John Stewart recommended we go to a site for HP archived info including manuals and schematics........
Go to this link-
www.hparchive.com
to see the HP 207A Wide Range Oscillator listed under the equipment
heading.
20 Hz to 20 KHz in one twist of the dial. Uses a modified Wein Bridge.
Cheers to All, John Stewart
The 207A is described as an audio sweep oscillator, which I assume means the F starts at 20Hz and over a time period the F is smoothly changed up to 20kHz, enabling the audio F response of filters and speakers etc to be seen on a CRO screen. The 207A is at
http://www.hparchive.com/Manuals/HP-207A-Manual.pdf
I have not yet looked closely at the 207A schematic but there is a typical 4 gang tuning gang in a Wien bridge with each gang having C max = 500pf.
Its not a wide band instrument because it omits F below 20Hz and above 20kHz which prevents an audio engineer to examine the extremely important behaviour of audio gear below 20Hz and above 20kHz.
The HP 209A is a more interesting wien bridge oscillator with multi bands and 4Hz to 2MHz, but it uses all discrete bjts and j-fets. Hm, my latest little Oskar Layta goes 1Hz to 2MHz, but its oscillator amp is all tubed. Seems like HP couldn't seem to manage getting down to 1Hz, but lemme tellya, its dead easy now with discrete parts, and 0.1Hz is possible, and waves will have less THD than if you use the alterantive multi-pin signal gene chip which simply uses a pot to control DC on a pin chip to set the F, as in BWD161 function gene I restored&upgraded recently. The BWD161 calibration isn't very good. They seek to give too much too easily, and budget price and hence toylike quality that was affordable by hobbyists.
Getting down to 0.1Hz with a tubed amp and Wien bridge oscillator circuit may be difficult because the tube amp needs to be RC coupled, and hence is bandwidth limited, so perhaps a 0.1Hz-1Hz and 1Hz-10Hz may be best done with a phase shift oscilator, using R+C parts used in the biasing of the tubes, as in many guitar amps for tremelo.
Patrick Turner.