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Sansui 1000a restoration & Modification questions

Discussion in 'Exclusively Sansui' started by birdyman, Jan 25, 2011.

  1. birdyman

    birdyman New Member

    Messages:
    26
    I got a revived Sansui 1000a with new caps ( replaced all oils and electrolytics) and new rectifier diodes. It is singing like a canary at this point but I am planing to to follow updates.
    1) replace caned power caps- New multisection Vs. stuffed with modern single caps- Pros & Cons??
    2) Replace all Germanium diodes- replace with new germanium Vs. Silicon equivalents-- Pros & Cons??
    3) Mod to 4 pot bias circuit - mine is single pot bias- Pros & cons??
    4) Switching on & off tuner and MPX section while they are not in use by putting a relay or triac mod- Pros & Cons??

    Any opinions and help is greatly appreciated.
     
  2. kevzep

    kevzep Its all about the Music Subscriber

    Firstly welcome to AK and the Sansui Forum!! You have come to the right place......

    1> New Caps are always a good idea in an amplifier this old, the old ones have all but dried up most likely.....

    2> Silicon is regarded as more reliable than Germanium especially when the Germanium is as old as it is in your amp, others may have varying opinions...

    3> 4 pot bias circuit, I think this is a good idea because you can get the Bias absolutely correct for each Tube, even with matched tubes there is variance, so it is a good thing. Will contribute to the extended life of your tubes

    4> You could switch it of when not used, I would leave it on, tubes like to be on rather than power cycled all the time, only an opinion though.......

    Hope that helps......
     
  3. birdyman

    birdyman New Member

    Messages:
    26
    Thank kevzep

    I will copy 4 pot bias circuit to update my one pot for sure. Stuffing existing caned caps with modern single cap can be fun but it will resulting better psudo multisectional caps than NOS multies.

    1) What would you use for Silicone replacement of germanium diodes? Is 1N4148 ok to replace germaniums?
    2) Using Triac to switch on & off heater circuit of tuner section:- Too noisy??
     
    Last edited: Jan 25, 2011
  4. kevzep

    kevzep Its all about the Music Subscriber

    1n4148 might do the trick, I would have to check the schematic though for voltage....

    I would leave the tuner unmolested personally, you could use a triac, or even a CMOS chip can be used to drive a FET to switch the actual supply.....might be a elegant way to do it......its all a bit high tech for such a lovely old amp though.....
     
  5. markn2wae

    markn2wae Mark T N2WAE

    Messages:
    2,910
    Location:
    Scotch Plains, New Jersey
    Two items not recommended IMHO.

    Two items I would not recommend doing.:no:

    A) Replacing the germanium diodes with other types, yes you can get schotky diodes with a low forward Voltage drop but what about junction capacitance?

    They may not work properly at high frequencies.

    B) Switching on & off tuner and MPX section while they are not in use....

    The tuner is already being switched off when not using AM\FM (you may hear a "POP" when switching to tuner from other modes as the B+ gets fed to the tuner section)

    As for the MPX section it uses little power and you won't save much on tube life anyway (unless there is a fault, the small signal tubes last a long time, driver\output tubes do "work harder" then most of the other tubes.

    Put a pair of fans on the back (your choice on how to power them).:thmbsp:

    Mark T.
     
  6. birdyman

    birdyman New Member

    Messages:
    26
    Mark T.
    Thanks for your recommendation . I guess I was " a novice killed cat". I read about 4 pot bias mod and 10 ohm serial resistors to measure current. Where should I put what size and type resistors??
     
  7. kevzep

    kevzep Its all about the Music Subscriber

    Oh well that makes two of us then.........good call on the fans, I guess Sansui may have used them if they had "small quiet" fan technology back in the day..........

    Yeah I had a suspicion the tuner turned off but I havent looked at the diagram for a while.......

    Fair call on the diodes actually now I think about it, you have a good point there.....

    However, its not a bad idea to do the individual bias pots, I would use a 1ohm resistor so its easy to calculate the current, some guys use 10ohm but I think 1ohm is easier.........they are in series with the cathode....

    see what Mark thinks........
     
  8. birdyman

    birdyman New Member

    Messages:
    26
    With 4 independent bias set up, is it OK to use non-matching op tubes or should I match at least pairs not quads?
     
  9. kevzep

    kevzep Its all about the Music Subscriber

    I would always use at least matched pairs.......most definitely...
     
  10. nerdorama

    nerdorama AK member Subscriber

    Messages:
    2,208
    Location:
    Seattle area
    Regarding the 10 or 1 ohm cathode resistors: Unless your meter is very accurate at the low voltages that 1 ohm provides, you may get better results with the 10. The 10 also creates a bit more internal heat and acts better as a safety fuse if a tube red plates.

    John
     
  11. birdyman

    birdyman New Member

    Messages:
    26
    According to a thread on this forum, two version of 4 pot bias are discussed. Should I repeat 1 pot bias version four times ( with 20uf caps with 10k pots) or use later version of Sansui 1000a -four 10k pots design(no 20uf caps)?
     
  12. kevzep

    kevzep Its all about the Music Subscriber

    Fair call John, I have 1ohms in my guitar amp, running KT88's, I have had in 26years only one red plate incident so my thinking is a bit different.........My guitar amps are something solid and unique.......
    Ive just never thought I needed something fusible on the cathodes.....since Ive only had one incident.......

    Lets face it, a lot of damage can occur if a tube red plates........


    Birdyman, not sure which would be the best option for your bias mod, Im sure someone with a 1000A will chime in and advise.....
     
  13. conaway

    conaway Active Member

    Messages:
    202
    Agree with Markn2 - the germanium diodes are used in high frequency circuits for two reasons - they require very little current to detect signal or 'turn on', and they work well at high frequencies. Silicon diodes generally have very different electrical properties. You can still find germanium diodes electrically equivalent to those that Sansui use - Mouser sells them.

    I had to replace several in my 1000a - used mouser part number 526-NTE-109. Germanium diodes generally have a low max inverse voltage compared to silicon ones and can and do short.

    if you replace use silicon diodes to replace them, the detector and mpx circuits may not work correctly.

    Jim
     
  14. kevzep

    kevzep Its all about the Music Subscriber

    Thank you Jim thats very interesting......
     
  15. birdyman

    birdyman New Member

    Messages:
    26
    Thank you for diode info.

    I still have more questions.
    Sansui 1000a uses BJT RIAA phono stage. Any one replace them with like this. This uses nice jfets with cascode. Good match with tube section?
     
  16. kevzep

    kevzep Its all about the Music Subscriber

    If I had a 1000A I would totally keep it stock standard, modifying the Bias circuit is an improvement and a valid modification keeping in with tube technology, but to butcher it with a JFET phono pre is something I just wouldnt do to be honest........
    If I wanted something like that I would build it as a stand alone kit so as to keep the 1000A unmolested, just my opinion though......
     
  17. JHS2RT

    JHS2RT and Miss Sasha Subscriber

    Messages:
    5,520
    Location:
    Vero Beach,Florida
    I agree~~~
     
  18. conaway

    conaway Active Member

    Messages:
    202
    agree with JHS and Kevsep - the 1000a, when recapped and serviced to replace out of spec compenents - sounds fantastic with phono and FM stereo functions. I also have a cd changer connected to the aux input - sounds great - can;t imagine needing to modify the circuits.

    Performance specs (freq response of audio section, fm sensitivity and mpx separation) are quite good by todays standards.
     
  19. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    38,168
    Location:
    Hertfordshire, UK
    Why don't you build "like this" into a separate box? - then you can try it out with any amplifier you wish and compare :smoke:
     
  20. birdyman

    birdyman New Member

    Messages:
    26
    Thanks to great modular design of phono section, it is possible to build simple phono PCB and substitute original. Original does not need to to be destroyed or molested. If I don't like JFET RIAA, I can just put original back and call it a day.
     

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