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    Very Vintage ST70 Bias Upgrade

    Peter W.

    Posts : 785
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Very Vintage ST70 Bias Upgrade

    Post by Peter W. on Mon Feb 13, 2017 2:30 pm

    This unit has the cursive Dynakit label as an indicator of age. But the second-generation driver board with the additional low-value caps.

    Just for giggles (and peace of mind), this Sunday, I took an hour to do the bias diode, cap and resistors upgrade as previously detailed herein.

    1N4007 diode.
    67 @ 140V 105C caps.
    10K @ 1/2 watt resistor
    22K @ 1/2 watt resistor

    That latter value put the travel of the adjustment pot almost exactly dead-center to 1.5V. The suggested 20K value skewed it slightly.

    I am running German Siemens "thin-man" EL34s with about 4,500 hours on them at best-guess. Two matched pairs, and as verified on the Hickok still 'close enough' for filament and plate current at the correct bias (all user-inputs/outputs on the 539B). Still lots of getter-flash. The 5AR4 is a British Mullard, the 7199s are Sylvanias with the same date code.

    I could watch the current drawn on the unit drop from about 1.5 A to about 1.4A when I reduced the bias from the approximately 2.5V (after resistor replacement).
    I am running now at 1.4V, steady as a rock after 15 minutes of warm-up. So, setting the proper bias has more just an affect on the output tube life.
    I also took the opportunity to test each connection with a dental-pick, and clean the switches and pots. The mono-stereo switch was failing - the back phenolic section split), so that is now bypassed. I re-soldered three connections and rebuilt two traces on the driver board. None yet failed but certainly getting there.
    The OEM 4 section cap tests out for capacitance *and* ESR. Imagine that! Not that I do not have my eye on it.

    The unit is now sitting in my test shelf being fed by a PAS3x, FM3 & CD player. The next few times I am working at the bench, it will provide sound, until I have about 20 more hours of steadiness completed. Then it will shift back into permanent service.

      Current date/time is Sun Jul 15, 2018 10:41 pm