Thursday, May 2, 2013


     Finished the 6V heaters and started on the crystal oscillator:
     One thing this is teaching me is that it has been a long time since I used a large soldering iron.  I'm getting better but still have to remind myself to take care.

     Remember the broken 40 meter JEL (B&W) coil?  Here it is repaired.
     The shiny stuff is home-made "Q-dope," liquid polystyrene, some of which dries white if it picks up contaminants from the old plastic.  The polystyrene coil support is a strip cut from a plastic box that just happened to be the right material.

     The toluene in the Q-dope is so obnoxious that all work with it I have done outdoors and tried to choose times when there was a steady breeze.


  1. Thank you!

    I keep wondering if I should check the power transformer with 110 now; but it's riskier to run them with no load. Ohmmeter checks were okay. So I'll probably not apply power until it is done, and even then, through a 40W light bulb current limited in series with the incoming AC.

  2. I would. The usual treatment I do is to put the whole transformer in the oven at 180ish and bake it to drive out all moisture. Once cooled I then put a lamp in series and apply power. If the insulation is
    good no load is not an issue. But for safety I have been known to use a variac before the lamp and transformer.


  3. it's riskier to run them with no load.

    I didn't know this. What's the failure mode? I can see running a DC supply open-circuit being a problem because the peak voltage on the filter caps may be higher. (And the amount of smoke old PC switching supplies used to put out in that state was impressive.)

  4. What!? You don't own a variac?

    tsk! tsk!

  5. I own two Varics, but they're historical. This business of "running it up slowly with a Variac" is bunkum. A) It's not a current limiter B) running tubes at low heater volts with (low) B+ doesn't do them any good.

    I use a series (incandescent) light bulb. Always have, always will. It is a current limiter and it'll let you know if there's excessive current being drawn: it lights up. Chose bulb wattage for maximum current limit.

    Failure mode for an unloaded power transformer is arc-over. Especially old ones.

    Testing after wiring, if it's okay, it'll work and if not, I'll order a replacement from Hammond and (with luck!) fit it in. Next option after that would be a toroidal type, under a metal can to make it look more suitable.

    * Strictly speaking, I own only one Variac: the word is a trademark of General Radio. The other one is, h'mm, a Powerstant, maybe?

  6. Thanks. I wasn't aware of that. I guess I've been lucky with the transformers I've played with.

    I believe the generic term for that foot-crushing thing with the knob on it is "variable autotransformer." Not only does it not limit current, it also doesn't isolate from the line. (By the way, did you know scope lead ground clips are fusible?)

  7. :) Sadly, I did.

    These days, I worry more about ruining the graphite "brush" on the autotransformer with excessive current. A lot of the mfrs. are gone and it's not easy to gnaw out a good replacement.

  8. Yup. Incandescent light bulbs are great for that. The Bose 601 Series II tower speakers I just restored have 24v bulbs in the crossovers as current-limiters. If you have the volume really cranked up, you can see the bulbs doing their thing.

    Which explains why you'll sometimes find pin-socket Mazda lamps on the chassis of vintage vacuum tube radios and amplifiers...