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Old 13th February 2012, 02:26 AM   #1
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Default Removing a motor from a tube turntable.

Hi, I'm working on removing the motor from a tube record player and I'm not sure what I have to do. I'm fairly inexperienced so expect naivety.

I made the mistake of simply bypassing the motor which blew the heater on the tube. Here are the schematics.

Thanks for the help.

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Old 13th February 2012, 03:19 AM   #2
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Stop, right now! That thing is a DEATH TRAP. Like many an old unit, there isn't any power transformer. Turn the plug the wrong way and the chassis is connected to the "hot" leg of the AC mains.

FWIW, the tube's heater and the motor are connected in a 300 mA. series string. Notice that the 90 V. motor and the 25 V. heater add up to 115 V. The North American standard for house current has been, and remains, 105 to 125 V. However, average mains voltages have risen over time.

What do you want the motor for?
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Old 13th February 2012, 03:29 AM   #3
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Before I tell you what you must do to successfully remove and eliminate the motor, please know this. These little amplifiers work directly from the AC line and are dangerous! It is not reccomended to use these as they are. To safely use, an isolation transformer is necessary! Since you admit that you're inexperienced, please do not use this amplifier unless you are prepared to purchase an isolation transformer for your protection. You could be seriously shocked or killed!

The motor is used in series with the tube filament. If the motor is removed, you must replace it with a suitable power resistor. Assuming a 120 volt line, the resistor needed to substitute for the motor is 316 ohms. The closest standard value is probably 320 or 325 ohms. And it will dissapate 28 watts, so you will need at lease a 50 watt resistor. And it will run very hot so it must be mounted carefully. A 100 watt would be even better if not big and cumbersome.

It would be much better to install a small power transformer that provides 125 volts and a 6.3 volt filament winding and then switch to the 6 volt tube version, 6EH5
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Old 13th February 2012, 03:33 AM   #4
kevinkr is offline kevinkr  United States
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Eli is absolutely right - you must use an isolation transformer with this live chassis amplifier.

This represents a severe electrocution hazard, discussing such amps without acknowledging the need for such isolation as is provided by an isolation transformer is also a serious violation of forum policy and will get the thread closed. Please acknowledge that you understand this in your next post.

Edit: I removed my incorrect suggestion on filament resistor replacement and instead recommend Hollowstate's excellent suggestion to use the 6EH5 with a power transformer with secondaries providing 125 volts and 6.3 volts.

Also plan on replacing the rectifier with a UF4007 or 1N4007, and all electrolytic caps.
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Old 13th February 2012, 04:10 AM   #5
benb is offline benb  United States
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Quote:
Originally Posted by kevinkr View Post
Eli is absolutely right - you must use an isolation transformer with this live chassis amplifier.

This represents a severe electrocution hazard, discussing such amps without acknowledging the need for such isolation as is provided by an isolation transformer is also a serious violation of forum policy and will get the thread closed. Please acknowledge that you understand this in your next post.
(above quoted to emphasize safety)
Quote:
Originally Posted by kevinkr View Post
Finally measure the DCR of that motor and replace it with a resistor of approximately equal value - given today's generally higher line voltages a few % higher would not hurt. Wattage rating should be at least 50W..
It seems to me the thing uses the motor's inductance as well as resistance to drop the voltage, and the motor's DCR would not be enough by itself to drop 90 volts at the desired heater operating current. One should calculate the needed resistance and power based on dropping 90V and the heater current used.
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Old 13th February 2012, 04:13 AM   #6
kevinkr is offline kevinkr  United States
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Absolutely correct.. Oops.. Calculating the resistance required based on operating voltage and filament current is the right way to go about this, but I still think Hollowstate's suggestion is best since the original tube is fried anyway.
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Old 13th February 2012, 08:28 AM   #7
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lets be honest.

Cut the plug off the mains lead now.

Remove the output transformer from the amplifier and hold it in your left hand.

You are now holding the sum total of redeemable components from this unit.

Anything NOT in your left hand can now be sent to the metal recyclers.

This will leave you with one hand free to search here and elsewhere for a circuit to match the 3k:4 ohm opt in your left hand.

Buy a lottery ticket while you are searching - you are VERY lucky to still be here.
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