NR86

NR86 is a variable-mu tube for RF and IF amplification, belonging to the KTW63 family. Admiralty Signal Establishment specification AD/CV1195, Issue 5 of 11 June 1947, explicitly places NR86 and ARP15 beneath the common CV1195 number and identifies KTW63 as prototype. It classifies the tube as a variable-mu HF tetrode. This primary identification explains why it should not automatically be treated as an ordinary pentode, despite some reference descriptions using that term. 1947 dates the consulted edition, not an introduction date established by that sheet.

The cathode is indirectly heated, with a 6.3 V, 0.3 A heater. The envelope is unmetallized glass. The octal base carries heater on 2 and 7, anode on 3, screen on 4 and cathode on 8; 1 and 5 are unconnected and pin 6 is omitted. The control grid is on the top cap. In particular, the free pin 5 must not be treated as a third-grid terminal wired according to another type’s pinout. Photographs of the retained specimen show its faint marking and, from above, a copper control-grid support.

The CV1195 specification sets maxima of 250 V anode voltage and 125 V screen voltage. It gives a characteristic transconductance of 1.5 mA/V at 250 V anode, 125 V screen and 0 V control grid; the note’s conditions are part of the figure. Capacitances measured with an external screen include at most 0.006 pF grid-to-anode, 7.5 pF anode-to-cathode and 4.7 pF grid-to-cathode. Specifying the measurement screen is significant for an RF tube: these numbers do not describe every possible mounting arrangement indiscriminately.

Page two separates characteristics from acceptance tests. With 6.3 V heater, 250 V anode, 100 V screen and −3 V grid, it permits 5.5–9.5 mA anode current and 1–2 mA/V transconductance. The 125 V screen figure from page one must therefore not be inserted into tests requiring 100 V. Another test sets the grid to −40 V and checks residual anode current between 2 and 170 µA: microamperes, not milliamperes. This documents behavior in the strongly reduced-gain region without making −40 V a universal cutoff voltage. Specification maximum dimensions are 120 and 40 mm under the reference drawing, distinct from measurements of the individual photographed specimen.