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Old 6th April 2005, 01:30 PM   #1
dnsey is offline dnsey  United Kingdom
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Default 50V line trafos for tube O/P?

I have a number of large transformers removed from 'industrial' solid state PA amps. The original topology was a bridged pair of (estimated) 200W amps driving each transformer via seperate primaries, with two secondaries in each case feeding 50V line output terminals.
The amp modules work very well indeed directly driving 8 Ohm speakers.
Are the transformers likely to be useful for tube output, used 'n reverse'? Any ideas for matching output devices (quality rather than power preferred)?
Many thanks,
Dave.
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Old 6th April 2005, 04:31 PM   #2
PRR is offline PRR  United States
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> 200W amps driving each transformer via seperate primaries, with two secondaries in each case feeding 50V line output terminals. Are the transformers likely to be useful for tube output, used 'n reverse'?

No.

50V rating means 71V peak. The peak swing of a 10 Watt 6V6 plate is about twice that, of a 100 watt tube amp about six times that. Exceeding the design signal voltage increases distortion. For geometric reasons, a 400 Watt industrial amp core may have decent bass response, but doubling the signal voltage will be bad and six times the voltage will just go mushy.

Or figure 2*200W at 50V. I get about 6 ohms, which is way low for any tube. (These may just be 1:1 isolation transformers!)

6080 triode has Rp about 280 ohms, and if you run it at about 150V B+ you can pick a load-line with about 70V peak swing in 280 ohms per plate, about 8 watts per pair. About 45 twin-tubes gives appropriate impedance and 350W(!) power at fairly high distortion; more tubes are needed to give proper THD numbers. This is also 708 Watts of heater power, which may be awkward. (If you trust RCA's heater insulation, you could series-string the whole four dozen directly off the power line...)

One thought, though it seems stupid. Connect as autotransformer to step-up low impedance speakers to the higher impedance favored by hollow-state transformerless power amps.
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