• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Play iPod into 1955 Record Player's Tube Amplifier

benproiii

Member
2010-08-29 1:12 pm
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.
 

bob91343

Member
2010-03-11 10:43 pm
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.
 

Pano

Administrator
Paid Member
2004-10-07 6:05 am
Panama
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?
 

bob91343

Member
2010-03-11 10:43 pm
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.
 

Pano

Administrator
Paid Member
2004-10-07 6:05 am
Panama
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.
 
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?
 
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.

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.
 
i modified the circuit will this work i just want to make sure
 

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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

I assume you mean you are going to use a proper isolation transformer. You don't need to do anything with the B- under such a case, although grounding B- through a 3 wire cord is not a bad idea, and would add an extra measure of safety as long as it is connected to an actual safety ground.. (not a cheater)
 
Note that you should replace the electrolytics, probable selenium rectifier and all of the coupling caps before you use this. All are subject to catastrophic failure due to age. The electroytics can explode, and the selenium rectifier when it fails will emit extremely toxic smoke. Coupling cap failure will ruin your output tubes.

The selenium rectifier can be replaced by a 1N4007 or UF4007, that 47 ohm resistor may need to be slightly increased in value. I'd replace those 150V electrolytic caps with at least 200V parts.
 
The interstage coupling caps will be ceramic disk and not so prone to failure as as much as they are just terrible for audio coupling.

Change them to MKP or other film caps of the same value, 200V or greater rating. Your B+ power supply will end up around 175V so anything over 200V is fine.

The input caps in the circuit up to the first grid can be film caps above 25V, as the grid is close to ground.

Personally, I'd bypass the treble circuit and just use the volume control.