Differential Reaction Condensor

[ed731pdh]

Has anybody come across one of these, and is there a modern equiv? I have never seen one, but am increasingly coming across a lot of references in documents as of late.

[pwdrive]

Two seperate stators opposite each other, as the rotor is turned the capacity of one decreases as the capacity of the other increases. Pretty sure you can still get them but not sure where.

lawrence.

[pwdrive]

http://golbornevintageradio.org/forum/at...p?aid=4517

Just an addition to show one of these capacitors in use in a receiver, if you study the circuit diagram of the xtal filter section you will see a differential capacitor as two arms of a xtal filter bridge using two xtals.

Lawrence.

[Aerodyne]

Google Jackson Brothers - I believe their successors still supply variable capacitors of both air-spaced and solid dielectric type, so I imagine they'll have a differential type. I have one, retrieved from an early 1930s homemade set. I believe I could also lay hands on an air-spaced low capacity type as well (I'd have to rummage).
Of course you don't need a differential for reaction purposes - a minor mod to the circuit will allow the use of a 'standard' component. The differential design was supposed to prevent (or limit) reaction chasing, the effect that forces slight retuning as reaction is adjusted.
-Tony

[Aerodyne]

Having had a look at Jackson via Google, I can't find any differential capacitors. There are some split stator types and they are still making the solid dielectric 'Dilecon' range - the typical reaction capacitor - among others.
-Tony

[AlanBeckett]

There's one in my Pye MM which I suspect may have a problem - see attached. I don't know what it's value is and it's part of a complicated 'Reaction Assembly'. Pye recommended that if there was a problem it should be sent back for repair - some chance.
They appear in lots of old books and yet I don't remember them being particularly common as components.
I'm still looking.
Alan

   

[pwdrive]

Hello Alan, not seen that one before, the only thing I can think of at the moment is that it affords a smoother control when going in and out of regen. ie. not so fierce.

Lawrence.

[AlanBeckett]

Lawrence,
Yes, it's an odd circuit. The capacitor is ganged with the bias control to the rf amplifier. The control is labelled Volume. Presumably the idea was to give a smooth control without having to twiddle separate Volume and Reaction controls. Unfortunately it doesn't work very well, at least on this set.
Any comments gratefully received, but preferably on the MM Thread.
Alan

[Paula A]

I can imagine how you could make one easily from a pair of reaction condensers back to back on the same shaft.. or a little mental gymnastics reversing which parts you see as a pair and which parts move

I have seen one only once.. in a VHF tv amplifier/booster thing where it was used as the output level control.

That is one strange circuit.. I can well understand why it doesn't work very well, in fact I'm surprised it works at all. What is more puzzling is why.. why design it that way when there are all the components for a straight forward volume control instead of making an uncontrollable oscillation machine as soon as it's not loud enough for the cup final.. It's hardly more than one step away from varying the filament temperature to control the output!!

[Aerodyne]

C20 only starts to work once R10 is at maximum. R10 is a bias control to set the gain of V1 - AC/SG - by varying the level of HT- in its chassis return. So, reaction increases gain only after full gain is reached 'conventionally' via bias application, without reaction. An odd situation when looked at in today's terms but it isn't hard to see how it works, really. Provided, of course, that the thing isn't faulty...
I would have expected V1 to have been the vari-mu version but according to Trader this isn't so.

-Tony