07-10-2021, 12:10 PM
I've been building a superhet radio with Russian rod pentodes this year, inspired by some of the posts by Trevor (Murphyv310) and Mike Watterson. The aim is a no-compromise, high-end design - and one of the issues I faced is provision for automatic volume or gain control.
I ended up with something which holds the output rock-steady between 1mV and 230mV RF input, and within 3db between 500μV and 500mV – that’s a 60db range.
Unlike pentodes with helically-wound grids (DL31, DL96, 1T4 etc) these little valves don't come in variable-μ flavours. I suspect it is theoretically possible (tapered or curved rods?) but I guess it's just too difficult to manufacture!
Anyway! To include AVC, two matters need to be dealt with. One is electronic control of gain; the other is deriving a control voltage or current.
For the first, some gain control is possible by varying grid bias. But, reducing gain by taking the grid nearer cut-off, reduces signal handling ability, just when we need it more. So this is a non-starter.
Varying filament voltage would probably work, but I don't like under-running filaments unless I can reduce other voltages and currents too, to avoid operating the valve in saturation-limited conditions.
Varying the screen voltage does work, and well. But it has the effect of reducing the cut-off voltage, thus reduces signal-handling capacity, though not so severely as biasing g1 more negative.
Applying negative voltage to the suppressor affects the balance between screen-grid current and anode-current, thus potentially reducing gain to anode. Unfortunately, it has the side-effect of reducing anode resistance, thus damping any anode tuned circuit the more, and reducing selectivity.
The strategy adopted was a combination of screen and suppressor control, and having comparatively low impedance tank circuits in the anode, damped by resistors to give the selectivity I wanted, so that the variable ra damping is swamped anyway. The consequential lower gain is overcome by having two IF valves (they're plentiful and cheap!), this also has the benefit that only moderate gain variation in each stage (10:1) is needed to give a large variation overall (100:1).
So, how to derive a control voltage? Several tens of volts swing is needed (+10 to +40V on screen-grid, and -20 to 0V on suppressors), so considerable amplification must be provided. And the screens take some hundreds of μA. I didn't want the AVC circuit loading the audio detector, and possibly causing distortion, so I used a buffer IF amplifier to isolate.
First attempt was to use an anode-bend detector, biased-off as a delay voltage. This worked, but the control voltage was modulation-dependent (note that the traditional double-diode triode topology suffers from this defect too). So I changed to a leaky-grid detector, filtering-out the audio with a (very) low-pass filter. This gives a control voltage of the wrong sense, so I then had to use another valve to invert it. But, it gives as a bonus additional DC amplification. I introduced the delay voltage at this point. With huge gain available, the circuit is capable of holding the output at a highly constant level even with big changes of input. Tracy (BusyBee) had considered an op-amp in one of his/her posts: this performs equivalently.
Eyebrows may be raised at the -60V bias line, but it's needed for other stages... And at very low power. It's easily derived from the heavily-screened and filtered switchmode converter which powers the whole thing from a 12V SLA battery.
I also fed some control voltage to the frequency changer (a single-balanced mixer using a pair of rod pentodes, with a third rod pentode as local oscillator driving suppressors in antiphase). Cutting gain in early stages prevents overloading in later stages – the final IF stage has only partial AVC applied to the screen. Of course, for lower-level signals it’s better to run early stages at full gain and then cut it in later stages because then you gut the noise too. It’s all a compromise!
Performance?
I ran a couple of tests: static audio output versus RF input, using a modulated RF generator and precision attenuator; and dynamic response by switching the RF input between two levels (6db apart) and examining how the output recovered to its controlled level (the 'scope photo shows: Upper trace, RF input; Lower trace, AF output from detector).
See below for circuits, and results.
Comments invited!
I ended up with something which holds the output rock-steady between 1mV and 230mV RF input, and within 3db between 500μV and 500mV – that’s a 60db range.
Unlike pentodes with helically-wound grids (DL31, DL96, 1T4 etc) these little valves don't come in variable-μ flavours. I suspect it is theoretically possible (tapered or curved rods?) but I guess it's just too difficult to manufacture!
Anyway! To include AVC, two matters need to be dealt with. One is electronic control of gain; the other is deriving a control voltage or current.
For the first, some gain control is possible by varying grid bias. But, reducing gain by taking the grid nearer cut-off, reduces signal handling ability, just when we need it more. So this is a non-starter.
Varying filament voltage would probably work, but I don't like under-running filaments unless I can reduce other voltages and currents too, to avoid operating the valve in saturation-limited conditions.
Varying the screen voltage does work, and well. But it has the effect of reducing the cut-off voltage, thus reduces signal-handling capacity, though not so severely as biasing g1 more negative.
Applying negative voltage to the suppressor affects the balance between screen-grid current and anode-current, thus potentially reducing gain to anode. Unfortunately, it has the side-effect of reducing anode resistance, thus damping any anode tuned circuit the more, and reducing selectivity.
The strategy adopted was a combination of screen and suppressor control, and having comparatively low impedance tank circuits in the anode, damped by resistors to give the selectivity I wanted, so that the variable ra damping is swamped anyway. The consequential lower gain is overcome by having two IF valves (they're plentiful and cheap!), this also has the benefit that only moderate gain variation in each stage (10:1) is needed to give a large variation overall (100:1).
So, how to derive a control voltage? Several tens of volts swing is needed (+10 to +40V on screen-grid, and -20 to 0V on suppressors), so considerable amplification must be provided. And the screens take some hundreds of μA. I didn't want the AVC circuit loading the audio detector, and possibly causing distortion, so I used a buffer IF amplifier to isolate.
First attempt was to use an anode-bend detector, biased-off as a delay voltage. This worked, but the control voltage was modulation-dependent (note that the traditional double-diode triode topology suffers from this defect too). So I changed to a leaky-grid detector, filtering-out the audio with a (very) low-pass filter. This gives a control voltage of the wrong sense, so I then had to use another valve to invert it. But, it gives as a bonus additional DC amplification. I introduced the delay voltage at this point. With huge gain available, the circuit is capable of holding the output at a highly constant level even with big changes of input. Tracy (BusyBee) had considered an op-amp in one of his/her posts: this performs equivalently.
Eyebrows may be raised at the -60V bias line, but it's needed for other stages... And at very low power. It's easily derived from the heavily-screened and filtered switchmode converter which powers the whole thing from a 12V SLA battery.
I also fed some control voltage to the frequency changer (a single-balanced mixer using a pair of rod pentodes, with a third rod pentode as local oscillator driving suppressors in antiphase). Cutting gain in early stages prevents overloading in later stages – the final IF stage has only partial AVC applied to the screen. Of course, for lower-level signals it’s better to run early stages at full gain and then cut it in later stages because then you gut the noise too. It’s all a compromise!
Performance?
I ran a couple of tests: static audio output versus RF input, using a modulated RF generator and precision attenuator; and dynamic response by switching the RF input between two levels (6db apart) and examining how the output recovered to its controlled level (the 'scope photo shows: Upper trace, RF input; Lower trace, AF output from detector).
See below for circuits, and results.
Comments invited!
