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Tube amp fried computers sound card...

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Diy1995

Member
Hello,
I extracted tube amp (el84 single tube) from old records player. I plugged it in phone and everything was OK, the same with laptop and mp3 player. Than i plugged it in my PC.
First fuse in my house turned off and there was loud bang, i believe that it comed from tube, i tought that one pin of tube was touching metal frame, so I straigten the tube and make sure that no pin is touching frame.
I turned it on again and fuse stayed on, but there was a bang in my pc. I had tube amp connected in front panel, bang comed from back of motherboard (i saw the flash). I didnt find any damage on motherboard, but jack that was plugged in the back was fried (my other amp) and sound card is dead.
After that I plugged in my old mp3 player, that i dont care about it and its playing fine, also with the phone i tried next.
Is there any way that i can check if amp destroyed pc since it was all good with phone, mp3 player and laptop?
I installed PCI sound card in my PC, but im affraid to plug amp back in any other source than old mp3!
Thank you for anwsers
 

Les Jones

Well-Known Member
Most Helpful Member
Can you post the schematic of the EL84 amplifier. I am particularly interested in the power supply part. I suspect the ground rail (Which is not the same as mains ground) is live. There would have been no input or output connections on the original record player so it would not be a problem if everthing was insulated. The first three items you connected to it were not connected to mains ground so they were not destroyed. The signal ground on your PC would be at about the same potential as mains ground resulting in the mains live finding a path to mains ground which caused the fuse to blow. You could have been electrocuted if you had touched the connection from the amplifier and mains ground. Most metalwork ( Taps, pipework and radiators.) in your house would be connected to mains ground also.

Les.
 
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Diy1995

Member
Can you post the schematic of the EL84 amplifier. I am particularly interested in the power supply part. I suspect the ground rail (Which is not the same as mains ground) is live. There would have been no input or output connections on the original record player so it would not be a problem if everthing was insulated. The first three items you connected to it were not connected to mains ground so they were not destroyed. The signal ground on your PC would be at about the same potential as mains ground resulting in the mains live finding a path to mains ground which caused the fuse to blow. You could have been electrocuted if you had touched the connection from the amplifier and mains ground. Most metalwork ( Taps, pipework and radiators.) in your house would be connected to mains ground also.

Les.
Hello, i dont have schematic, but i will try to draw it. One cable from mains is connected direcly to chassis, i dont get how i dont get shocked when i touch the chassis since it depends on how you turn mains plug, one time zero is connected to chassis, but other time its the phase that is connected to chassis.
Thanks
Oh dear, i was always unsure about realy old electronics, but it fascinates me.
 

Les Jones

Well-Known Member
Most Helpful Member
You have proved what I suspected so there is no need for a schematic. You have had a lucky escape just killing your PC instead of yourself.

Les.
 

Les Jones

Well-Known Member
Most Helpful Member
You could re build the amplifier using a power supply with a mains transformer. I think it would be chaeper to use an amplifier based on an audio amplifier IC.

Les.
 

Diy1995

Member
You could re build the amplifier using a power supply with a mains transformer. I think it would be chaeper to use an amplifier based on an audio amplifier IC.

Les.
I guess i could try that, or i will use tgis amp only with mp3 player. I know ICs are cheaper, but I have many IC amps much much more powerfull than this one, but the sound you get from tube amp is wonderful plus the charm of glowing tube :) This is my first tube amp and it is realy interesting to me.
Thanks for help
 

atferrari

Well-Known Member
Can you post the schematic of the EL84 amplifier. I am particularly interested in the power supply part. I suspect the ground rail (Which is not the same as mains ground) is live. There would have been no input or output connections on the original record player so it would not be a problem if everthing was insulated. The first three items you connected to it were not connected to mains ground so they were not destroyed. The signal ground on your PC would be at about the same potential as mains ground resulting in the mains live finding a path to mains ground which caused the fuse to blow. You could have been electrocuted if you had touched the connection from the amplifier and mains ground. Most metalwork ( Taps, pipework and radiators.) in your house would be connected to mains ground also.

Les.
Hola Les

When you come to a final conclusion on what actually happened, could you make a brief recap and eventually add a simple schematic? This subject is a recurrent stumbling block for me.

Maybe then you could say if people in the past, using that amp was risking electrocution or not.
 

cowboybob

Well-Known Member
Most Helpful Member
... but the sound you get from tube amp is wonderful plus the charm of glowing tube :) This is my first tube amp and it is realy interesting to me.
I agree.
You could re build the amplifier using a power supply with a mains transformer. ...
I agree.
And I think it would be worth the effort, Les.

Diy, just one example for a power supply (within red square):
upload_2017-1-25_18-55-52.png (From this site)
I modified the transformer configuration.

The amplifier schematic is a fairly common design for an audio amp, although possibly different than yours. But the power supply will, generally speaking, provide adequate filament (6.3VAC) and DC voltages appropriate for a tube type amp of this kind. There are many, many other power supply designs out there.
 
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KeepItSimpleStupid

Well-Known Member
Most Helpful Member
the phono amp will likely have RIAA equalization and very low levels unless it's a ceramic cartridge. It won't sound very good.
 

Les Jones

Well-Known Member
Most Helpful Member
Hi Diy1995,
Re your post #08 Don't even use the amplifier with your MP3 player. The input connector to the amplifier can still be connected to mains live (Depending on which way the mains plug is inserted into the socket.) So it can still kill you. (Although the MP3 player will probably survive.)

Hi atferrari,
In post #04 Diy1995 says that one side of the mains is connected directly to the chassis so there is a 50/50% chance it will be connected to mains live. The record player would have no external connections as the turntable , amplifier and speaker were all in the same box so in normal use it would be safe.

Hi KISS,
I am almost certain it would have used a ceramic cartridge as they gave a much higher output than a magnetic cartridge so it could save one stage in the amplifier. (They were also cheaper.). If Diy1995 meant that there was only a single valve (tube) (Rather than it was single ended output rather than push pull.) then it would need the high output of a ceramic cartridge to drive the EL84 directly. If it was only a single valve then they still would probably have not had enough gain to allow negative feedback to be used to improve the quality. I think this unit was built as cheap as possible for the bottom end of the market so I dont think would be worth building into a safe to use unit. I think if the OP wants a good quality valve amplifier he should build a push pull design using a pair of 6L6's, KT66's, KT88's or EL34's These were the range of valves being used when I last built valve amplifiers when I was still a school in the early 1960's

Les.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I am almost certain it would have used a ceramic cartridge as they gave a much higher output than a magnetic cartridge so it could save one stage in the amplifier. (They were also cheaper.)
I'm absolutely certain it requires a ceramic or crystal cartridge (most probably crystal, as they were cheaper and lower quality) - I repaired hundreds (if not thousands?) of these kind of items (although most weren't as lethal as this one) 'back in the day' - I never saw one that had the capability for using a magnetic cartridge.
 

Diy1995

Member
Hello, yes it used crystal cartridge.
I managed to solve the problem by adding bluetooth speaker circuit as input source. Now i can connect via bluetooth so there is no physical connection between player and amp. I also solved another problem, that was with low input, now amp is twice as loud :) My final solution will be buying high quality bluetooth module and making preamp for higher input.
Thank you for your time and help
 

Diy1995

Member
Hello, yes it used crystal cartridge.
I managed to solve the problem by adding bluetooth speaker circuit as input source. Now i can connect via bluetooth so there is no physical connection between player and amp. I also solved another problem, that was with low input, now amp is twice as loud :) My final solution will be buying high quality bluetooth module and making preamp for higher input.
Thank you for your time and help
 
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