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RE: Losses in Output Transformers

Paul, the transformers are specified as 6.6K, and that's what I measured on the bench in testing last year. There's a small difference between the 4-8-16 ohm taps. Based on the voltage drop I mentioned earlier, which occurs at roughly 45 mA idle per tube, I believe the primary DCR is 250-300 ohms. These are not as large as ST-70 transformers.

I worked on the amp today to further optimize the feedback and response. At the end, I made another power measurement. With +430V on the CT of the OPT and +350 on the screens (full output, one channel driven), maximum output is about 30W. The tubes are set to idle at 45-50 mA. Current at full output is about 85 mA per tube, measured with a DVM across the 1 ohm cathode resistors.

The original outputs used with this transformer set (PT and OPTs) were 7591. The manufacturer rated the amp at 35W/ch, probably with one channel driven. I changed the outputs to 6L6, then designed a new driver with higher gain. Open loop gain of the driver is about 350, more than twice that of the original 7199. Output headroom at the 6L6 grids is also more than 6dB, relative to the 60V pk-pk required for full output.

Again, this isn't the first time I've encountered this. I'm becoming somewhat skeptical of power tube ratings in terms of the "typical operating conditions" presented in the sheets. It's also not clear to me how that data is acquired by the manufacturers, and exactly what it represents.






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