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