The DH81 is a double diode triode first seen at the 1947 National Radio Exhibition.
The design use was as detector, AGC rectifier and first audio voltage amplifier in broadcast superhet receivers and effectively was the 6Q7GT on the new B8B or Loctal base.
Type DH81 was obsolescent by early 1951. The DL82 was similar and pin compatible but had a lower amplification factor and a vari-μ characteristic.
The Osram brand label. The diode anodes are seen close to the base below the triode.
The diode anodes are folded and welded to the support. Each anode surrounds the common cathode tube. The triode anode is stitched together and blackened to radiate heat efficiently.
The anode is screened by the vertical plate on the left of the diode anodes. The cathode connection is at the top.
The wide glass tube envelope is 32 mm in diameter, and excluding the B8B base pins is 56 mm tall.
References: 1040 & 4018. Type DH81 was first introduced in 1947. See also1947 adverts.
