BUCKING TRANSFORMER VS POWER RESISTOR to lower the TODAY Line Voltage to our old radios.

PRO'S and CON'S

Simplicity and follows the radio forever +1 Resistor
Cost of parts. +1 Resistor $3 to $4 per radio

Power Wasted through heat. Again the RESISTOR wins this one. +1

WHAT that resistor gets HOT. That can't possibly be true. Yes, nearly the same watts burned in the bucking transformer. Just that those heat watts are in a much smaller package therefore yes the temperature of the resistor is higher than that of the bucking transformer. +1 for resistor.

Watts are still Watts current times voltage = watts burned up as heat.

If you think the resistor is burning up more watts then try this. Hook up your bucking transformer and also hook up your Kill-A-Watt or your Fluke meter in current mode. Radio running through bucking transformer = watts.

Now turn OFF your radio and re-read your Fluke / Kill-A-Watt and see who many watts your burning up in heat with the Bucking transformer. Yes it gets left turned ON many times. It keeps on wasting WATTS.

I have over 200 restored radios in our collection. Do I want 200 bucking transformers in external boxes setting around with each of those radios? What if I sell a radio to some unsuspecting buyer, Will he want to take the bucking transformer and the box along and always plug the radio into it? Likely not. However if it has an internal resistor it will always be running at slightly reduced voltage and the odds are the radio will last many more years than without the resistor. +1 resistor

Yes usually when you get to a 15 tube radio your probably better off using a bucking transformer. Also with many 15 tube and larger radios you can hide the bucking transformer under the chassis and rewire the On/Off switch so the Bucking transformer is turned off by the radio On/Off switch.

Don't forget sooner or later all those radios you have worked so hard to get back to looking nice and playing are going to be in the hands of someone else. Do you want your effort to last for near eternity or do you want to see it go up in smoke from over voltage someday?

Radios with motor driven dials the MOTOR needs to continue to run on FULL Line Voltage. So wire it that way when you install the Power Resistor to the Power Transformer. +1 Resistor

Flame Suit On
John Goller k9uwa

I was just listening to my 1934 Bosch. Rather than a resistor cord it has a resistance riveted to the back of the chassis. Metal frame and pretty long. And despite the holes in the back it is played for short periods, occasionally. When I got it, it had been recapped some time in the 70s as far as electrolytics, not the paper caps. Lot of stuff was turned blackish looking due to the heat buildup under the chassis. I only listen to one radio at a time, so I can use my variac or one of my transformers with several taps/settings for like 105V and up. In the instance it seems to be needed. You can do whichever method seems most fitting to the particular situation. Other than the fact that I will not use an old resistance cord under any circumstances.

In the above I didn't mention Curtain Burner cords or large resistors in early AC/DC radios. So since you brought it up. Have you looked at this page?
https://www.vintage-radio.com/repair-restore-information/valve_dropper-calcs.html

If you wish skip all the math and go to the bottom of the page. Download the little program that is a 3 method calculator for replacement of resistors / resistance wire cords.
You didn't mention the model number of your Bosch radio. Get that Calculator and fill in your line voltage cycles 60 and the filament string plus light bulb total voltage for your Bosch. Then take a look at the Capacitor method of replacing the curtain burner cord. Zero Heat.
Or use this On-Line calculator. http://electronixandmore.com/resources/calculator/index.html

Assume you have 68 volts at 300 mA for your filament plus dial lamp. This calculator says 8 mfd film cap and your done. This capacitor if 8mfd 250vac rated film cap $4.20. Small enough to put it under the chassis.
https://www.digikey.com/en/products/detail/EXH2E805HRPT/493-13831-ND/4694508

We have over 100 Emerson Ingraham radios in our collection. I have lots of experience in dealing with series string filament radios.

John k9uwa

I am struggling.

Assume you need 100W to supply your radio. At 120V, let's round that to 1 Amp of line current.

You want to drop 10V ? 10V @ 1 Amp is 10 Watts burned in the resistor. Better use a 20 W resistor.

Buck down 10V ? That should reduce the radio power input by 10 Watts (see above). A small transformer is about 70% efficient, so 30% of the 10 Watts ( 3 watts) is consumed in the bucking transformer. Sounds like a better solution to me.

Where you mount your dropping resistor makes a big difference. Don't put it near sensitive components or any of the power tubes.

Rich

For completeness, there is also "Auto-Buck":
https://antiqueradios.com/forums/search ... mit=Search

the is no reason that you can't tweak the filament voltage with s series resistor or capacitor---once the B+ is in the desired range

I once did some testing with precision meters on this. Even a modern, cheap Radio Shack 12-volt, one amp filament transformer is capable of 97% to 98% efficiency. Instead of three watts a bucking transformer of that size will dissipate about a watt, making the argument for transformers even better. Keep in mind that in a bucking connection the radio power only goes through the secondary winding. If the voltage ratio is 10:1 (which would be the case with a 12-V transformer) then the power taken by the primary of the transformer is 1/10th of what the radio draws at the lower voltage.

I think there's a mistake here. A resistor definitely dissipates the unwanted volts & current in heat. A bucking transformer gets rid of some voltage by opposing magnetic fields. I don't think it actually dissipates all the difference in heat. A bucking transformer does have to carry the current & ohmic & core losses warm it some but I think that's the source of the heat vs the reduction in line voltage.

I would suggest a 50 watt wirewound resistor;
https://www.mouser.com/ProductDetail/Vi ... esLacR8%3D

I like my Raytheon Voltage Stabilizer! Had to replace the Oil filled cap in it but it does good! Does get hot though!




Todd

I've used the bucking transformer in every instance. I even use them to power my vintage series Christmas light strings. So far the same two strings has got 5 years without a blown bulb... Not all perfect, for some reason one of the Radio-Shack transformers is a bit of a "hummer" , hey, want do I want? Both transformers were free for cleaning out the SK's estate of radio gear...

For Zenith collectors that deep 1930's chassis has no problem hiding the transformer inside...

A high quality transformer can approach 100% efficiency in a bucking or autotransformer configuration.
If the application is to drop 5 to 10 volts for a high tube count radio, the transformer wins by a mile on power dissipation.

Jay

To use an analogy, it's like the old "mileage vs HP" comparison. If low cost, compactness, and contained within the radio chassis is the goal, the resistor.

If the fact that it's an external device, or it's cost is NP of course the efficiency of the bucking transformer is the choice.

No one size fits all, here.

John Goller k9uwa,

I have used the power dropping resistor per your advise on a few of my radios with transformers. The transformers are usually only warm to the touch. The dropping power resistor is somewhat hotter but not my much. This is when I play my radios for around 1 hour. I like this system, and I am planning on using it on many more of my radios.

Thanks,

Joe Filipczak

For AA5 sets, the series resistor really is a good bet. The total line current is small, and the resistor solution is very simple. 10% of 25 or 30 W is not very much heat.

An appropriate bucking transformer will waste less powe when the set is on, but does have to be switched off with the radio to avoid waste 24/7. And the savings when the set is on is small for an AA5 sort of radio.

Radios drawing lots of power do benefit from the bucking transformer, since a resistor then generates lots of heat. The resistor still has the advantages of simplicity and low cost. Also, it provides a slightly softer start, and a small extra reduction in the RMS current.

Watts do equal Volts X Amps, for DC. With a non-resistive load, AC is more complicated. An efficient transformer may have considerable primary current with no load, yet be drawing almost no power. A Kill A Watt unit can make the right sort of measurements, but the values you will see with a small transformer barely register.

Ted

My rule of thumb is that if a radio is going to draw close to 1 amp or more, I go with a bucking transformer. Anything below, I use the resistor. I don't have any AC/DC sets that use a ballast or curtain burner cord, but if I did, I'd use a dropping capacitor. No heat and it will fit under the chassis in most cases.

I have a series string radio where someone swapped the tubes so that they were equivalents with lower filament voltage, so they only totaled around 90 volts. I put a single diode in the filament string. Works great, no heat, and the heaters don't seem to mind! Saved me having to buy three tubes for the thing. Yes, it does heat up a little slower, but not significantly so.

I prefer bucking transformers. And it has the added benefit of saving wear and tear, and cutting down on power loss the way I build them as they are switched. So no power loss when off as I switch the hot as it comes in to the box before the transformer. And I save wear and tear on the radio's 70 some year old power switch. I just leave the radio switch on, and flip the switch on the transformer. Bonus is that I can encase them and stain them to match my wood trim.

IlikeTech mentioned using a diode to drop the series string voltage.

Yes, adding a diode in the string (provided there are no filter caps in the string itself to smooth the pulsating DC upwards) drops the equivalent heating voltage to 0.707 times line RMS (120), in this case 85 volts equivalent. This is also a good trick if trying to replace an unobtainium ballast or curtain burner. Add resistors as needed by assuming the new voltage is actually 85. Your voltmeter will not measure it accurately. (Except possibly a true RMS voltmeter.)

This trick was used in some overhead projectors using 86 volt lamps. If the lamp blew instantly, diode was shorted.

-- Rich

In my area of the world the power line voltage is remarkably consistent, ranging between 119-1/2 to 121 volts, at least on a run of the mill digital multimeter. So in the case of most five tube AC/DC sets there isn't really a problem with heater voltages, 12.6 x 3+35+50=122.8 volts, I don't worry about the B+ voltage as it is well under the ratings for the tubes. Many of my AC sets have 25 cycle transformers, the voltage may be running higher on those, originally rated at 115 volts, 24 cycles, but the transformers don't seem to care with the amount of iron they have in the cores, and the tube heaters seem to be at 6.3 as they should be.
The only sets I am concerned with are some Canadian RCA and G.E sets from the early 40s, they run somewhat warm, and seem to be just adequately sized, however in running them at a reduced voltage on a Variac doesn't seem to make them run any cooler, so I think that the core size is the issue and not the line voltage. One thing I have observed is that a transformer, or field coil speaker can run hotter then normal if the power output stage is biased incorrectly and the tube is being overdriven, so resistors need to be checked even if the set plays well.
From what I have read, many Brand Z seems to have been notorious for using transformers that are barely able to handle the load, hence their propensity towards using vertical mounted transformers, and their power supply designs do not help in this. If I sought to own one I would probably go for one with a smoked power transformer, and replace it with a modern overrated unit for a guitar amp from Hammond or Edcor, rather then fussing with a dropper of some kind to keep the one on the edge of failing alive.
Regards
Arran