Dynaco built high quality amplifiers and this exhibit was made in the USA but not by them. The Dynaco ST-70 stereo amplifier, introduced in 1959, used a 7199 in each channel to drive the output valves. Between 1959 and 1969 more than 350,000 ST-70 stereo amplifiers were produced.
The 7199 triode pentode was designed specifically for audio use and the data-sheet dates to 1959. The 7199 was made in small quantities compared to the similar VHF mixer triode pentodes of the day.
The 7199 was also used in guitar amplifiers with the pentode as the input stage and the triode as the phase splitter in the same way as the ST-70 used the valve.
The article above on the use of a triode pentode for better performance in audio phase splitting is dated 1962. Bailey used the plentiful VHF triode pentode ECF82 for his design.
The 7199 has no direct equivalent but with changes to the base wiring the 6U8 and 6U8A can be substituted. In Europe that corresponds to the ECF80 and ECF82.
The pentode on the left and the triode on the right have shapes and sizes comparable with the ECF82.
The sections are separated by an electrostatic screen. The interesting point is the heater at the base of the triode. This is formed as a bifilar pair of twisted wires in the same way as the Mullard's EF86 to reduce the external magnetic field and reduce hum. The ECF82, as a VHF mixer has no such hum reducing adaptation.
Bifilar winding in diagram.
Circuit of of one channel of the Dynaco ST-70 stereo amplifier.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 48 mm tall.
Reference: Data-sheet & Wikipedia. Type 7199 was first introduced in 1959. See also1959 adverts.
