Differential Reaction Condensor

[AlanBeckett]

Tony,
I think you've got a couple of typos in there re Resistor numbers - Rs about face
R12 varies the bias on V1 Grid, so V1 is at Maximum gain when R12 is at Minimum value - shorted. It does actually work as far as that goes - the bias on V1 varies nicely, and stays at a low value after about 90 deg. rotation. My book of notes is in the garage so I can't tell you the actual value. The problems start with further rotation. The volume actually drops and there is some evidence of oscillation. The bias on V1 however remains constant. Attached is the Pye description of it.
C18 tests fine, both value and ESR, and substitution makes no difference. The voltages on V1 are also fine and the Rs and Cs measure OK. So that leaves C20 as suspect. I'm not sure in what way it might be faulty. I'm reluctant to take it to bits unless I have to. If I could buy/beg/borrow/steal a suitable substitute I'd tack it in and see what happens.
Alan

   

[Aerodyne]

Alan wrote 'R12 varies the bias on V1 Grid'
...Not on the Trader sheet, Alan. The grid bias control is definitely R10 (mind you, it is on your circuit too). R12 on the Trader sheet is part of a a screen potentiometer. I suspect you are confusing Pye's numbering with that of Trader.

I have to admit that I haven't followed your main thread about the set so I'm not in a position to comment upon any other fault - I was referring specifically to the function of the combined volume/reaction control.

With the pot effectively shorted the bias will remain constant on V1 grid as the reaction is turned. I don't suppose somehow that the reaction winding could have been connected in reverse? That certainly would cause a drop in gain.
Watch out for the mazak that self-destructs.
-Tony

[pwdrive]

Not sure how I arrived back on this thread...?

So far as I can work out: If a regen has a differential capacitor as the regen control, the total capacitance across the feedback coil remains constant due to the differential action of the capacitor but the feedback applied to the coil is varied by that same capacitor as it is acting as a variable shunt divider across the feedback winding by the same differential action.
This would reduce the amount of frequency pulling on the grid tank circuit due to mutual coupling when advancing the differential capacitor regen control because the feedback coil is fixed in value and the capacitors value across the feedback coil is kept at a relatively constant value at any setting, this reduction in frequency pulling would be more difficult to attain with the more familiar series connected throttle capacitor arangement.

Lawrence.

[Aerodyne]

I'm sure you are right, Lawrence. Reaction'chasing' is a nuisance with a standard variable capacitor - the need to retune slightly as reaction is increased or decreased, hence the introduction of the differential type. I have one of these solid dielectric differential controls somewhere in my junk but it was badly corroded when I last clapped eyes on it (years ago) so I suspect it is likely to be in an unusable condition.
Tony

[pwdrive]

Tony, I've searched the net for some early info on that arrangement but there's not much about it, it's the type of application where a patent might have been taken out possibly?
Yes regen tuning can be a pain, I have built several SW regens over the years and have always found the Hartley with screen grid control in a pentode to be the most reliable and smoothest to use.
I have never used a receiver with the diff capacitor arangement and I'm wondering how it compares.
The Pye MM that Alan's got uses that system, I wonder if any other UK manufactures used it?
The last regen I cobbled up was a regenerodyne type for use on SW, that had an RF amp and osc/mixer feeding into the regen det grid tank which tuned a 500khz band centered on a conversion frequency 3.5 Mhz, the det was operated at low anode and screen volts (approx 20v) that along with a low value of grid leak capacitor (22pf) and high value grid leak resistor (3.3m) enabled it to go in and out of regen with virtualy no discernable drift on reception of both AM and SSB.

I have also seen the diff cap being used as an RF attenuator on the aerial input to a receiver.

Lawrence.

[pwdrive]

The Osram Four New Music Magnet used the diff cap reaction c 1930.

Lawrence.

[AlanBeckett]

The esteemed Mr. Camm was quite keen on them - see attached.
That's from the 12th Edition of 1941 although I fancy it's mostly the 1938 Version.

I can only suppose that back then they were readily available.

Alan


  PWC No 13.pdf (Size: 891.64 KB / Downloads: 57)

[pwdrive]

Thanks Alan, I don't know if the diff caps are still being made, I think split stator and butterfly caps are or at least are readily available.

Lawrence.

[AlanBeckett]

I think it's interesting that it's simply listed as 'Reaction Condenser' as though everyone, or at least your local Radio man, would know what you meant.

Alan

[pwdrive]

Yes, not very descriptive...Camms Comics and all that.

Lawrence.