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Old 4th September 2010, 09:54 PM   #1
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Smile Play iPod into 1955 Record Player's Tube Amplifier

How would i play my ipod (low level input device) into the amp that is in a old record player. i got the amp to turn on removed the record player part, the tubes glow and the speaker has a faint hum. it has a 12AX7 preamp and two 50C5 at the output stage it even has the circuit diagram. the amp in in a little metal box and it has a 120vac power cord on it and two wires where the record players stylus connects could i connect the two wire in series with a 2meg resistor to lower the volume of the ipods audio signal so it would amplify the ipods audio signal.
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Old 4th September 2010, 10:03 PM   #2
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You are courting disaster. That amplifier is not isolated from the power line and you may experience the delightful feeling of power line voltage connected to your body. Or to some other part of your house, perhaps causing a short or starting a fire.

If you have an isolation transformer you can do the job all right. And no, you won't need any extra parts, just connect the units.
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Old 4th September 2010, 10:11 PM   #3
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Bob, how do you know it's not isolated? Were they all that way?

Whatever the case, going into the phono input is going to be touchy. It's not designed for a line level signal like the iPod. If you're lucky, it will be set up for a ceramic cartridge, which could make it easier. The 2 meg in series in not likely to do the trick.

Do you have more details? About the unit?
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Old 4th September 2010, 10:23 PM   #4
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Anything with that tube complement was connected as a series string across the line. Add them up, 112.6 Volts and there is probably a series resistor of maybe 39 Ohms, 1 Watt or so, to take up the extra few volts.

No, they weren't all that way. Only the cheapest of them were. Good stuff used power transformers and all 6.3 Volt heater tubes.

I think the phono input would work fine. Anything that cheap utilized a ceramic cartridge with output nearly one Volt, and that's the order of signal one might expect from a random line level unit. Even a speaker output would work, as that's not so much higher and can be backed off. One of the leads from the cartridge is ground, the other the signal lead.
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Old 4th September 2010, 10:32 PM   #5
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Good to know! Was not sure how much output the ceramics had. 1V is a lot.

Having run 350V from a non isolated amp thru my portable CD player, I do NOT recommend it! It would be good bye iPod and maybe good-by Ben.
Isolation transformer = good, safe idea.
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Old 4th September 2010, 10:33 PM   #6
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The amp in not isolated from the mains i could isolate it with a transformer or could i ground the b-

here is the circuit diagram

http://benproiii.webs.com/Phonograph.jpg
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Old 4th September 2010, 10:44 PM   #7
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Click the image to open in full size.
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Old 4th September 2010, 11:42 PM   #8
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Quote:
Originally Posted by bob91343 View Post
You are courting disaster. That amplifier is not isolated from the power line and you may experience the delightful feeling of power line voltage connected to your body. Or to some other part of your house, perhaps causing a short or starting a fire.
I'm very impressed by the deduction Mr. Holmes!! You may also have saved another tube fan, so double congratulations are in order!

Not so much safety going on in the '50s was there?
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Old 4th September 2010, 11:46 PM   #9
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so if i isolate the amp and ground the b- can i plug the ipod with 100k and 0.047uf in series with signal input into the amp's signal input
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Old 5th September 2010, 12:24 AM   #10
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Quote:
No, they weren't all that way.
Yes, they were. If you see a 50C5 there is a 95% chance it is powered directly from the line. Some of the cheap ones connected one side of the AC line right to the metal chassis. The only thing between the user and the wall outlet is the plastic knobs.

I got an Electrolab guitar amp at a flea market. It has a 50C5, and yes it is a shocker! I added a little Triad isolation transformer $15 from Mouser.

Quote:
I think the phono input would work fine. Anything that cheap utilized a ceramic cartridge with output nearly one Volt, and that's the order of signal one might expect from a random line level unit.
I used the headphone output on my Sony Discman to drive a Magnavox stereo amp robbed from an old console. I connected the Sony to the phono cartridge input. The old Magnavox never sounded so good.
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