Lesson learned

RhythMick

Member
2015-10-20 8:20 pm
Posting to help other beginner builders. Plus writing it down helps reinforce the lesson to myself.

New build phono amp, balanced with balanced power supply, separate psu with umbilical cable. Long in the research and design phase, long in the careful build. I'm fairly new to builds so spent a long time on safety measures: built a series safety lamp and tested it, wrote out a list of around 150 tests and measurements to perform in stages (powered, heaters only, ht). Lots of reading about how to test safely.

Safety wise it was all good. Somehow, in the final hookups between valves and circuit board I think I managed to hook an anode load to one of the grids. Still not sure how and no evidence that I did but the voltages were not where I expected on one of the channels so I powered off to investigate. In my measurements and continuity checks I realise that the HM3 step up transformer on that channel is now open circuit on the secondary. Definitely wasn't before and I think I just fried it with the anode voltage.

In hindsight (and the path I'm now going down) startup testing should have been with the hm3s disconnected and no valves, measuring all voltages at the pins. I can use resistors to provide a dummy load across the anode/cathodes to draw the expected current, but I can check everything looks right per design without risking valves or sensitive SUT.

If only I'd realised that earlier.
 

RhythMick

Member
2015-10-20 8:20 pm
I'm obviously missing something about transformers. A continuity test across the primary coils tests closed circuit (continous) , but the secondaries test open. Both of them. And the brand new Tributes I have lined up also test same : primaries closed, secondaries open. Surely both are just coils of wire and should test closed?

All tests done are very brief, as I don't want to apply a dc voltage for anything more than the briefest of touches - just to hear the beep.

Is the test invalid? Should the secondaries test closed or open?
 

RhythMick

Member
2015-10-20 8:20 pm
It's a very odd feeling, the combination of utter stupidity and relief that I may not have burned out the HM3s after all. Thank you.

I've always shied away from actually testing the resistances with a DC multimeter. I resorted to this briefest of continuity tests in one of those "digging for the problem" moments. Is this a valid fear ?
 
No, it is not. As was mentioned above, you needed to use the resistance scale, not continuity, because above a certain resistance threshold, you get no beep.

Meanwhile, your meter resistance scale cannot possibly harm a transformer. The currents it uses are minuscule, and won;t hurt the transformer, which is after all, a piece of wire wrapped around a core.

If you wired 300v across the tiny transformer winding, that could harm it potentially, but your meter? Never.

But you are right about testing. When you wire up a circuit, power it first with no tubes and nothing you consider fragile (real or imagined, if you fear, then don;t) Do you get proper heater voltage at each socket heater pins? and do you get B+ voltage on the plate pins and nowhere else it ought not be? Be aware that without tubes, the plate pin voltages will be full B+ supply voltages, not the much lower voltage that happens with tubes.

If that does not blow a fuse and the voltages seem appropriate, then you can install tubes and retest.
 

RhythMick

Member
2015-10-20 8:20 pm
No, it is not. As was mentioned above, you needed to use the resistance scale, not continuity, because above a certain resistance threshold, you get no beep.

Meanwhile, your meter resistance scale cannot possibly harm a transformer. The currents it uses are minuscule, and won;t hurt the transformer, which is after all, a piece of wire wrapped around a core.

If you wired 300v across the tiny transformer winding, that could harm it potentially, but your meter? Never.

But you are right about testing. When you wire up a circuit, power it first with no tubes and nothing you consider fragile (real or imagined, if you fear, then don;t) Do you get proper heater voltage at each socket heater pins? and do you get B+ voltage on the plate pins and nowhere else it ought not be? Be aware that without tubes, the plate pin voltages will be full B+ supply voltages, not the much lower voltage that happens with tubes.

If that does not blow a fuse and the voltages seem appropriate, then you can install tubes and retest.

Thanks Enzo. Followed this and got the expected voltages on heaters, anodes and cathodes (higher actually, 285ish vs 265, but this must be because the valves aren't drawing current so the chokes are saturating). I then put valves partway in and used those little hook clips to measure onto the exposed legs. All is safe. 1 channel not working to design so some things to track down, but safe.

My fear was whether the meter could damage the SUT by magnetising the coils.