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

Neil Purling · 13th Dec 2010, 9:23 am

Mentioned on a 'suggested application' diagram for a Denco 'C' coil set.
I have the coils and I have a regular 300pf single-gang tuning capacitor as well as the reccomended 500pf twin-gang.
I have never encountered this type before. Were all variable capacitors used for reaction controls this sort?

Herald1360 · 13th Dec 2010, 1:42 pm

In a word- no. It's just one way of doing it. Presumably with some desireable feature or it wouldn't be worth the extra complexity and cost. Or maybe it got round someone's patent, back in the days.....

kalee20 · 13th Dec 2010, 2:02 pm

The advantage is that it always gives you some RF bypass at the detector's anode. Either the RF is bypassed straight to earth, or it's bypassed to earth via the reaction coil.

If you didn't have the differential reaction capacitor, then at minimum reaction you'd have a fair bit of RF on the anode, which might upset something. And if you tried to bypass it with a fixed capacitor, when you really wanted the RF (for reaction), you wouldn't have it.

TriMan66 · 14th Dec 2010, 12:52 pm

Hi,

I recently picked up a sent of Denco coils with the associated circuit diagram. This is shown below with the differential capacitor mentioned.

regards
Craig

13th Dec 2010, 9:23 am	#1
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Differential Variable Capacitor Mentioned on a 'suggested application' diagram for a Denco 'C' coil set. I have the coils and I have a regular 300pf single-gang tuning capacitor as well as the reccomended 500pf twin-gang. I have never encountered this type before. Were all variable capacitors used for reaction controls this sort?

13th Dec 2010, 1:42 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: Differential Variable Capacitor In a word- no. It's just one way of doing it. Presumably with some desireable feature or it wouldn't be worth the extra complexity and cost. Or maybe it got round someone's patent, back in the days..... __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

13th Dec 2010, 2:02 pm	#3
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: Differential Variable Capacitor The advantage is that it always gives you some RF bypass at the detector's anode. Either the RF is bypassed straight to earth, or it's bypassed to earth via the reaction coil. If you didn't have the differential reaction capacitor, then at minimum reaction you'd have a fair bit of RF on the anode, which might upset something. And if you tried to bypass it with a fixed capacitor, when you really wanted the RF (for reaction), you wouldn't have it.

14th Dec 2010, 12:52 pm	#4
TriMan66 Pentode Join Date: Jun 2009 Location: Royston Posts: 120	Re: Differential Variable Capacitor Hi, I recently picked up a sent of Denco coils with the associated circuit diagram. This is shown below with the differential capacitor mentioned. regards Craig Attached Thumbnails