Need help with series filament 25l6GT amp

[GavinPaddock]

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?

[Shoog]

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

[GavinPaddock]

Resistors are cheaper then transformers where I live. Do you have any help along the lines of what I'm trying to do?

[payitforwardeddie]

Originally Posted by GavinPaddock

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.

[dcgillespie]

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

[Rod Coleman]

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

.

[GavinPaddock]

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!

[dgta]

Originally Posted by GavinPaddock

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 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.

[Soonerorlater]

Replace your motor with a fan and keep things nice and..........

[nigelwright7557]

Originally Posted by GavinPaddock

Resistors are cheaper then transformers where I live. Do you have any help along the lines of what I'm trying to do?

Trouble is you need a 30 watt resistor ! that is going to put out some serious heat.

I would go for a small transfomer.