EL821=6CH6 Brimar UK black plate like new 1956 in white boxes

1 tube) 35.2ma Gm=11,100

1 tube) 35.0ma Gm=11,000

Tested on Amplitrex

Vplate=250

Vscreen=250

Vgrid=-4.5V

Good Iplate 35-40ma

Good Gm = 10,000-11,000

The ECH81 was designed by Mullard and was used as a "VIDEO OUTPUT PENTODE" with high mutual conductance (Gm)

Vfil=6.3V @ 750ma, Vplate MAX = 275V, Pout MAX = 12 watts

6CH6 APPLICATION DATA:
---------------------
Class Va Vg2 Vg1 Ia Ig2 Ra Gm
A S/E 250 250 -4.5 40 6 50,000 11

This valve was used as the audio output valve in the Rees-Mace receiver AKA
the Pye marine receiver AP100339 that was built in the 1950s and extensively used by British and Dutch navies. The question remains as to why the 6CH6 was the valve of choice for this receiver when so many purpose designed audio valves could have been selected. Sources specifically list it as a video amplifier. Note the high value of gm, necessary in order to obtain reasonable gain and power output with wideband signals, and for correct matching of video into coax cable. TV
studio equipment racks used to be full of 6CH6s.