Variable Capacitor differences - UK Vintage Radio Repair and Restoration Discussion Forum

Neil Purling · 13th Dec 2015, 7:08 pm

in selecting a tuning capacitor for a valve superhet:
I never counted the plates of the two gangs. Correct me if I am wrong... Should the oscillator gang have one or two more pairs of vanes?
I am just wondering here as in my first effort I had a 1000pf trimmer in series with the osc coil primary to get the offset, which seems very wrong.

ionburn · 13th Dec 2015, 7:19 pm

Not sure how general things are but if the oscillator frequency is higher then it will, if different, have less vanes

13th Dec 2015, 7:34 pm

Most superhets had the same vane setup for both oscillator and aerial, others (usually portables) had specially shaped oscillator vanes to save the padder capacitor and probably to make the tracking better for a hi-q aerial circuit.

turretslug · 13th Dec 2015, 10:14 pm

As MM says, it's possible to make a gang variable with an oscillator section profiled for perfect tracking at a constant IF difference from the aerial input tuning- often seen in older transistor portables. This only works for one band though, i.e. the MW band in a simple battery portable (though there would be nothing to stop such a set having VHF FM as well, as a separately tuned front end is needed here anyway). For multi-band sets, the oscillator section will almost certainly use the same profile vanes and total capacity and tracking is achieved with padding and trimming- it can be got close over a band span but it's always a compromise, hence the quoting of particular alignment frequencies by designers to achieve minimal deviation from ideal.

Don't confuse the gang capacitors used in some receivers as apparently having different capacitance between sections when the physically rather larger oscillator section may have a somewhat wider air-gap between vanes to minimise capacitance change with temperature. Drift of a few kHz would be hopeless in an oscillator but is relatively insignificant in the input circuits, hence the difference in capacitor sections. The R1155, Collins TCS12 and Pye CAT all had markedly larger oscillator gang sections, although the capacitance swing was similar to the other gang sections- the Pye has two gangs in parallel for the oscillator, giving the superficial appearance of four-gang tuning.

13th Dec 2015, 7:08 pm	#1
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Variable Capacitor differences in selecting a tuning capacitor for a valve superhet: I never counted the plates of the two gangs. Correct me if I am wrong... Should the oscillator gang have one or two more pairs of vanes? I am just wondering here as in my first effort I had a 1000pf trimmer in series with the osc coil primary to get the offset, which seems very wrong.

13th Dec 2015, 7:19 pm	#2
ionburn Heptode Join Date: Dec 2014 Location: Lincoln, Lincolnshire, UK. Posts: 583	Re: Variable Capacitor differences Not sure how general things are but if the oscillator frequency is higher then it will, if different, have less vanes

13th Dec 2015, 7:34 pm	#3
Guest Guest Posts: n/a	Re: Variable Capacitor differences Most superhets had the same vane setup for both oscillator and aerial, others (usually portables) had specially shaped oscillator vanes to save the padder capacitor and probably to make the tracking better for a hi-q aerial circuit.

13th Dec 2015, 10:14 pm	#4
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,398	Re: Variable Capacitor differences As MM says, it's possible to make a gang variable with an oscillator section profiled for perfect tracking at a constant IF difference from the aerial input tuning- often seen in older transistor portables. This only works for one band though, i.e. the MW band in a simple battery portable (though there would be nothing to stop such a set having VHF FM as well, as a separately tuned front end is needed here anyway). For multi-band sets, the oscillator section will almost certainly use the same profile vanes and total capacity and tracking is achieved with padding and trimming- it can be got close over a band span but it's always a compromise, hence the quoting of particular alignment frequencies by designers to achieve minimal deviation from ideal. Don't confuse the gang capacitors used in some receivers as apparently having different capacitance between sections when the physically rather larger oscillator section may have a somewhat wider air-gap between vanes to minimise capacitance change with temperature. Drift of a few kHz would be hopeless in an oscillator but is relatively insignificant in the input circuits, hence the difference in capacitor sections. The R1155, Collins TCS12 and Pye CAT all had markedly larger oscillator gang sections, although the capacitance swing was similar to the other gang sections- the Pye has two gangs in parallel for the oscillator, giving the superficial appearance of four-gang tuning.