No announcement yet.

Output transformer tech questions

  • Filter
  • Time
  • Show
Clear All
new posts

  • Output transformer tech questions

    I have a pair of Grommes 7.2k EL84 push pull outputs. They measure very closely in primary DCR and primary inductance. This make sense to me- similar output transformers for a stereo amp hopefully means similar sound on both sides. Having used them in a stereo situation I found them to sound nearly identical.

    I have another pair of 6BM8 6k2 push pull output tf's from an inexpensive Japanese hi-fi. I've used one of them in a push-pull EL84 output amp for a while- I built the amp in '98 and I've used it heavily till a few weeks ago when I got the urge to try some different things with the amp. Both tf's are fairly similar in DCR but I hooked them up to my LCR meter and one reads almost twice the primary inductance of the other! I removed the outer metal covers and I see nothing that leads me to believe that one is substantially different from the other. In the past (before I had the LCR meter) I did a test for shorted turns and found none. When I connect each up to a signal (11.4 volts ac from a wall wart) both halves of each primary produce nearly identical output voltages at any given secondary tap. Why would the primary L be so different? What effect would this have on the tone of each?

    I have another two transformers that are the same size and almost the same impedance ratio but have substantially different DC resistances. One is in the 400 ohm range and the other is in the 150 ohm range, measured center tap to plate tap. What is likely to be the sonic different between them? The lower DCR one is from a Bogen PA with 7868's and the other is from a small hi-fi with 6973's. Once again, they're different. I assume the higher DCR one is using smaller wire with more interleaves...right?

    I do plan to do listening comparisons but I thought I'd ask before I formed any opinions on my own because it's bugging me.


  • #2
    Not an expert at all - typically, the transformers with the highest primary inductance are 'better' due to better low frequency response.

    Inductance isn't the easiest thing to measure, and in todays world, even the less expensive transformers can deliver good frequency response at rated power.

    OTOH, in a guitar amp, I'm not sure that a really good 5 - 40KhZ rransformer is required.