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needtubes 14th March 2005 05:17 AM

Interstage transformer as OPT?
I just saw an interstage transformer rated at 600-ohm primary to 4 and 8-ohm secondaries, 30Hz-20KHz, 12W max. Would it be conceivable to use this transformer as an OPT for a ~1-2W SET? With a tube that likes a 600-ohm plate load, of course.

I also had another thought... Would it work well if one used the ultrapath topology on the output of an amp?

Thanks all

SY 14th March 2005 12:50 PM

It will probably work if it's designed for a single-ended input- some ITs are made to be driven push-pull and will saturate if you use them for SE service. One could consider one of the variants where the transformer is couple to the tube plate via a capacitor (to get rid of the saturating DC), but that will have to be a pretty hefty sized cap. And I don't know what reasonable tube will be happy with a 600 ohm plate load...

needtubes 14th March 2005 04:28 PM

I had in mind the Hammond P-T119DA. And was thinking the 6AS7 for the output tube... it usually calls for a ~1k load, and with a slightly larger cathode resistor should be good for a watt or so on a 120-150V supply at about 50mA bias. Just approximations... The transformer doesn't mention being for push-pull use, bu I wonder about running it at that high of bias. I guess I could try and see, though it could be a costly mistake.

SY 14th March 2005 04:36 PM

That transformer is not made to see a DC current- it's used as a second transformer between 600 ohm outputs and 8 ohm speakers for ham receivers (where 300-3000Hz frequency response is the norm).

needtubes 14th March 2005 04:59 PM

Hmm.... That's what I figured. The mroe I read, the more it looked not so good.

Will it stick with its rated 30-20KhZ +/-1dB if I used it as part of a parafeed with SRPP 6AS7 (capacitively coupled, of course...)? (I was looking for a good excuse to use two 6AS7's anwyay). Thanks

SY 14th March 2005 05:14 PM

Depends on the source impedance of the SRPP circuit. Have you calculated that?

needtubes 14th March 2005 09:52 PM

Umm... no... you lost me there. :rolleyes: What exactly would I be calculating?

I found a link to a 6AS7 SRPP and SEP that I thought was interesting. I would want it at a lower voltage, though. In this circuit, though, shouldn't the cap be before the transformer?

SY 14th March 2005 09:59 PM

Since the cap and the transformer primary are in series, it doesn't much matter which order they're in: a + b = b + a.

By "source impedance," I mean the series resistance if you convert the output stage to its Thevenin equivalent, i.e., the "looking back" impedance that the transformer sees. If the source impedance is low with respect to the primary impedance, the transformer distortion will be much lower than the opposite case.

needtubes 14th March 2005 10:14 PM

Ahh, I see now. Well, the source impedence would be higher than the transformer impedence, I suppose... Each 6AS7 plate is around 280-ohms and each cathode is 250-ohms, plus the resistor which would have to be about 1.5k... We'd be sitting at about 2560-ohms total for the SRPP. So I am guessing this would not be so good afterall...

Eventually I have to have an idea that'll actually work.

SY 14th March 2005 10:20 PM


Eventually I have to have an idea that'll actually work.
I keep telling my wife the same thing. She doesn't believe it.

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