Happy Holidays, folks! I've been quite busy the last month putting together a 'super deluxe' testing rig. Diytube has been lucky enough to get a HP 8903 Audio Analyzer from the northern Illinois Motorola diaspora - still NIST certified, too. I've made a pretty decent GPIB controller out of an old PC and have written some custom diytube software. Sweeping these tube amps now is a breeze. The speedy, volumous data permits many tweaks to see gain, distortion and frequency response changes.

To put the rig through the paces I figured a transformer shootout would be just the ticket. Plus it answered some questions I had in my mind - maybe it will answer some of yours, too.

Important Note: Now this is not an ultimate test of these transformers - rather it is a test of these transformers plunked into a stock diytube ST35 circuit. I didn't tweak the circuit for any of these.

The test setup was as follows for each:

- 400mVRMS input was used for tests at normal output levels (green data)

- 850mVRMS input was used for ~1% distortion output levels (red data)

- Hammond 272HX power transformer with B+ at 391VDC

- bias set at 35mA per tube

- 19.2K negative feedback from the 8ohm tap

I tested these output transformers (all purchased by me, except the MQ which is on loan from a customer):

- Hammond 1620

- Hammond 1650F

- Magnequest MQ-565

- Dynaco Z-565 (dated 1965, cloth leads)

- Handwound 7.6K PP 25W

The data columns are: frequency tested, output VRMS, gain, percentage of distortion & noise, and finally power into an 8ohm load.

Dynaco Z-565

020Hz 5.25V +22.1dB .1778%THD+N 3.458W @ 8ohm

050Hz 5.28V +22.2dB .0979%THD+N 3.551W @ 8ohm

100Hz 5.32V +22.2dB .0893%THD+N 3.538W @ 8ohm

200Hz 5.32V +22.2dB .0737%THD+N 3.525W @ 8ohm

500Hz 5.34V +22.2dB .0704%THD+N 3.564W @ 8ohm

01kHz 5.32V +22.2dB .0747%THD+N 3.525W @ 8ohm

02kHz 5.34V +22.2dB .0891%THD+N 3.564W @ 8ohm

05kHz 5.29V +22.1dB .1513%THD+N 3.498W @ 8ohm

10kHz 5.24V +22.0dB .2760%THD+N 3.432W @ 8ohm

20kHz 5.16V +21.8dB .3050%THD+N 3.315W @ 8ohm

30kHz 4.92V +21.5dB .5300%THD+N 3.014W @ 8ohm

40kHz 4.29V +20.4dB .9850%THD+N 2.279W @ 8ohm

020Hz 10.50V +21.7dB 3.43%THD+N 13.81W @ 8ohm

050Hz 10.65V +21.8dB .841%THD+N 14.18W @ 8ohm

100Hz 10.68V +21.9dB .786%THD+N 14.23W @ 8ohm

200Hz 10.69V +21.9dB .792%THD+N 14.26W @ 8ohm

500Hz 10.68V +21.9dB .783%THD+N 14.26W @ 8ohm

01kHz 10.68V +21.9dB .840%THD+N 14.23W @ 8ohm

02kHz 10.68V +21.8dB 1.07%THD+N 14.23W @ 8ohm

05kHz 10.63V +21.7dB 1.94%THD+N 14.10W @ 8ohm

10kHz 10.54V +21.5dB 3.04%THD+N 13.86W @ 8ohm

20kHz 10.27V +21.1dB 3.77%THD+N 13.13W @ 8ohm

30kHz 09.60V +20.2dB 2.67%THD+N 11.47W @ 8ohm

40kHz 07.86V +18.2dB 3.14%THD+N 7.683W @ 8ohm

Magnequest MQ-565

020Hz 5.34V +22.5dB .2685%THD+N 3.498W @ 8ohm

050Hz 5.32V +22.5dB .1384%THD+N 3.538W @ 8ohm

100Hz 5.34V +22.5dB .1096%THD+N 3.551W @ 8ohm

200Hz 5.34V +22.5dB .0856%THD+N 3.578W @ 8ohm

500Hz 5.38V +22.5dB .0799%THD+N 3.605W @ 8ohm

01kHz 5.38V +22.5dB .0820%THD+N 3.605W @ 8ohm

02kHz 5.38V +22.5dB .0970%THD+N 3.605W @ 8ohm

05kHz 5.41V +22.5dB .1460%THD+N 3.645W @ 8ohm

10kHz 5.38V +22.4dB .2355%THD+N 3.605W @ 8ohm

20kHz 5.16V +22.0dB .3350%THD+N 3.315W @ 8ohm

30kHz 4.80V +21.5dB .4640%THD+N 2.868W @ 8ohm

40kHz 4.05V +20.0dB 1.139%THD+N 2.040W @ 8ohm

020Hz 10.58V +21.8dB 3.570%THD+N 13.97W @ 8ohm

050Hz 10.74V +22.0dB 1.047%THD+N 14.47W @ 8ohm

100Hz 10.80V +22.0dB .9680%THD+N 14.55W @ 8ohm

200Hz 10.81V +22.0dB .9250%THD+N 14.61W @ 8ohm

500Hz 10.86V +22.0dB .9480%THD+N 14.72W @ 8ohm

01kHz 10.87V +22.0dB .9920%THD+N 14.77W @ 8ohm

02kHz 10.87V +22.0dB 1.241%THD+N 14.74W @ 8ohm

05kHz 10.85V +21.9dB 2.147%THD+N 14.72W @ 8ohm

10kHz 10.74V +21.7dB 3.110%THD+N 14.42W @ 8ohm

20kHz 10.24V +21.1dB 3.040%THD+N 13.08W @ 8ohm

30kHz 9.550V +20.2dB 2.088%THD+N 11.35W @ 8ohm

40kHz 7.410V +17.6dB 3.570%THD+N 6.845W @ 8ohm

Hammond 1650F

020Hz 5.27V +22.2dB .2742%THD+N 3.419W @ 8ohm

050Hz 5.25V +22.2dB .1219%THD+N 3.445W @ 8ohm

100Hz 5.26V +22.3dB .0980%THD+N 3.458W @ 8ohm

200Hz 5.27V +22.3dB .0768%THD+N 3.485W @ 8ohm

500Hz 5.31V +22.3dB .0745%THD+N 3.525W @ 8ohm

01kHz 5.31V +22.3dB .0764%THD+N 3.525W @ 8ohm

02kHz 5.30V +22.3dB .0913%THD+N 3.498W @ 8ohm

05kHz 5.26V +22.1dB .1563%THD+N 3.458W @ 8ohm

10kHz 5.06V +21.8dB .3180%THD+N 3.188W @ 8ohm

20kHz 4.52V +20.8dB 1.139%THD+N 2.543W @ 8ohm

30kHz 3.91V +19.6dB 2.830%THD+N 1.901W @ 8ohm

40kHz 3.07V +17.6dB 5.570%THD+N 1.170W @ 8ohm

020Hz 9.910V +21.2dB 10.08%THD+N 12.20W @ 8ohm

050Hz 10.58V +21.8dB .7360%THD+N 14.07W @ 8ohm

100Hz 10.65V +21.9dB .6800%THD+N 14.15W @ 8ohm

200Hz 10.66V +21.9dB .6690%THD+N 14.15W @ 8ohm

500Hz 10.69V +21.9dB .6950%THD+N 14.28W @ 8ohm

01kHz 10.70V +21.9dB .7210%THD+N 14.28W @ 8ohm

02kHz 10.68V +21.9dB .8840%THD+N 14.23W @ 8ohm

05kHz 10.58V +21.7dB 1.500%THD+N 13.97W @ 8ohm

10kHz 10.27V +21.4dB 2.061%THD+N 13.16W @ 8ohm

20kHz 9.290V +20.3dB 3.070%THD+N 10.76W @ 8ohm

30kHz 8.120V +18.8dB 8.520%THD+N 8.222W @ 8ohm

40kHz 6.370V +16.3dB 18.05%THD+N 5.040W @ 8ohm

Hammond 1620

020Hz 5.20V +22.2dB .3350%THD+N 3.354W @ 8ohm

050Hz 5.23V +22.2dB .1172%THD+N 3.406W @ 8ohm

100Hz 5.24V +22.2dB .0983%THD+N 3.432W @ 8ohm

200Hz 5.24V +22.2dB .0764%THD+N 3.432W @ 8ohm

500Hz 5.24V +22.2dB .0731%THD+N 3.419W @ 8ohm

01kHz 5.24V +22.2dB .0773%THD+N 3.419W @ 8ohm

02kHz 5.23V +22.2dB .0930%THD+N 3.406W @ 8ohm

05kHz 5.17V +22.0dB .1501%THD+N 3.341W @ 8ohm

10kHz 5.00V +21.7dB .2470%THD+N 3.125W @ 8ohm

20kHz 4.49V +20.8dB .3950%THD+N 2.509W @ 8ohm

30kHz 3.91V +19.6dB .7880%THD+N 1.901W @ 8ohm

40kHz 3.13V +17.8dB 4.340%THD+N 1.217W @ 8ohm

020Hz 09.27V +20.6dB 16.18%THD+N 10.65W @ 8ohm

050Hz 10.48V +21.7dB 1.266%THD+N 13.83W @ 8ohm

100Hz 10.56V +21.8dB 1.183%THD+N 13.94W @ 8ohm

200Hz 10.57V +21.8dB 1.140%THD+N 13.97W @ 8ohm

500Hz 10.61V +21.8dB 1.183%THD+N 14.07W @ 8ohm

01kHz 10.62V +21.8dB 1.279%THD+N 14.10W @ 8ohm

02kHz 10.61V +21.8dB 1.560%THD+N 14.02W @ 8ohm

05kHz 10.50V +21.7dB 2.431%THD+N 13.76W @ 8ohm

10kHz 10.13V +21.2dB 2.909%THD+N 12.83W @ 8ohm

20kHz 08.98V +20.0dB 2.550%THD+N 10.06W @ 8ohm

30kHz 07.77V +18.4dB 2.828%THD+N 7.547W @ 8ohm

40kHz 05.87V +15.6dB 11.80%THD+N 4.278W @ 8ohm

Handwound 7.6K Push-Pull 25W

020Hz 5.12V +22.0dB .3610%THD+N 3.239W @ 8ohm

050Hz 5.17V +22.1dB .1563%THD+N 3.328W @ 8ohm

100Hz 5.18V +22.1dB .1190%THD+N 3.354W @ 8ohm

200Hz 5.19V +22.1dB .0882%THD+N 3.354W @ 8ohm

500Hz 5.19V +22.1dB .0809%THD+N 3.354W @ 8ohm

01kHz 5.19V +22.1dB .0834%THD+N 3.367W @ 8ohm

02kHz 5.19V +22.1dB .1654%THD+N 3.354W @ 8ohm

05kHz 5.19V +22.1dB .9550%THD+N 3.354W @ 8ohm

10kHz 5.14V +21.9dB 3.560%THD+N 3.290W @ 8ohm

20kHz 4.78V +21.3dB 6.660%THD+N 2.844W @ 8ohm

30kHz 3.97V +19.8dB 4.540%THD+N 1.960W @ 8ohm

40kHz 3.15V +17.8dB 2.325%THD+N 1.232W @ 8ohm

020Hz 09.76V +21.1dB 6.260%THD+N 11.83W @ 8ohm

050Hz 10.23V +21.5dB 1.881%THD+N 13.18W @ 8ohm

100Hz 10.32V +21.6dB 1.769%THD+N 13.31W @ 8ohm

200Hz 10.35V +21.6dB 1.677%THD+N 13.39W @ 8ohm

500Hz 10.37V +21.6dB 1.647%THD+N 13.44W @ 8ohm

01kHz 10.38V +21.6dB 1.757%THD+N 13.47W @ 8ohm

02kHz 10.40V +21.6dB 2.257%THD+N 13.52W @ 8ohm

05kHz 10.38V +21.5dB 4.710%THD+N 13.47W @ 8ohm

10kHz 10.15V +21.2dB 10.72%THD+N 12.90W @ 8ohm

20kHz 08.17V +19.1dB 25.39%THD+N 8.364W @ 8ohm

30kHz 08.00V +18.7dB 16.62%THD+N 7.920W @ 8ohm

40kHz 06.16V +16.0dB 7.950%THD+N 4.728W @ 8ohm

Epilogue

As one would expect, the Dynaco Z565 sets the bar. The Magnequest does meet or beat the Z565, which I would attribute to less losses in the wire - a fine transformer and the teflon coated wires really take the heat (yep, I tested!). The Hammonds really are quite a value. The bottom end performance isn't as spectacular as the 565s, but these Hammonds are still excellent performers to 30Hz. The Handwound transformer unfortunately underperforms at the higher frequencies in this stock circuit.

The screen feedback capacitor, a 20pF cap, does sharply drop the high frequency performance. Lowering its value (eg 15pF) may make the Hammond units even flatter at 20kHz. I'll do some twiddling with this in the near future, but that was not the purpose of these tests. Finally, I did remove this screen feedback cap during testing of the Handwound unit to see if I could achieve better distortion numbers, but while the 400mVRMS tests looked better, my 10kHz to 20kHz performance at 850mVRMS looked worse.

Do post your questions.

Shannon