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Thread: Radford STA100 Restoration Blog

  1. #11
    Join Date: Apr 2012

    Location: South West of England

    Posts: 263
    I'm Will.

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    Skipping forward a bit.

    We got the boards cloned onto modern FR4, with the inclusion of a solder resist mask. They are a true mirror image pair for each channel.

    Some of the resistors have been specified much higher in terms of power rating compared to the originals - they run hot, specifically in the phase splitter circuitry. I believe the original resistors were 2W rated, whereas now I used 7W vitreous enamel wire-wound types instead. All non-electrolytic capacitors are polypropylene types. All resistors are metal film aside from aforementioned wire wound types.

    Perhaps shockingly the smallest resistors in the picture are actually 1W rated, despite looking like 1/4w or 1/2w types. They are minuscule for their supposed power rating but they are used where 1/4W resistors were specified and aren't dissipating any power. I would struggle to treat these as true 1W resistors, they are so small!

    All values are as originals - except for one exception in order to run the output valves a bit cooler. The original configuration runs the output valves right on the edge of their dissipation limits. This addresses this.

    Radford Revival
    www.radfordrevival.co.uk

  2. #12
    Join Date: Apr 2012

    Location: N E Kent

    Posts: 51,624
    I'm Geoff.

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    Looks very neat. Will you be getting the transformers rewound?
    It is impossible for anything digital to sound analogue, because it isn't analogue!

  3. #13
    Join Date: Apr 2012

    Location: South West of England

    Posts: 263
    I'm Will.

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    Quote Originally Posted by walpurgis View Post
    Looks very neat. Will you be getting the transformers rewound?
    If it's necessary, we have all the specs to wind any of the transformers in the amp, including the output transformers. No anxiety about irreplaceable transformers here!
    Radford Revival
    www.radfordrevival.co.uk

  4. #14
    Join Date: Apr 2012

    Location: South West of England

    Posts: 263
    I'm Will.

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    The STA100 in its original form contains numerous minor design flaws. One of which is a zener in the power supply section that holds the bias rail steady. Sounds like a good idea in theory, but it actually makes the bias stability worse because you forgo the equal-but-opposite percent change in bias voltage to HT voltage as the mains fluctuates - meaning the bias currents are more at the whim of the mains voltage fluctuations than without it.

    Here is an original bodged power supply board (mentioned zeners not visible, some time in the distant past they were replaced and relocated to the underside of the board as a completely un-insulated zener string that could touch something else at any time, fantastic!)




    Here is my re-engineered board. The bias supply has been made entirely conventional with no zener regulation, high voltage fast recovery diodes used for the main supplies and fuses also accommodated. The layout has also been made much more logical. The original STA100 has many unintentional internal earth loops and they are known to hum and buzz themselves. I have since gotten one of these amps working and they are now completely quiet with my improved layout.

    Radford Revival
    www.radfordrevival.co.uk

  5. #15
    Join Date: Apr 2012

    Location: South West of England

    Posts: 263
    I'm Will.

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    The valve holder assemblies were of course re-zinced and populated with fresh valve sockets



    Radford Revival
    www.radfordrevival.co.uk

  6. #16
    Join Date: Apr 2012

    Location: South West of England

    Posts: 263
    I'm Will.

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    Here is the first amp nearly complete. There are still numerous bodge wires, namely the input, feedback and output transformer wiring (they are temporary and are being swapped for new ones). The mains switch wiring is also missing as the legend panels (front and rear) are yet to come. No point in wiring it up and having to undo it again to get the panels on. I am getting some new ones made as they originals were visually a mess.

    As I said this NOT the finished article, there are things to finish and tidy up, but it powers up and works incredibly well. It sounds fantastic.







    Radford Revival
    www.radfordrevival.co.uk

  7. #17
    Join Date: Apr 2009

    Location: Near Saffron Walden, Essex

    Posts: 7,087
    I'm Dave.

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    Great work as always Will.

    When will you be making commercially available amplifier(s) ?

  8. #18
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 6,089
    I'm Alan.

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    Quote Originally Posted by Radford Revival View Post
    It sounds fantastic.
    I bet! Great work Will.

  9. #19
    Join Date: Apr 2012

    Location: N E Kent

    Posts: 51,624
    I'm Geoff.

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    Grand job Will. I'd love to hear it.
    It is impossible for anything digital to sound analogue, because it isn't analogue!

  10. #20
    Join Date: Sep 2014

    Location: Northern Ireland

    Posts: 1,403
    I'm John.

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    Ooh, that looks right proper Will.

    Nice work

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