Bias Probes. Is more really better?

  • Thread starter Thread starter kylendm
  • Start date Start date
I've got the little Eurotubes probe, it works just fine. I've A/B'd it with each individual tube slot, all 2 or 4 tubes (depending which amp I was working on) as well as the bias calculations I picked up from Aiken's site for biasing* by reading the resistance of each side of the transformer taps and the voltage drop between transformer taps when powered up. I also used two DVOM, my $40 DVOM and my $400 Fluke from work, double checking/cross checking all readings with each other. Boring story short, the reasonably priced Eurotubes socket bias adapter with a decent/accurate DVOM is all you need.

Also, the Krank I have, there is a resistor on the main board where you measure the voltage in mV, it corresponds to the bias current, that also cross-checked with the readings from the Eurotubes probe, as well. So, I'll reinforce the above with the idea that the DVOM's accuracy is the key here. And this is assuming you have one bias pot, a multiple socket system might be more convenient if you have more than one bias pot.



*From here, down to the "Plate/cathode current method," third bullet-point down.
 
Check out this thread. I think the cheap ones will work fine, I used some cheap probes I got on ebay for years. I love the Compubias though, much easier than trying to measure plate voltage with my meter. viewtopic.php?f=3&t=65729&hilit=bias
 
If I was doing it for a living/working on other people's gear a lot, I'd definitely opt for something like the Compubias, that is a sweet concept. You do still have to know what you are looking at, though. I bought my Blackmore from someone that owned a Compubias, and he had put KT-66's in it, using that unit to set the bias. I thought it sounded funny when I took possession of it, so I tested the bias, and it was running just about 80-90% plate dissipation with the pot adjusted as far down as it would go, pulling what I thought was way too much plate voltage (I forget the exact numbers now) for the amp's transformer, in my limited experience. So...

...Four new 6L6GC and screen resistors later, with a proper biasing, it sounded good. :doh: That isn't the Compubias' fault, of course.
 
Just one more question. What's a good multimeter to use. I'll pass on Flukes multimeter since I don't have the money to spend on them but how about in a 0-$50 range?
 
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