch wood with strengtheners--like extra wood at the rear edges which strengthens the wood and also makes it look thicker. This continues right into the 1940s. By the 1950s, only consol radios are made of wood, and these usually have thick wood. The economy is great by this time, though, and if anyone actually wants to buy a consol, they usually want to pay extra for it, since radio is no longer the big thing anymore. A consol radio of the 1930s and 1940s, once something that everyone wanted, is often replaced with a television set. The consol of the 1950s is often high-end, so extra money is going to be spent on the cabinet. Those who don't want (or need) this high-end technology will buy a cheap or moderately priced table model radio.

Thomas

Thomas, what you say makes sense, but I'm still scratching my head on this set:

I presently have a Kolster K-20, c. 1928, on my bench. This is an AC-powered, 7-tube, table-model TRF. Its nameplate says 60 cycles, 50 watts. This bruiser weighs in at 50+ lb (excluding any external speaker). Criminey!

At 50W, this thing doesn't take much more power than most prewar AC superhets that weigh maybe 10-15 lbs.

I think the technology just improved so they were able to make things smaller and lighter. I had one of the very first "CELL" phones that came out and I'm not sure that was the correct term but it was in a suit case and it weighed about 50lbs.
Greg

:Thomas, what you say makes sense, but I'm still scratching my head on this set:
:
:I presently have a Kolster K-20, c. 1928, on my bench. This is an AC-powered, 7-tube, table-model TRF. Its nameplate says 60 cycles, 50 watts. This bruiser weighs in at 50+ lb (excluding any external speaker). Criminey!
:
:At 50W, this thing doesn't take much more power than most prewar AC superhets that weigh maybe 10-15 lbs.

Well, just because the transformer is huge doesn't necessarily mean that it's going to draw a lot of current. Remember that minus the inefficiencies of the transformer, a transformer only draws what is being drawn from it on the other side. Does this transformer get hot or does it barely get warm? If it barely gets warm, chances are it's overkill. This set was built pre-Depression. A lot of radios were made overkill prior to this time. Just look at every part in one of these pre-1930 radios. Some of them use grossly oversized wire. The tube sockets and condensers are elaborate with heavy casings, etc. (not every one, but a lot). It is wiser to overbuild things anyway. This puts less stress on the parts. Of course when everyone is broke, you need to cut corners. As it stands, quite a few of the hot running transformers are still working to-day. Unfortunately many did burn out because of this, too.

WOW! I just took a look at your schematic, too. No wonder your set is so heavy. Just look at how it's built! It's WAY overkill! First of all, since it employs filamentary tubes, separate filament windings are used. The reason for this is because the cathodes are all biased differently. Since the cathode in each tube is also the filament, the filament wiring must be separated for separate biasing. You have FIVE different filament windings (including the rectifier winding). Filament windings use fairly heavy wire, so they are not compact--they make the transformer large.

Now take a look at the rest of the schematic. This radio is loaded with all of those 1920s things which got omitted from most sets in the 1930s due to expense. You've got transformer coupling in the audio. These are heavy. You have choke type filtering in the power supply along with your speaker's field coil. I didn't look, but if this is the parallel type instead of the series type, it's going to be quite large.

Looking at all of this it is easy to see that transformer technology did not improve much if at all by the 1930s. Your set is simply overbuilt, which is great. Be proud of it. Many an old radio book from the 1940s, 1950s, and 1960s, talks about the skill, quality, and craftsmanship which went into these old radios, and how they should be appreciated even though they lack superheterodyne technology and compactness. To-day not many people appreciate this anymore. I can say that I do usually, but even I am a victim to modern economization, which is good, I guess, to a degree. I tend to look at things and think of ways to economize on materials and fuel. I guess this is because I hear so much about environmental problems and such. I also tend to be thrifty, and I get every last drop out of my soap bottles. At any rate, if we didn't have to worry about pollution and such, it'd be great if everything was made like 1920s radios. If cars were made this way, they'd be so much more rugged. As things stand, though, cars of to-day last much longer than any car of prior times, so I guess all is good.

Thomas

OK, thanks Thomas for your analysis of the schematic.

I've got the set working, after a fashion, but I need to do a little more work. The cabinet is pretty good.

Here's another thing about these old AC-powered TRFs. Very often, the original filter caps in the power supply are OK - which is good, because they're often potted in tar.

These filter caps were usually not electrolyics, and were rather small, electrically - typically a few uFs. I wonder what they used for the dielectric material? If paper, they must of used very good quality paper, unlike in the later type of paper caps.

Hi Thomas:... sorry to jump in here on this...but that can't be exactly true...can it?..
Why does the primary of my isolation transformer draw 200ma with a totally unloaded secondary?

Just the inductive reactance (expressed in ohms) of the primary by itself, at 60Hz, is a curent drawing load...no?
peter

:Well, just because the transformer is huge doesn't necessarily mean that it's going to draw a lot of current. Remember that minus the inefficiencies of the transformer, a transformer only draws what is being drawn from it on the other side.

Peter: a transformer has two different losses. One is the copper loss, the I^2R loss thru the windings; this loss is zero when the secondary is open.

The other is the iron (or core) loss, which is the power consumed to continually magnetize and remagnetize the core. This loss is what causes you to measure a primary current when the secondary is open. As a percent of full-load losses, this is usually just a few percent.

I misspoke. The core losses will be more than "just a few percent" of full-load losses. But, the current to supply the core losses will be a few percent of the full-load current.

But you're right - there will also be a current thru the primary, with the secondary open, which is related to the inductive reactance of the primary winding. This current will be out of phase with the voltage, and will produce essentially no power.

While I write a lengthy and not exactly well written explanation, Doug comes up with the perfect answer, which is short and to the point. I envy some people's brains. My brother has a brain like that, too. He gets all As in his nursing school. Mine's complex and confusing.

Thomas

So let's see... that's interesting... if I am watching my electric meter and it's not moving at all... then I plug in my isolation transformer primary... (even tough my amp meter reads 200ma) you say my electric meter will not move?

:But you're right - there will also be a current thru the primary, with the secondary open, which is related to the inductive reactance of the primary winding. This current will be out of phase with the voltage, and will produce essentially no power.

No, your kWh meter will move - to supply the core losses of the xfmr. If the connected load were a pure inductance (or pure capacitance), the kWh meter would not move.

Okay.. I think I get it... however all these non-power thingie still make that meter move and I'll still get a bill.
Let's see.. humm... my door bell transformer draws about 50ma unloaded... we pay 13 cents per/kwh here so thats about $6 a year waiting for somebody to ring my bell!!
Gee I should help the national economy a little... if I move the door bell button to the primary side... I'll save that 6 bucks!!!
Now we've got about 250 million people in this country with about 2.5 people per family... all living somewhere ... that's 100 million doorbells at 6 bucks wasted a year...that's 600 million dollars I can save the American public with just one simple idea!
...Hey! president Bush... listen to this new idea I just got....

Well, Peter, don't move the doorbell switch to the primary side. The switch and the wiring are not meant for that kind of voltage. Not to mention that when it rains on the switch, you'll have a mess anyway. Either way you look at it, it's a fire hazard and a shock hazard.

What you can do, though, is make up extension cords with line switches on them. You can get these snap switches at your hardware store. Plug all of your new stereo and television equipment into these switches. Whenever you're not in the room, turn the switches off (one per entertainment center, not one per device...that'd be excessive). Modern televisions and such (like since the early 1970s) do not completely turn off. The only problem with this is that a lot of older televisions and some new televisions will lose memory if you unplug them. If you program your favorite television stations, and your preferred color and brightness settings into the television, they may be lost when you turn off that extension switch. Some devices with a clock display will not keep time, either.

Anyway, there's a lot more than just your doorbell transformer. It's annoying, too. They should put batteries in the sets for the memory. The batteries would recharge while you watched television. If they didn't, oh well. The remote control system must also be kept on, though, for obvious reasons. Philco's Mystery Control had it down--you could only turn the radio off by remote control. You couldn't turn it back on. I like that.

The neon bulb in those glow switches (the ones which light when you turn off the switch) consume an extremely small amount of current. This isn't something to really think about. I don't know if it will actually turn your meter. If your meter is digital, it may, but with a mechanical meter, there may be a small amount of frictional force to overcome before the meter starts turning.

Also, since alternating current is used in your home, there are capacitive losses through the wiring. If you ever have one of those neon glow switches in your home, unplug and remove all lights, etc. from the circuit which the neon glow switch controls. You'll notice that the neon bulb dims down, but doesn't go out. The reason why it still stays lit even though it doesn't have devices for current to flow through is because of the capacitive losses through the wiring in your home (the hot and return wires are next to eachother, which forms a capacitor).

As I said before, those little plug in AC-DC converters (box like plug with a thin cord that plugs into cell phones, etc.....a cell phone charger for instance) consume current even when nothing's connected to them.

Thomas

Sure Thomas.. I realize that.. my post was purely tongue-in-cheek.
I would never move the existing low-voltage switch to the primary side..Zap! of course... although there are a few doorbell ringers I'd like to zap... like vote-seeking politicians and then there's those Jehova witness folks...
But thinking "green" ... why don't they make newly designed doorbell stuff like that with the switch on the primary?... probably a better idea than the brick in the toilet tank idea to save water. After all water IS a renewable resource.

:Well, Peter, don't move the doorbell switch to the primary side. The switch and the wiring are not meant for that kind of voltage. Not to mention that when it rains on the switch, you'll have a mess anyway. Either way you look at it, it's a fire hazard and a shock hazard.

Never did get the whole water saving notion. Here in Milwaukee we have a huge lake. In a desert it's a wise idea to conserve water. Here it makes no sense. They make these toilets which you have to flush two or three times anyway, so where's the water savings? I like the toilets at my parents'. I restored all of them...they even have brass floats. They're all from the 1940s and they flush like Niagra Falls. If you take a drill bit you can clean out the water holes around the rim. You can clean the rust stains with Wink, and the calcium with Wink or vinegar. I see people throw away old toilets all the time--beautiful ones, not the ugly crap sold to-day. It is amazing how many unintelligent (brainless) people there are who follow anything any so-called expert says. People will get in heated arguments with me over their beloved new toilets. Well, I'll get in a heated argument right back at them for my attractive toilets which actually get the job done. My sister's nextdoor neighbor threw out her 80 year old toilet (magnificent toilet in a magnificent Craftsman house which was once owned by a sweet lady) because the flush valve was leaking. I could have given her a few choice words (or a hard smack upside the head), but she really doesn't have a brain at all. I just didn't think it would be worth my time. I initially offered to fix the toilet and after I got a moronic answer, I left the issue alone. Sad.

Not sure what to do about doorbells. You could use rechargable batteries and a small solar cell mounted on your roof. Since you almost never use the doorbell, the batteries would have plenty of charging time. I don't know how long they'd last. Perhaps they'd last a long time without use, provided that they're sealed properly. Dried out batteries don't work well. Give it a try. See what happens. You need about 10 or 15 volts, though since batteries are direct current, you may only need 10 volts or less. It takes more alternating current to operate a doorbell than direct current. This is due to the choking transformer action of the bell coil.

Thomas

Your electric meter, the one on the side of your house, will be moving when you plug in your isolation transformer. Current is flowing through the primary of your transformer (which also flows through the utility meter). Current is not flowing through the secondary unless it is loaded with something (direct short, lightbulb, radio, fan, etc.). A transformer is much like a generator. A moving magnetic field will generate current in a nearby coil of wire. The moving magnetic field is like the moving magnets in a generator, and moves because alternating current passing through a coil switches flow direction 60 times a second. Now, if you have nothing connected across the secondary, current cannot flow through the secondary. If the secondary was a water pipe with water that wanted to move back and forth, it could not unless something was connected across it to allow it to flow in a ciruit.

Now just because the current in the secondary cannot flow doesn't mean that the primary isn't going to conduct current. It's still connected to the line, and makes a complete circuit.

Thomas

When we speak of "xfmr losses," it's "real" power - the stuff you get charged for by the utility company. The current supplied to a pure inductive (or capacitive) load does not consume real power (only reactive power - except for the I^2R loss thru the conductors), and your kWh meter won't turn.

Transformers do draw current regardless of whether they are loaded or not. As you said, the inductive resistance (alternating current impedance) allows some current to flow through. Upon this depends the efficiency of the transformer (along with other things). Adding more iron pieces to the transformer core, either by making a larger core with existing style iron pieces or by making a core identical in size to a less efficient one, but with thinner iron pieces, allowing for more pieces, will make the transformer conduct less when unloaded. Increasing the primary turns (which requires increasing secondary turns in order to keep turn ratio constant, and also requires increasing wire gauge in order to keep DC resistance the same--amperage will go down if there's too much DC resistance due to too much wire) will also increase transformer efficiency.

Transformers lose energy due to currents being generated in the core, which happens less when the core is made up of more insulated segments, and also lose energy due to current loses through the primary if the primary is not large enough. A transformer is essentially a choke with a secondary which is magnetically influenced by the primary. As we know, the higher the frequency imposed upon a choke, the higher the choke's alternating current impedance. I'm not going to go through all that again, because it takes a lot to explain it, and I'm not very good at it. At any rate, you can also increase a transformer's efficiency by raising the frequency of the current....to a degree. There is a point at which some transformers will not pass certain frequencies efficiently (not the same as an inefficient primary), when these frequencies are above this point. The *transfer* efficiency tapers off at this point. The transformer does not consume more current due to this type of inefficiency, though.

Anyway, taking all of this into consideration gives you some idea as to what you're working with. Those little black (or other color) power supplies for your cordless telephones, cell phones, etc., which plug in the wall, will get hot regardless of whether they're plugged into their consuming devices (telephone or other load) or not. This is because they're inefficient. They're designed to be small. This makes it necessary to cut back on wire and iron. Turn count is not necessarily cut back on, but the overall length of the wire is going to be less due to the small size of the iron core. Though this won't cut down the AC impedance much, it'll cut down the DC resistance, which, at a low frequency like 60 cycles, will have a great affect on the efficiency of the transformer

Thomas