The following snippet is from the Christchurch groups latest meeting minutes by our faithful scribe, David Fahy... I thought I'd throw it in here to see how many people can figure it out (spoiler alert, I probably can't)...
The workings of the A.V.C. circuitry in a Gulbransen 872 being restored by Albie Smith proved to be something of a conundrum, eventually unravelled by Murray Clark.
Check this circuit on Riders Volume 5 GHI Page 40/85 and see if you can fathom how this A.V.C. works. The N.Z.V.R.S. could run a competition for the most succinct and accessible explanation of this bit of valve technology wizardry. (Explanations must include the role of the screen and suppressor grids to gain marks). And the question remains – could there have been an easier way? Perhaps a Peter Lankshear explanation might be forthcoming.
You have 1 hour. Any talking or looking at other peoples work will see you ejected and your grades nullified. You may begin the examination now.
There are no personal problems that can't be solved with the liberal application of high explosives
Here Goes: Looks like an early attempt to provide AGC without the use of "double diode triode" tubes that came later? The Audio detector looks like a Plate detector which isnt suitable in this instance for AGC, so they have used another tube who's cathode is biased at -150Volts. The IF comes off a separate winding and fed to the grid, via a pot to adjust the gain (also Vol control). The anode has a large Cap to bypass any Audio to ground leaving the DC, the anode voltage range would be a around -3VDC down to minus approx -8v which is cutoff for the 57 tube. Have I got close ?
Post by radioholic on Jun 27, 2020 23:03:37 GMT 12
Here is my take: 1. the grid bias voltage of the first 2 valves comes from the anode of the AVC valve.
2. A steady DC current (say 50 mA) flows from GND through the volume pot R15 , to the transformer centre tap, so there is an IR drop across the volume pot. The sweeper on the pot feeds a negative bias voltage to the grid of the AVC valve, depending on the setting of the pot.
3. First consider the case with the volume pot set to max volume (sweeper to the left on the diagram). The bias on the AVC valve is then 50 mA across 150 ohm = 7.5V. This is the cutoff voltage for the 57 valve, so the AVC valve draws zero current in absence of any signal.
4. the anode voltage on the AVC valve is due to the IR drop across R10, plus the IR drop across R14. The IR drop across R14 is 60 ohms x 50 mA = 3V. The IR drop across R10 =250 Kohms x Ia where Ia is the anode current drawn by the AVC valve. Since Ia = 0, the anode voltage is -3V which sets the bias voltage on the controlled valves.
5. In the presence of a signal the positive peaks move the AVC valve grid into the conducting region, so current flows through the AVC valve, generating an IR drop across R10 proportional to the strength of signal. The AC component of this voltage is filtered out by C11, leaving the DC component, which then adds to the -3V grid bias on the controlled valves.
6. With volume pot set to minimum volume (sweeper to the right), there is zero grid bias on the AVC valve, so it conducts maximally, causing a large voltage across R10, which biases the controlled valves past their cutoff points.
7 With an intermediate setting of the volume pot, the effect is between these extremes.