I've picked up an old Montgomery Ward record player in a small suitcase, and the mechanical parts (stylus arm, tray for record to sit on, etc) are all shot/missing/broken. I've decided to add an isolation transformer and turn it into a little micro guitar amp.
The motor that ran the spindle turning the records still works, and was wired in series with the filament of the single 25L6GT tube. I don't want a motor (or motor coil) sitting in the box taking up space, so I want to replace it with a proper sized resistor.
The mains (soon to be wired after the iso transformer) comes in, goes to a rectifier diode to half-wave rectify to generate B+ for plates, and also to the filament of the tube, then to the motor, then it completes the loop. The motor's DCR is 24 Ohms. I initially made the mistake of jumpering where the motor used to be, and toasted a filament. I assumed that if I put a high power resistor where the motor coil used to be at the same ohmage, I'd be fine...but doing some calculations...
120V from the iso trafo - 25V drop for the filament (@ 300mA) = 95V to drop on a series resistor. Current is the same through every element in a series circuit, so 95V/.3A= 316 Ohms.
This is pretty far off from 24Ohms, which if I do put in, will allow over 3 Amps to flow. Not good for filaments either. I AM neglecting the single rectifier diode/resistor/capacitor network and further down the line tube resistances...but what gives?
The motor that ran the spindle turning the records still works, and was wired in series with the filament of the single 25L6GT tube. I don't want a motor (or motor coil) sitting in the box taking up space, so I want to replace it with a proper sized resistor.
The mains (soon to be wired after the iso transformer) comes in, goes to a rectifier diode to half-wave rectify to generate B+ for plates, and also to the filament of the tube, then to the motor, then it completes the loop. The motor's DCR is 24 Ohms. I initially made the mistake of jumpering where the motor used to be, and toasted a filament. I assumed that if I put a high power resistor where the motor coil used to be at the same ohmage, I'd be fine...but doing some calculations...
120V from the iso trafo - 25V drop for the filament (@ 300mA) = 95V to drop on a series resistor. Current is the same through every element in a series circuit, so 95V/.3A= 316 Ohms.
This is pretty far off from 24Ohms, which if I do put in, will allow over 3 Amps to flow. Not good for filaments either. I AM neglecting the single rectifier diode/resistor/capacitor network and further down the line tube resistances...but what gives?
You would be better off giving it its own little filament transformer. Less heat - less waste.
Big resistors are expensive, salvaged 24V transformers are cheap.
Shoog
Big resistors are expensive, salvaged 24V transformers are cheap.
Shoog
Resistors are cheaper then transformers where I live. Do you have any help along the lines of what I'm trying to do?
Do you know how much heat your resistor will dissipate inside your small case.
A transformer would be much safer.
Have a look on Ebay, I found this item 260774620626.
If you insist on using a resistor, to show how impractical that is, to drop 95 v @ .3 amp current draw will require a 317 ohm resistor. Nothing special about that, except that it would also be dissipating 28.5 watts (!), meaning (in reality) you would need to (likely) use a 300 ohm 50 watt resistor. Against that scenario, a small little transformer must be more practical and economical.
Dave
Dave
You may be able to use a motor-run MKP capacitor to do the drop for you. The capacitor's reactance drops voltage as surely as a resistor, and this way, you lose very little in heat.
Check out this page to do the calculation for the C value, and associated padding resistor.
About halfway down the page, under the header 'Capacitive Dropper'
UK Vintage Radio Repair and Restoration - Dropper Calculations
.
Check out this page to do the calculation for the C value, and associated padding resistor.
About halfway down the page, under the header 'Capacitive Dropper'
UK Vintage Radio Repair and Restoration - Dropper Calculations
.
OK...looks like transformer it is. My local electrical salvage place has some 24V trafos for a few bucks...and they look like I can jam them in the box somewhere.
I realize now that the original design with two prong power cable was really a "curtain burner" resistive line cord. So the 25l6GT filament and 24Ohm motor were both in series with a big long resistor in the power cable itself. Thats why the math didn't add up with just those two components in-line. Only after I cut the old cord and installed my three prong did I notice a strange third wire in the power cord...did a little research and found out about curtain burners...
Thanks for your help!
I realize now that the original design with two prong power cable was really a "curtain burner" resistive line cord. So the 25l6GT filament and 24Ohm motor were both in series with a big long resistor in the power cable itself. Thats why the math didn't add up with just those two components in-line. Only after I cut the old cord and installed my three prong did I notice a strange third wire in the power cord...did a little research and found out about curtain burners...
Thanks for your help!
You just discovered that a motor is not a resistor 🙂 In simple terms, the motor windings are not resistors turning electrical energy into heat. A lot of that energy is converted into mechanical energy.
Trouble is you need a 30 watt resistor ! that is going to put out some serious heat.
I would go for a small transfomer.
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