• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

OPTs of questionable utility

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Being the comparative tube newbie (tuwbie?) that I am, I in my zeal for finally figuring things out for myself bought a pair of old telefunken console OPTs off of ebay.

However, this was done at something like 3am, when I wasn't terribly lucid and most certainly shouldn't have been shopping for toys online.

These things are purportedly 12k/4, and were advertised as being acceptable for an EL34 single-ended application. Knowing I had some EL34s kicking around, I pulled the trigger without properly considering that I don't actually have any sort of 4-ohm speakers sitting around to go on the other side of the transformer. So my question to you, dear reader, is what in the world do I do with these? I don't fully understand the process of matching the output stage to the transformer primary (see point #3, below).

Some thing to keep in mind, here:
1- I'm a total tuwbie (totuwbie? God I hope that catches on), but I'm also an EE that knows sort-of what's going on.
2- I've done my best as to due diligence on this before posting. I found some literature on the google, but none of it seemed really authoritative and the explanations seemed aimed slightly above my head.
3- The copy of Jones is in the mail :)
There was a seller from Germany offering pairs of small open frame Telefunken transformers with these specs. (12K/4) that were said to be used with EL84s....not EL34s. Are these the units you speak of? If so, they probably are not the best match up to your EL34s and they also look a bit under powered.

And I noticed that he claimed to measure the windings with a multimeter to get these numbers. Using this type of meter to measure DC resistance does not relate to the dynamic (AC) impedance. This needs to be done with AC to get the turns ratio and then compute the impedance (N²) or with a bridge measurement. So his numbers may not be correct.
My mistake. That's what I get for trusting my hazy recollection of this happening in the first place. The transformers in question are indeed the ones of which you speak; apparently intended for operation in an EL84 or EL95 SE configuration. Thankfully, I also believe I've got EL84s kicking around.

12k/4 seems like an awfully odd number. But what do I know? NOTHING! That's what.

Since this'd be my first from-scratch project, I'm quite alright with building something on the low end, with regards to power. I have some reasonably sensitive speakers, and huge numbers of amps and volts flying all about the place quite frankly scare the bajeezus out of me.
Ok, if you use these with EL84s (6BQ5s) you'll be fine since that's what they were originally used with. Output tubes have a range of load inpedances they will work into. Somewhere between 8K to 10K is typical for this number, but 12K (if that's what they actually are) is close enough for audio. And a proper measurment may prove them to be a little lower.

I'll post pictures as soon as I get them in the post. As they're coming from Germany, and I only paid for them a few days ago, it might be a while. The transformers themselves looked quite decent in the auction picture. Much nicer, at any rate, than anything I've pulled from equipment of similar age.

I think I'll be trying the RH84, since I have most of what's required to make it go. Hooray, learning!
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2wo said:
If it was measured with an ohm meter it may well not be a "4 ohm " winding. The last trans I checked was about 4 ohms on the 8 ohm tap. Probably be fine…John

The pair i had like that were just marked as such.



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If I read correctly (and judging by the photo), this is not a p.p. transformer. In SE cases the optimal Zl is about 4 - 6k.ohm, so one will have quite a low output at 24k.ohm.

Then I suspect that the impedances themselves are 12K/4 ohm, not dc resistance, although doubt was expressed. The dc resistance of any 4 ohm winding will be sub-ohm, while primary dc resistance would be of the order of a few hundred ohm maximum (otherwise you only have a door stop!).

Other parameters like primary inductance, leakage etc. are also relevant - it is not a simple matter to buy an OPT worth its money. Mostly not enough specifications are available.
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