Hammond 125ESE Part 2
Test bench results for the Hammond 125ESE
[Tango U808 used as a reference point]
"Reflections of a scope jockey"
These tests are by no means definitive and I still rely on my ears more than anything else. However the techniques I employ have served me pretty well as short cuts in troubleshooting.
Although some tonal aberrations can be recognized on the scope, it should not be taken as the last word. I have tested amplifiers that looked good on the scope but failed to satisfy me musically or two amplifiers with dissimilar circuit topologies that looked and measured the same but sounded totally different from each other. But these were mostly amps with a lot of negative feedback. Negative feedback can extend the bandwidth of an "ill conceived" circuit or output transformer while overshoot and ringing at high frequencies can be "cured" by judicious use of picofarad value caps. So keep these "band-aids" in mind.
I passed up on several opportunities to purchase a good harmonic distortion meter cheap at hamfests, mainly because I do not think it has anything to do with music. On page 32 of the Radiotron 3rd edition, I quote, "The total harmonic distortion is not a measure of the degree of distastefulness to the listener and it is recommended that its use should be discontinued", this was written by someone in 1938 and knew better then.
To a certain degree measurements can be helpful just like what you will see in the case of unit # 2. I also take measurements and observe the behavior of recently completed amp/preamp projects to make sure the circuit is functioning properly. As long as the waves look clean and the math [operating point and etc.] works out right then the unit is ready for an audition.
In the following, all transformers were installed and tested in SE2A3 amps with the same 76-DC-6SN7 input/driver circuit cap coupled to a 2A3 running at 17-18W plate dissipation [60ma. DC unbalanced current through the primary] with no NFB [negative feedback] employed.
Frequency response in 2.5K mode @ 1W output into an 8 ohm load [0 dB = 1W]
Hammond 125ESE [all dated 02/27/01]
- Unit # 1: 20hz - 20khz = flat +/- 1 dB, @ 50khz [ -3.5 dB], rolloff is smooth.
- Unit # 2: 20hz - 20khz = started rolling off at 10khz [ -.3 dB] @ 50khz [-6 dB], rolloff is rough
- Unit # 3: 20hz - 20khz = flat +/- 1 dB, @ 50khz [ -4 dB], rolloff is smooth.
- Unit # 4: 20hz - 20khz = flat +/- 1 dB, @ 50khz [ -3.5 dB], rolloff is smooth
Tango U808 [2.5K mode]:
- Both units: 20hz - 20khz = flat +/- .5dB, @ 50khz [-3 dB], rolloff is very smooth.
NOTE: Typically the bandwidth of any "universal" type output transformer will diminish when used to step down at the secondary taps. So expect less bandwidth at 5K and even more at 10K primary Zs. However, I do not think this would have any serious sonic consequence in the case of the 125ESE units 1, 3 and 4.
Hammond 125ESE square wave response at 100hz, 1khz and 10 khz
Top trace is the signal from the audio generator at less than .1V rms and the bottom trace is the amp output at around 2.2V rms into an 8 ohm load.
|OPT 1 @ 100hz|
|OPT 1 @ 10khz|
OPT 1 - looks good overall!
|OPT 2 @ 100hz|
OPT 2 - the spike at the leading edge of the 1khz square wave is a manifestation of the harshness heard in the treble. At 10khz the rise time is comparatively slow compared to the other 3 units. The overshoot is an artifact of that seen @ 1khz. a slow rise time tend to curtail transient signals resulting in poor treble resolution.
|OPT 3 @ 100hz|
OPT 3 - slightly slower rise time at 10khz compared to OPT 1 and OPT 4 but still fine. There is a tendency to ring but it is well damped. These slight aberrations were not detected by my ears and sound identical to OPT 1 and OPT 4.
|OPT 4 @ 10khz|
Tango U808 [both units looked identical] - less "tilt" at 100hz [greater inductance?] compared to all the 125ESEs and may well be the reason why the bass sounds more articulate and extended. Tango had tight quality control procedures based on several models and samples I've tried.