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 Post subject: Re: Bucking Transformer VS Power Resistor Today's Line Volta
PostPosted: Oct Thu 08, 2020 9:53 pm 
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Joined: Apr Sun 08, 2007 5:47 am
Posts: 5961
nick3092 wrote:
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.


I like that box you constructed, seems practical and leaves the radio unmolested electrically. I have the same habit, except I use a power bar, or unplug the set when I have finished using it. Lightning strikes have killed more then a few radios by leaving then plugged in full time. I have more radios then most people on here, and I think that rotating the "daily driver" makes more sense then equipping each radio with a bucker, how many people have 100+ electrical outlets in their house anyhow? Most people who have an old radio, other then myself, only turn on a set to show it off to visitors anyhow, the only real concern I have is for the future of oddball sets like a 32 volt radio where people could very easily connect it to the full 120 thanks to the very normal looking line cord and plugs many came with, that I would alter to prevent that sort of thing.
Regards
Arran


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 Post subject: Re: Bucking Transformer VS Power Resistor Today's Line Volta
PostPosted: Oct Thu 08, 2020 10:46 pm 
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Joined: Dec Sun 14, 2008 2:33 pm
Posts: 1340
Location: TOKYO JAPAN
For transformer PS radios, put two diodes, wired in parallel and cathode to anode, in the heater line. Drops heater voltage about .7V, perfect for 6V heaters. However, the resultant pulsing DC waveform sometimes makes noise, particularly in SW receivers. A small cap, say .01 ~ .047uF, across the diodes or shunting to ground usually takes care of that.

Rob


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 Post subject: Re: Bucking Transformer VS Power Resistor Today's Line Volta
PostPosted: Oct Fri 09, 2020 3:01 am 
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Joined: Oct Sun 09, 2011 4:50 am
Posts: 501
Location: Westerville OH 43081
I have taken to using thermistors. (inrush limiter). Inexpensive.
The CL-90 or CL-80 from Mouser. 2 amp and 3 amp respectively. They get hot so need air circulation or inside a substantial metal chassis.
Some of my Scotts have hi-lo switch which I run in Hi (house voltage over 115 volts) for cooler operation.
For those later Scotts with no hi-lo switch, the thermistor provides for a softer start and knocks 3 or 4 volts off the working voltage and proportional reduction in B+.
The thermistor resistance presents a voltage drop initially, but resistance quickly drops as it heats up - hence the soft start, but a few ohms resistance remains.

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Dave Poland


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 Post subject: Re: Bucking Transformer VS Power Resistor Today's Line Volta
PostPosted: Oct Fri 09, 2020 4:14 am 
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Joined: Jun Wed 14, 2006 10:49 pm
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Location: Zellwood Florida
OK time for some comments since I am the one that started this thread.

For those who don't know I have done radio restorations as a business for about 30 years or so. I have done in excess of 3,000 old radio restorations. Most of those radios were customer owned radios that were only in my shop for restoration. Every radio that leaves my bench carries a years warranty for anything that goes wrong with the radio. Out of all these radios I have replaced exactly ONE Power Transformer and that was a Zenith 7s363 that I had done the restoration on it some 20+ year prior to the time the transformer went out. It was also before I began replacing ALL of the 6X5 tubes in Zenith radios with diodes and resistors instead of the tube. I had the customer ship me the chassis and I replaced the transformer and returned it to the customer. He asked me what he owed me and I said ZERO. Nothing. I should have put diodes in that radio to start with me error.

Every radio that I restore gets at least an 8 hour or longer solid ON RUNNING while I observe operation as well as temperature of transformers. Many get DAYS of operation before they are returned to the owners.

The last thing I need is to have a failure on a customers radio after they have had it in operation for some time and I get to pay Shipping plus I get to fix whatever is wrong with the radio for FREE. And at the least the customer is upset a bit because they had to take it apart, pack up the chassis and send it to me. Out of the 3000+ restorations that number stands at FOUR chassis returns. TWO of those were well past the one year warranty but due to what was wrong with them I warrantied the repairs and cost to customer was ZERO. Watch it. Mica Caps can become NOISEY after they leave my bench. 30 years ago Mica Caps were not much of a problem. Today a bit different. Particularly if you move or bend the leads on one while replacing other parts in the same area of the chassis. It is called moisture ingress due to lead seals to the plastic case being compromised.

To those with questions about Curtain Burner power cords. AC/DC radios with large resistor in the chassis or a ballast tube in the chassis. The simple answer is become familiar with this webpage calculator for filament voltage reduction methods. http://electronixandmore.com/resources/calculator/index.html
Look at the Voltage Drop for Series Tube Filaments section. Fill in the blanks for your radio and read below the values needed for the various methods of filament voltage reduction. Diode + Resistor, Resistor only, Capacitor. Film Caps with AC ratings of 250v are readily available in the $3 to $5 price range. Zero Heat. Small size and your done. No Flash Bulb effects from dial lamps at turn on is a bonus with the capacitor method. A few volts spread over 5 tubes not a big deal.

To those who say... I RUN MY RADIOS FROM A BUCKING TRANSFORMER BOX. I say someday that radio will go off to a new owner. Will the new owner want that box along with the radio? Or will that new owner simply plug it into the wall and yes the radio plays very well. Perhaps you could care less what happens to your radios once you sell them or you pass away and someone buys them at your estate sale.

To those who say I seldom run my radios more than a few minutes or an hour or so... I say OK sooner or later someone else is going to own that radio and they might just run it all day every day. Will your restoration hold up under those conditions?

Flame suit still on !!

Mine will
John & Jean Goller, k9uwa

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http://www.JohnJeanAntiqueRadio.com


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 Post subject: Re: Bucking Transformer VS Power Resistor Today's Line Volta
PostPosted: Oct Mon 12, 2020 6:53 pm 
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Posts: 1219
Location: Houston, Texas
Hey John,

I agree with most of what you say here. I think in most cases for average table top radios a resistor makes the best choice. However the following is simply not true from your original post.

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

A bucking transformer is really just a slightly modified version of an auto-transformer and does not reduce the voltage thru heat dissipation it does it thru the magnetic properties of a transformer.

Here is a very good page describing it all.

With that.. i'm putting a 3 ohm power resistor setup in my 19 tube Magnavox because 1 i don't have a 3a filament transformer i want to give up for this purpose 2. I do have the power resistors on hand and 3. it takes up less space 4. is a more permanent solution.

Down side is an extra 12 watts of dissipation being absorbed into the chassis. Not a big deal.

Steve


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 Post subject: Re: Bucking Transformer VS Power Resistor Today's Line Volta
PostPosted: Oct Tue 13, 2020 9:43 pm 
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Joined: Jan Wed 15, 2020 11:29 pm
Posts: 2477
Location: Queensland, AU
targeteye wrote:
Hey John,


A bucking transformer is really just a slightly modified version of an auto-transformer and does not reduce the voltage thru heat dissipation it does it thru the magnetic properties of a transformer.

Steve


Yes, the fundamental difference being that the magnetic field of the power transformer (feeding the 115V power) to your street/home & GPO's is not linked with that of the core of the bucking transformer. The effect of this is to degrade the voltage regulation and create a higher internal impedance voltage source.

If say an auto-transformer is used instead it is a little superior to the bucking transformer in terms of efficiency and voltage regulation as the fields are shared. An autotransformer, depending on its size & design can be made close to 98% efficient. I have an autotransformer on the street feed to my home which can approach 250V(with solar input to the grid from surrounding homes) it reduces the line feed to 230V which is our standard value in AU. It is 15kW rated and its efficiency was calculated by the designer at close to 99%.

The common type of Variac is merely a toroidal design autotransformer where it is made variable by tapping off the turns with a sliding carbon brush. A 180 to 200W rated 120V Variac is quite a compact item maybe only 3 or so inches in diameter, and quite efficient too, but if you want to mount them inside a radio chassis, usually there is not enough room and a bucking transformer occupies less space. So in order of efficiency likely it is Variac, then bucking transformer then resistor.

The series diode idea seems attractive at creating low power loss rms voltage drop where as noted the effective rms value becomes 0.7071 of the rms input, but is not good on series heater chains where initial current surges are problematic.

The idea of a power NTC resistor is good as it limits initial surge currents brilliantly and this protects a series heater chain. In the UK, nearly every vintage TV set (designed for AC/DC use) used a power NTC resistor in series with the heater chain, the classic part was called a CZ1 "Brimistor"


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