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Mike's Stereo 20 refurbishment thread.

Discussion in 'classic' started by Mike P, Apr 23, 2019.

  1. Stunsworth

    Stunsworth pfm Member

    Her latest album? If so it's a cracker with some lovely middle eastern undertones on some of the songs.
     
  2. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    "Freedom Highway". Check out "Our Native Daughters" if you've not heard it. Easily the best album of 2019 so far IMHO. Giddens and other female USA folk musicians but Giddens is lead vocals on most tracks and it kind of sounds like another Giddens album, even for all the collaborators!
     
  3. Stunsworth

    Stunsworth pfm Member

    I'm thinking of "There Is No Other", released in May this year, well worth checking out. She has a wonderful voice. I'll grab a listen to the album you mention - I agree Our Native Daughters is excellent.
     
    Arkless Electronics likes this.
  4. Mike P

    Mike P pfm Member

  5. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

  6. Mike P

    Mike P pfm Member

    A couple of weeks ago I ordered a trio of Sovtek 12AX7LPS to try and today I finally got around to trying them.

    First impressions are that they're very good indeed. Hopefully the durability is good too.

    If you're in the market for some decent ECC83 type tubes at a sensible price give the the 12AX7LPS a try.
     
    Arkless Electronics likes this.
  7. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    I'm still liking the silver anode EI's here:)
     
  8. Mike P

    Mike P pfm Member

    I added a 33uF cap at C10 today, the same as I did at C12 previously.

    It could be my imagination but I really felt it added significant extra grip in the bass and a small improvement across the board. Very worthwhile.
     
  9. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Pleased to hear it. Wait till you (if you...) make C10,11 and 12 all 1000uF huge things laying next to the amp on short leads (they're big) ;)
     
  10. Mike P

    Mike P pfm Member

    andy831 likes this.
  11. Tony L

    Tony L Administrator

    Excellent, everyone should have at least two! Looks pretty tidy on top, what’s the bottom like?
     
  12. Mike P

    Mike P pfm Member

    The top side isn't as nice as my first one but better than average.

    The underside is exactly as I'd want, totally original and no sign of melted wax under the transformers or other nasty overheating.

    [​IMG]P1110223 by Michael Pickwell, on Flickr

    [​IMG]P1110224 by Michael Pickwell, on Flickr
     
  13. Tony L

    Tony L Administrator

    That looks really good, exactly what one wants to see!
     
  14. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Pair of 20W monoblocks?
     
  15. Mike P

    Mike P pfm Member

    How much modification would that involve?
     
  16. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Non.
     
  17. Mike P

    Mike P pfm Member

    OK, let me think about that for a moment...:confused:

    So...use a Y piece splitter on each of the incoming RCA leads so that the left channel can be fed to both sides of one amp and the right channel fed to both sides of the second amp.

    Then use stacking banana plugs (or whatever) to connect the L+R outputs of each amp together.

    Et voila, monoblocks.

    Have I got that right?
     
  18. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    .... and set the output transformers for 16R if using 8R speakers... Yep.

    The NFB is taken from the 16R taps so that's another advantage. Channels will need to be well matched!

    You may well get a touch over double the normal power.... 22-24W ish
     
  19. Mike P

    Mike P pfm Member

    Thanks Jez.

    Well if it's as simple as that, it'd be rude not to at least try it :D

    I've already started ordering the parts to rebuild the second amp in the same spec as the one I already have.
     
  20. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    For best results you need the two channels being connected together to be very well matched otherwise they will "fight" each other. Valves should be selected to give identical outputs from each channel and/or negative feedback tweaked to match them. A variable resistor (a preset) could be added to the main feedback resistor and the resistor itself reduced in size slightly ie if it were say 22K then make it 21K and add a 2K preset so it can be varied from 21K to 23K. You could then feed anti-phase signals to the two channels and adjust gain on the variable channel to give a null in the summed outputs of the two channels. Sum them with say a 1K resistor from each channel connected together in the middle.
    Probably just choosing valves to give a decent match will do the trick though... dummy load on each channel (ideally also if doing the above) and feed an identical signal from sig generator to each channel at a level to give a convenient output of say 5.0V and swap valves around etc to get exactly the same from each channel.
     

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