Notices
Results 1 to 7 of 7
Like Tree3Likes
  • 1 Post By tk421
  • 1 Post By MacGyver1968
  • 1 Post By billvon

Thread: old audio transformer

  1. #1 old audio transformer 
    Forum Freshman
    Join Date
    Oct 2013
    Posts
    39
    I am looking for an output transformer for an old Heathkit A7 mono amp circa 1950 but I'm running out of places to look. Actually the transformer is missing from the chassis so I can't look for any numbers etc. My question is what do you need to know about a transformer to ID a gereric transformer that will likely work? What I do know is that the amp is rated at 6 watts, has selectable impedances of 4,8 and 15 ohms. According to the schematic it has three taps on each side. I have a users assembly manual but even that doesn't give anything on the transformer other than the part number is or was 51-18. I'm tired of emailing people and places so if somebody could possibly answer my question I would be greatful.


    Reply With Quote  
     

  2.  
     

  3. #2  
    Forum Isotope
    Join Date
    Feb 2012
    Location
    Western US
    Posts
    2,820
    Quote Originally Posted by deric View Post
    I am looking for an output transformer for an old Heathkit A7 mono amp circa 1950 but I'm running out of places to look. Actually the transformer is missing from the chassis so I can't look for any numbers etc. My question is what do you need to know about a transformer to ID a gereric transformer that will likely work? What I do know is that the amp is rated at 6 watts, has selectable impedances of 4,8 and 15 ohms. According to the schematic it has three taps on each side. I have a users assembly manual but even that doesn't give anything on the transformer other than the part number is or was 51-18. I'm tired of emailing people and places so if somebody could possibly answer my question I would be greatful.
    You have a challenging task ahead of you. A classmate of mine had a similar problem many years ago. I watched as he went through many, many iterations of trying things that didn't work. In the end, he used an old power transformer just to get his amp working. It sounded ok to me, but I'm sure those with better ears would have found fault with it.

    If you were making your own, here are some of the things you would need to know:

    1) Maximum current through the primary (DC + AC) -- you need to know this because you will have to ensure that the core will stay well away from saturation at maximum volume.

    2) Impedance transformation ratio. My friend used a 24V transformer (which would correspond to a 10:1 voltage ratio, as we were in the UK at the time). This choice was not the result of careful analysis. He found the transformer in the back of a cabinet in the lab, and it was "about the right heft."

    If you wanted to do it right, you'd analyse your circuit to estimate its output resistance. The transformer's job is to step that resistance down to a value that matches the speaker resistance (4, 8 or 16 ohms, I think you wrote). Then the needed voltage stepdown ratio of the transformer will be the square-root of that resistance stepdown ratio.

    3) Voltage rating of the primary (just to make sure that your valve/vacuum tube supply voltage doesn't toast your transformer.

    4) Current rating of the secondary (to make sure that you can push the ~couple of amperes through the speaker without burning up the transformer secondary winding).

    There are other considerations, but those will do for a start.


    deric likes this.
    Reply With Quote  
     

  4. #3  
    Forum Freshman
    Join Date
    Oct 2013
    Posts
    39
    Thanks I'll see what I can come up with. Maybe the right heft will work. I came across a good place so maybe the engineers will suggest something for me.
    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by deric View Post
    I am looking for an output transformer for an old Heathkit A7 mono amp circa 1950 but I'm running out of places to look. Actually the transformer is missing from the chassis so I can't look for any numbers etc. My question is what do you need to know about a transformer to ID a gereric transformer that will likely work? What I do know is that the amp is rated at 6 watts, has selectable impedances of 4,8 and 15 ohms. According to the schematic it has three taps on each side. I have a users assembly manual but even that doesn't give anything on the transformer other than the part number is or was 51-18. I'm tired of emailing people and places so if somebody could possibly answer my question I would be greatful.
    You have a challenging task ahead of you. A classmate of mine had a similar problem many years ago. I watched as he went through many, many iterations of trying things that didn't work. In the end, he used an old power transformer just to get his amp working. It sounded ok to me, but I'm sure those with better ears would have found fault with it.

    If you were making your own, here are some of the things you would need to know:

    1) Maximum current through the primary (DC + AC) -- you need to know this because you will have to ensure that the core will stay well away from saturation at maximum volume.

    2) Impedance transformation ratio. My friend used a 24V transformer (which would correspond to a 10:1 voltage ratio, as we were in the UK at the time). This choice was not the result of careful analysis. He found the transformer in the back of a cabinet in the lab, and it was "about the right heft."

    If you wanted to do it right, you'd analyse your circuit to estimate its output resistance. The transformer's job is to step that resistance down to a value that matches the speaker resistance (4, 8 or 16 ohms, I think you wrote). Then the needed voltage stepdown ratio of the transformer will be the square-root of that resistance stepdown ratio.

    3) Voltage rating of the primary (just to make sure that your valve/vacuum tube supply voltage doesn't toast your transformer.

    4) Current rating of the secondary (to make sure that you can push the ~couple of amperes through the speaker without burning up the transformer secondary winding).

    There are other considerations, but those will do for a start.
    Reply With Quote  
     

  5. #4  
    Cooking Something Good MacGyver1968's Avatar
    Join Date
    Aug 2006
    Location
    Dallas, Texas
    Posts
    2,051
    Good luck!...Let us know how it goes. Take a picture of the thing if you don't mind. I'd like to see it.
    babe likes this.
    Fixin' shit that ain't broke.
    Reply With Quote  
     

  6. #5  
    Forum Cosmic Wizard
    Join Date
    Aug 2013
    Location
    San Diego
    Posts
    2,222
    Quote Originally Posted by deric View Post
    I am looking for an output transformer for an old Heathkit A7 mono amp circa 1950 but I'm running out of places to look. Actually the transformer is missing from the chassis so I can't look for any numbers etc. My question is what do you need to know about a transformer to ID a gereric transformer that will likely work?
    The output impedance. In general a 1K to 8 ohm transformer should work. They are no longer common but available at places like DigiKey.
    deric likes this.
    Reply With Quote  
     

  7. #6  
    Forum Isotope
    Join Date
    Feb 2012
    Location
    Western US
    Posts
    2,820
    Quote Originally Posted by billvon View Post
    Quote Originally Posted by deric View Post
    I am looking for an output transformer for an old Heathkit A7 mono amp circa 1950 but I'm running out of places to look. Actually the transformer is missing from the chassis so I can't look for any numbers etc. My question is what do you need to know about a transformer to ID a gereric transformer that will likely work?
    The output impedance. In general a 1K to 8 ohm transformer should work.
    That sounds about right. The 10:1 voltage transformer my friend used would have produced a 100:1 impedance transformation, i.e., 800 ohms to 8. Close enough.
    Reply With Quote  
     

  8. #7  
    Forum Freshman
    Join Date
    Oct 2013
    Posts
    39
    Thanks for that all
    Reply With Quote  
     

Similar Threads

  1. transformer
    By sabari raja in forum Electrical and Electronics
    Replies: 6
    Last Post: October 29th, 2012, 08:03 AM
  2. step up transformer
    By usman33 in forum Introductions
    Replies: 4
    Last Post: October 23rd, 2012, 09:33 AM
  3. Real Transformer
    By rockfor in forum General Discussion
    Replies: 0
    Last Post: April 2nd, 2010, 07:31 AM
  4. Replies: 0
    Last Post: December 17th, 2009, 09:35 PM
  5. Transformer usage
    By Cold Fusion in forum Electrical and Electronics
    Replies: 22
    Last Post: March 26th, 2009, 11:52 PM
Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •