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Transformer output puzzle for tube pre-amp project

Discussion in 'General Electronics Discussion' started by Electro-Cat, May 25, 2017.

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  1. Electro-Cat

    Electro-Cat

    3
    2
    May 25, 2017
    Background:
    I have started working on this tube pre-amp project, and things are going well:
    See schematics below.

    …however, I have hit a barrier with the AC trafos.
    The PSU needs two inputs:

    If I measure the transformers (unloaded 230V input) the first one reads 11.6VAC and the second one 29.5VAC. The first one is OK as it is within the 9-12 range, but the second one is too high.

    I have read that transformers will give a higher output until a load comes on, but I doubt that 60 mA will get it under 24VAC. Also I have just returned from my local electronics store with a 21VAC, but this one reads 32.2VAC??! (unloaded). This is a jungle for me :-/

    Question:
    So the question is; how can I calculate or figure out which transformer to get if they all have a higher output than their specs – or said in another way, how much can I expect the voltage to drop under load? Or can you actually get ones which will not have a higher output than what’s written on them?

    Help!

    PSU:
    PSU.png

    Pre-amp:
    Pre-amp.png
     
  2. Audioguru

    Audioguru

    2,904
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    Sep 24, 2016
    Phono? Tubes? Why?
     
  3. Electro-Cat

    Electro-Cat

    3
    2
    May 25, 2017
    Phono indeed - Why? because it's a challenge (for me).
     
    davenn likes this.
  4. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    I did not see a schematic.
    You have stabilisers to control the output so why are you so fussy about the transformer volage?
    Keep the voltage as low as possible to keep dissipation low.
    Presumably the 6J5s will need 6.3V for the heaters or 12.6V if connected in series.
    Surely you will need about 100V for the anodes.
     
  5. 73's de Edd

    73's de Edd

    2,898
    1,231
    Aug 21, 2015
    Sir Electro-Cat . . . . . Hej!


    Don’t wurry ‘bout it “ . . . . . . . you'll be fine ! . . . . .just as it is


    "Reading" the PC boards . . . .

    As you can see, the lower voltage one of the two transformers is having its AC converted to DC and regulated and filtered to present PURE DC ~6.3 VDC to the filament supply pins 2 and 7 of the pair of 6J5 tubes, them being wired in parallel.


    The second transformer is taking its AC out voltage and then having its voltage tripled by the input diodes/ filter caps circuitry to then be filtered by the final 47 ufd at 100VDC to then feed down to be the w……i…..d…..e…… acceptable supply range of “ 70---------- 100 VDC” for the tubes plate circuitry supply.


    THEN . . . . . when you finish this . . . . You might then listen to it, and become converted to a full-fledged, membership card carrying . . . . . . TUBE PHREAK !

    (All HAIL the mighty '01 triode )

    73’s de Edd
     
    Last edited: May 25, 2017
    Electro-Cat and duke37 like this.
  6. Audioguru

    Audioguru

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    Sep 24, 2016
    The article about the tubes preamp mentioned its 6% harmonic distortion. I made an amplifier with tubes about 55 years ago and an amplifier clinic measured 10% distortion until its tubes were replaced. The tube replacements needed to be done quite often. I replaced it with a solid state amplifier that still works perfectly today, last year I replaced its 52 years old main filter capacitor.
     
    Arouse1973 and Electro-Cat like this.
  7. Electro-Cat

    Electro-Cat

    3
    2
    May 25, 2017
    Ahh - I'm a big fan of "Don't worry, you'll be fine just as it is". I was worried by too much heat if the AC input is too high.

    I'm sure I'll listen to it when done - can't wait. I'm sure I'll love it - a whole new world opening up!

    Thanks 73's de Edd.
     
    Arouse1973 likes this.
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