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SANSUI AU-70 or 1000A???

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Hello everyone.

I have a pair of SANSUI output transformers labeled “AU-70”. I am not certain which amplifier they came from. As far as size and dimensions, my OPTs are comparable to other OPTs that can deliver 30-35W.

First guess would be the Sansui AU70 integrated amp, which uses 7189’s in a PP configuration for the final stage. However, I estimated the transformers’ primary impedance to be 6,300 ohms, which is somewhat low for the 7189’s.

On the other hand, 6,300 ohms is suitable for a final stage using 7591’s in PP as used in the Sansui 1000A amp.

Could anyone, please, clarify the mystery and put an end to my dilemma? I would like to build an amp and use these OPTs, but I am not sure what tubes to use.

Thank you in advance for your help and advice.
 
@ Tony:

You are absolutely correct. Unfortunately I do not have all the transformer specs. I used a rather empirical method to measure the primary impedance, which is about 6,300 ohm. I have no idea about the power handling capabilities of these transformers. This is why I need to know which amp they came from. I would really hate to fry a pair of perfectly good transformers.

@Korey:

AU-70 is indeed the obvious first guess. However, as I said before, the 6,300 ohm impedance is rather low for the 7189 tubes used in the AU-70 (data sheet indicates a plate to plate resistance of 8,000 ohm, even 10,000 ohm in class AB1 – triode mode). On the other hand, the data sheet for the 7951 tubes used in the 1000A indicates a plate to plate load of 6,600 - therefore my dilemma.

Perhaps someone on this forum has one of these amps and is kind enough to check for me the label on the output transformers?............. It would be greatly appreciated!

Thank you.
 
I have output transformers from the Sansui 1000A receiver and their transformer number is 410-5234 and sometimes they are labeld 1000A and sometimes both numbers are labeled on the end bell. So most likely your transformers are from the AU70 integrated amp which uses 7189 tubes. Although according to this schematic the impedance should be 8K.
AU70_Schematic.jpg


Does your irons shape like these? Too bad I couldn't find a picture showing the transformer number...
http://images02.olx.com.my/ui/7/65/...be-amplifier-au-70-Electronics-1286515715.jpg
 
We all easily lose sight of the fact that the real working load of an amplifier is a loudspeaker, something very, very far from a resistor. Variations of 25%, that seem so important to us when designing, are *nothing* compared to the real world use. You certainly cannot hurt your transformer by a slight mismatch - if so, they'd all be dead already.

If you want to use them with 6BQ5/EL84's and are concerned with linearity, just keep working voltages down and idling current up, connect as triodes. Not tons of power output, maybe +7 or +8dBW, but clean.

All good fortune,
Chris
 
@directdriver

Thank you very much for your effort and the detailed information. The schematic you attached shows the OPT impedance as 8K, just as the 7189 data sheet. Judging by the labeling of you 1000A transformers that shows “1000A”, it appears that mine are indeed from an AU-70. I have to get to the bottom of this impedance discrepancy. Could my transformers be shorted? Who knows!!! I bought them as a set including the mains transformer, which was definitely shorted. Thanks again, directdriver!!!


@ Chris Hornbeck

I understand perfectly what you are saying and you are correct. The idea was to figure out if I can get 30-35W out of these babies instead of just 15-17W. Thanks for your input.


@Michael Koster

The method to estimate the primary impedance consisted of multiplying the impedance of each speaker tap with the turn ratio squared (turn ratio different for each of the 4, 8, and 16 ohm taps). Because of measurement inaccuracies, the calculated impedance values ranged between 6,150 and 6,600 ohm, the average being 6,300.
 
I made a mistake in my previous post. The average impedace is 6,300 ohm, not 6,600. Sorry.

By th way, I determined the turn ratio by applying 15, 10, and 5 volts and measuring the voltage on each speaker tap. I did a total of 9 tests, good enough for estimating purposes.

One measurement should be good enough under the right conditions: e.g. midband frequency 400 Hz or thereabouts, with a resistive load on the output, and preferably at at least a volt output or more. Also did you observe the output waveform on a scope?

What I'm saying is that low voltage unloaded measurements at certain frequencies could introduce gross voltage errors. Even the simple winding DCR can account for 10% difference in actual turns ratio vs. measured voltage ratio. If the 8K ohms specified includes say 10% DCR (primary plus reflected secondary), and you measure without a load, then your impedance ratio could be off by something like 20%. Add to this a little overshoot due to leakage inductance at low voltage and you may have explained your discrepancy.

In fact, if your most representative measurement was the highest, 6600 ohms, and your DCR component causes a 10% voltage error in an unloaded measurement, you have exactly 8K... Assuming the 8K is measured with the rated load.
 
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Thanks, Michael. You make a very good point. I ran the tests off a variable transformer at 60Hz, no load, no scope. I will redo the testing as you recommended. I need to find a generator that can supply a voltage high enough to get one volt on the secondary taps.

Now I have some work cut out for me!!!

Thanks again.
 
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