A 4120 / 7120 schematic with notes

A 4120 / 7120 schematic – OS/116 (adjusted) – with notes

updated 5th December

Click for a larger downloadable image

Worked up in 2010. It is not known at present which amp was tested and measured.  Note that the sheet has been digitally altered to accord with the build of the amp – it is stated that in the original schematic prepared by JMI, R92 is 20K ohms/10W;  R100 is 5K ohms/10W; and R109-R112 are 330ohms.  See below, this is not entirely correct.

Beware of the bias voltage mentioned – this could be too low for modern valves.  It should also be said that plate voltages may differ from amp to amp.


Above, a detail from OS/116, unmodified. Click for a larger much sharper image.  R92 is not 20K ohms/10W as the tester noted on the modified sheet, but 10K, which is the value he encountered in the amp in front of him.

A 4120 / 7120 schematic with notes | 2017 | blog