1A7: Difference between revisions

(From correspondence with John Addis, May 2021)

Type [[D]] was a mid 1950s DC coupled 1mV/div differential plugin (350kHz at 1mV/div, but 2MHz at 50mV/div) for the 530 and 540 series of mainframes. Similarly, Type [[E]] was a mid 1950s AC coupled 50uV/div differential plugin (30kHz at 50μV/div, but 60kHz at 0.5mV/div). 

By 1963 they had become dated by later plugins in the [[560-series scopes|560 series]] (e.g. [[3A3]], [[2A61]]).  Personally, I thought this was too bad as the 530/540 series was the premier scope line being overtaken by the lower-priced line.

[[Roy Hayes]] sat near me in [[Russ Fillinger]]’s group in Sunset South, Bldg 81 (45.5082 -122.7750), and he had been working on Type [[3A3]], a very nice dual trace, DC coupled 100µV/div (500kHz at all sensitivities) differential amplifier plugin for the 560 series.  It had very high common mode rejection due to some more sophisticated circuits than the D or E.  I really liked several circuit ideas in the 3A3.

I proposed to Russ a DC coupled plugin for the 530/540 series ([[500-series plug-ins (Letter-series and 1-series)|the "Letter" series]]) with the idea of combining the out-dated D and E plugins and extending the DC sensitivity from 1mV to 10µV.  Rather daring for a vacuum tube front end.  I included the high common mode rejection circuitry from the 3A3 and added a full range of bandwidth limit selections, at both the low and high frequency ends of the spectrum.  The amplifier low frequency limit included DC as the first selection.  Russ went for it even though it was not part of his focus, the 560 series.  It took exactly a year to design, and by the time I was finished, I had added another feature, wide range differential DC offset.

It was supposed to be the Type [[F]] plugin, because it did not have the bandwidth extension of the 1A series (50MHz in the 547), but to my chagrin, management decided that it would be a 1A series anyway and renamed it the 1A7.  (I probably have the only Type F plugin.)  Management also changed the nice clear-skirt VOLTS/DIV knob (a larger version of the 3A3’s knob) to a boring opaque-skirt knob like older letter-series plugins.  Late production versions of the 1A7 and 3A3 had JFET inputs.

Nevertheless, the exact combination of features of the 1A7 was repeated in the 1A7A (designed by [[Thor Hallen]]), in the venerable [[7A22]] (designed by [[Val Garuts]]), and in the [[3A9]] (with additional current probe input).  These later three plugins made use of JFETs and all three are of the same design.  That circuit, using feedback, is much better than the earlier 1A7 design.