Difference between reaction condenser and variable capacitor - UK Vintage Radio Repair and Restoration Discussion Forum

SurreyNick · 28th Jul 2014, 9:23 pm

Can someone tell me the difference between a reaction condenser and a variable capacitor, or are they the same thing by another name? They look different, but google wasn't very helpful in answering the question.

I ask because I am replicating F.J. Camm's 3-valve receiver and it calls for a 300pf tuning condenser (which I take to be a variable capacitor), and also a 100pf reaction condenser (which from the illustration looks identical to the attached picture). I can't find a 100pf variant like this so was wondering if I could substitute with a variable capacitor?

Thanks
Nick

julie_m · 28th Jul 2014, 9:30 pm

A reaction condenser is just a variable capacitor with a smaller range than a tuning condenser.

You could use a 0-300 pF capacitor where a 0-100 pF capacitor is called for, but all the control would be squozen into a small space at the "low" end of its rotation.

Station X · 28th Jul 2014, 9:31 pm

Variable capacitor is a generic term for any capacitor whose capacitance can be varied. This is done by increasing or decreasing the amount by which the plates overlap. Some variable capacitors have an air dielectric, whereas others, like the one in your picture have a solid dielectric.

Variable capacitors have many uses, the most common being as a tuning capacitor. Other uses are as (variable) reaction capacitors, aerial trimming capacitors, VFO (Variable Frequency Oscillator) capacitors, BFO (Beat Frequency Oscillators) capacitors and transmitter PA (Power Amplifier) loading capacitors.

Radio Wrangler · 28th Jul 2014, 9:35 pm

Your picture shows a variable capacitor. It can be used as a reaction control or for any number of other purposes.

It just happens to be be a miniature type which uses thin sheets of dielectric material between the vanes to increase the capacitance for its physical size. Jackson Brothers made them and sold them under the 'polivaricon' series name. Most probably that's where Radio Spares bought them from.

If you look inside most oriental small transistor radios you'll see even smaller dual variable capacitors used for RF and LO tuning of a basic superhet.

Most radios use two or three gang variable capacitors for tuning superhets, and most are air-spaced types which makes them a good deal larger in depth. Single variable capacitors get used in basic TRF designs and for tuning BFOs, VFOs in complex shortwave sets, and may be used to adjust the positive feedback strength (aka reaction) in regenerative sets.

Reaction controls can be a bit touchy, so I'd rather go for an air spaced variable capacitor than what you have there. The one you have could be dismantled and reassembled with fewer vanes to scale down the capacitance.

Any decent amateur radio rally ought to have a good selsctiion sprinkled around its stalls.

David

SurreyNick · 28th Jul 2014, 9:42 pm

Thank you for your replies.
This thread can be closed.
Nick

28th Jul 2014, 9:23 pm	#1
SurreyNick Heptode Join Date: Apr 2014 Location: Wigton, Cumbria, UK. Posts: 728	Difference between reaction condenser and variable capacitor Can someone tell me the difference between a reaction condenser and a variable capacitor, or are they the same thing by another name? They look different, but google wasn't very helpful in answering the question. I ask because I am replicating F.J. Camm's 3-valve receiver and it calls for a 300pf tuning condenser (which I take to be a variable capacitor), and also a 100pf reaction condenser (which from the illustration looks identical to the attached picture). I can't find a 100pf variant like this so was wondering if I could substitute with a variable capacitor? Thanks Nick Attached Thumbnails

28th Jul 2014, 9:30 pm	#2
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Difference between reaction condenser and variable capacitor A reaction condenser is just a variable capacitor with a smaller range than a tuning condenser. You could use a 0-300 pF capacitor where a 0-100 pF capacitor is called for, but all the control would be squozen into a small space at the "low" end of its rotation. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

28th Jul 2014, 9:31 pm	#3
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Difference between reaction condenser and variable capacitor Variable capacitor is a generic term for any capacitor whose capacitance can be varied. This is done by increasing or decreasing the amount by which the plates overlap. Some variable capacitors have an air dielectric, whereas others, like the one in your picture have a solid dielectric. Variable capacitors have many uses, the most common being as a tuning capacitor. Other uses are as (variable) reaction capacitors, aerial trimming capacitors, VFO (Variable Frequency Oscillator) capacitors, BFO (Beat Frequency Oscillators) capacitors and transmitter PA (Power Amplifier) loading capacitors. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

28th Jul 2014, 9:35 pm	#4
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,875	Re: Difference between reaction condenser and variable capacitor Your picture shows a variable capacitor. It can be used as a reaction control or for any number of other purposes. It just happens to be be a miniature type which uses thin sheets of dielectric material between the vanes to increase the capacitance for its physical size. Jackson Brothers made them and sold them under the 'polivaricon' series name. Most probably that's where Radio Spares bought them from. If you look inside most oriental small transistor radios you'll see even smaller dual variable capacitors used for RF and LO tuning of a basic superhet. Most radios use two or three gang variable capacitors for tuning superhets, and most are air-spaced types which makes them a good deal larger in depth. Single variable capacitors get used in basic TRF designs and for tuning BFOs, VFOs in complex shortwave sets, and may be used to adjust the positive feedback strength (aka reaction) in regenerative sets. Reaction controls can be a bit touchy, so I'd rather go for an air spaced variable capacitor than what you have there. The one you have could be dismantled and reassembled with fewer vanes to scale down the capacitance. Any decent amateur radio rally ought to have a good selsctiion sprinkled around its stalls. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

28th Jul 2014, 9:42 pm	#5
SurreyNick Heptode Join Date: Apr 2014 Location: Wigton, Cumbria, UK. Posts: 728	Re: Difference between reaction condenser and variable capacitor Thank you for your replies. This thread can be closed. Nick