09-12-2018, 06:52 AM
One feature of the T99 is the small circular subsidiary dial around the tuning knob. None of the pictures that I have found shows this very clearly. But it appears to contain a logging scale at its outer edge. Inside that appear to be six scales. My best guess is that these were part of a mechanical form of bandspreading, covering six of the SW broadcast bands, probably six selected from the 11, 13, 16, 19, 25, 31, 41 and 49 metre bands.
Looking at the valve line-up as shown at RMorg, the respective functions would logically appear to be:
W77 - RF amplifier
X79 - Mixer-oscillator
W77 - 1st IF amplifier
W77 - 2nd IF amplifier
D77)
D77) - Between them, demodulator, AGC rectifier, AGC delay
6BR7 - Gramophone preamplifier
Z77 - AF amplifier
6U5 - Tuning indicator
AGC delay was probably done in a separate diode from the AGC rectifier, to avoid differential distortion. That was the case with the following T139.
And again looking back from the T139, the Z77 and 6BR7 were probably triode-strapped.
The Brimar 6BR7 looks to be odd-man-out in an essentially Osram valve line-up. But at the time the T99 was developed (1951), the EF86 and Z729 were still in the future (1953 second half) whereas the 6BR7 (originally the 8D5) was newly available.
Cheers,
Steve P.
Looking at the valve line-up as shown at RMorg, the respective functions would logically appear to be:
W77 - RF amplifier
X79 - Mixer-oscillator
W77 - 1st IF amplifier
W77 - 2nd IF amplifier
D77)
D77) - Between them, demodulator, AGC rectifier, AGC delay
6BR7 - Gramophone preamplifier
Z77 - AF amplifier
6U5 - Tuning indicator
AGC delay was probably done in a separate diode from the AGC rectifier, to avoid differential distortion. That was the case with the following T139.
And again looking back from the T139, the Z77 and 6BR7 were probably triode-strapped.
The Brimar 6BR7 looks to be odd-man-out in an essentially Osram valve line-up. But at the time the T99 was developed (1951), the EF86 and Z729 were still in the future (1953 second half) whereas the 6BR7 (originally the 8D5) was newly available.
Cheers,
Steve P.
