Cannot identify resistor

Discussion in '"Boat Anchor" & Classic Equipment' started by KN4SMF, Aug 24, 2020.

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  1. KN4SMF

    KN4SMF Ham Member QRZ Page

    About 21 years ago I had an old Crosley console that belonged to a buddy in my living room/work area that I had just finished up. Back then I didn't know of the importance of replacing capacitors. I just fixed what needed fixing to get it up and running. I was pretty proud, so I let it sit there and play for hours. At one point I decided to have a looksee and saw this goop dripping out of it and a puddle on the floor. but it was still playing. Immediately I cut it off and with the sound of the radio now gone, I could hear the sizzling from the power transformer. It had no fuse, not that it would have done any good anyway. Fuses are no guarantee that they will protect against this sort of thing. But as sure as you breathe, had I left the house with that radio running, I would have come home to my house burned down. There's only so much you can do.
     
    KP4SX likes this.
  2. VK6ZGO

    VK6ZGO Ham Member QRZ Page

    I had a look at this website:-

    https://www.kevinchant.com

    Looking at the radios on that site, most of them use "back bias"like your schematic.
    The main advantage of this is that you can provide bias to all the stages with one resistor network, instead of individual cathode resistors.

    About the 14 volts-----from the info on that site, it was common to read this from the cathode of the tube to the chassis, so it may well appear as a positive value.

    As the grid is returned to chassis, it is the same as measuring cathode to grid with the VOM +ve probe
    pon cathode & -ve probe on grid.

    Remember, ordinary analog VOMs cannot read negative voltages, so the probes need to be reversed to do so.
    Many VTVMs can, but normally a polarity change entails switching to "negative volts".
    DMMs can, of course, read negative values, as can Oscilloscopes.


    14 volts is a very common grid bias value on all sorts of output tubes from the 42 to such things as 6M5s & others of that ilk.

    Amongst the manufacturers on the linked website, few are fussy enough to include the DC resistance of their output transformers in their info, but with those who do, the values are between 350 & 500 Ohms, with most occupying the "sweet spot" around 400 Ohms.
    It pretty much looks like you were correct about the 2k Ohm value being a sign of trouble.
     

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