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Unknown DIY valve pre build?

Discussion in 'classic' started by Dowser, Jul 3, 2019.

  1. Dowser

    Dowser Learning to bodge again..

    Bought this cheaply on a whim recently - DIY valve pre with an MM stage. I liked it because the build looked so good. Unfortunately the heater transformer broke loose during shipping, but as I opened before trying power no problem (the heater regulator was crushed and shorted, easily fixed).

    Anyone recognise the design or the valves? I have never seen such round logo RCA 7025s before?

    Haven't powered it up yet, job for weekend.

    Whole thing

    Power supply less heater transformer

    Heater transformer

    MM phono stage and valves (big round logo RCA 7025 valves, look grey plate)

    Line stage and National 12at7 wb black plates (strangely, different brand OP decoupling caps, but both 1uF)


    I have not fully traced wiring yet, but it looks like there are 2 outputs - one with a balnce control in cct, one without.

    I wonder how it compares to my Croft Micro II? :)

  2. Vinny

    Vinny pfm Member

  3. davidsrsb

    davidsrsb pfm Member

    That pot in the power supply section looks corroded away
  4. Dowser

    Dowser Learning to bodge again..

    Thanks both

    A quick google search didn’t show any big red rca logos, need to investigate a bit more!

    The pot, yes, but still doing it’s job - heater voltage is 11v DC.

    HT is 320v - output spikes to 40v unloaded at switch on, 0v with a 33ohm resistor and 9v with my fingers across it :)

    Time to try it in system I guess
  5. Dowser

    Dowser Learning to bodge again..

    Hmm - impressive. Initially imaged well but limited dynamic range. After 2 hours playing range has increased and it sounds good. Better than the Croft? Too early to say, I’d guess bass is less well defined. But definitely worth servicing with new caps (the Croft is modified).

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