TRF reaction control types. - UK Vintage Radio Repair and Restoration Discussion Forum

taipai_royal · 9th Nov 2005, 9:05 am

Please can anybody tell me why a variable condenser is employed to control reaction rather than a variable resistance, in these early sets.

quantum · 9th Nov 2005, 8:37 pm

Well, depending on the circuit you may find either method, but remember that in early TRFs the reaction control would get more use than a volume or tone control, and a variable cap would stand up to constant use a lot better than a variable resistor. Many leaky-grid detector circuits using reaction would also find the application of reaction smoother with a variable cap than using a variable resistor to control anode or screen voltage.

jim_beacon · 9th Nov 2005, 9:57 pm

The variable resistance control is normally applied to pentodes, where you use the varible resistor to vary the screen potential, and hence the gain.

Early sets used triodes, and here it is easier to apply a variable level of feedback, rather than vary the gain of the active device, hence a series tuned circuit is used to apply the feedback.

The TRF discussed in the current thread uses variable resistance to vary the anode potential of the triode detector, but I suspect that the control may be a little critical, depending on the coupling of the feedback and tuned circuits, it will also be very dependant on the slope of the valves characteristics.

Jim.

taipai_royal · 11th Nov 2005, 10:50 am

Thanks guys, The circuit I was hoping to modify supplies current from the anode of the triode detector then through the MW LW reaction coils via a variable condensers to earth. If I replaced the (faulty) condenser with a variable resistance pot, would it work?
Regards Graham.

quantum · 11th Nov 2005, 12:34 pm

No, not without modification. If you replace the variable cap with a variable resistor then the HT voltage can be shorted to earth through the resistor, and on its minimum setting you risk burning out the reaction coils. You would need a blocking cap in the lead connecting the anode to the reaction coil. Even so, even with the resistor at maximum some feedback would still be getting to the reaction coils and you might have trouble with maintaining stability. The cap normally controls the amount of reaction down to zero.

I think you might get a better response in the reaction if you connect the end of the reaction coil which normally connects to the variable cap (the other end of the variable cap going to the earth/chassis) directly to earth itself, and connect your variable resistor (about 50k or so) across the reaction coil, while disconnecting the connection which goes to the anode. The wiper of the variable resistor connects to a small cap (500pf) which is connected to the anode of the detector. Thus the resistor controls the amount of feedback going into the coil from zero.

taipai_royal · 12th Nov 2005, 10:04 pm

Thanks Quantum, I will take your advice and try the seconed option.
Regards Graham.

9th Nov 2005, 9:05 am	#1
taipai_royal Pentode Join Date: Apr 2003 Location: Didcot UK Posts: 121	TRF reaction control types. Please can anybody tell me why a variable condenser is employed to control reaction rather than a variable resistance, in these early sets. Last edited by Darren-UK; 8th Nov 2007 at 5:49 pm. Reason: For benefit of search.

9th Nov 2005, 8:37 pm	#2
quantum Retired Dormant Member Join Date: Mar 2003 Location: Birmingham, West Midlands, UK. Posts: 785	Re: TRF reaction control Well, depending on the circuit you may find either method, but remember that in early TRFs the reaction control would get more use than a volume or tone control, and a variable cap would stand up to constant use a lot better than a variable resistor. Many leaky-grid detector circuits using reaction would also find the application of reaction smoother with a variable cap than using a variable resistor to control anode or screen voltage.

9th Nov 2005, 9:57 pm	#3
jim_beacon Retired Dormant Member Join Date: Mar 2003 Location: Bishop's Waltham, Hants, UK. Posts: 939	Re: TRF reaction control The variable resistance control is normally applied to pentodes, where you use the varible resistor to vary the screen potential, and hence the gain. Early sets used triodes, and here it is easier to apply a variable level of feedback, rather than vary the gain of the active device, hence a series tuned circuit is used to apply the feedback. The TRF discussed in the current thread uses variable resistance to vary the anode potential of the triode detector, but I suspect that the control may be a little critical, depending on the coupling of the feedback and tuned circuits, it will also be very dependant on the slope of the valves characteristics. Jim.

11th Nov 2005, 10:50 am	#4
taipai_royal Pentode Join Date: Apr 2003 Location: Didcot UK Posts: 121	Re: TRF reaction control Thanks guys, The circuit I was hoping to modify supplies current from the anode of the triode detector then through the MW LW reaction coils via a variable condensers to earth. If I replaced the (faulty) condenser with a variable resistance pot, would it work? Regards Graham.

11th Nov 2005, 12:34 pm	#5
quantum Retired Dormant Member Join Date: Mar 2003 Location: Birmingham, West Midlands, UK. Posts: 785	Re: TRF reaction control No, not without modification. If you replace the variable cap with a variable resistor then the HT voltage can be shorted to earth through the resistor, and on its minimum setting you risk burning out the reaction coils. You would need a blocking cap in the lead connecting the anode to the reaction coil. Even so, even with the resistor at maximum some feedback would still be getting to the reaction coils and you might have trouble with maintaining stability. The cap normally controls the amount of reaction down to zero. I think you might get a better response in the reaction if you connect the end of the reaction coil which normally connects to the variable cap (the other end of the variable cap going to the earth/chassis) directly to earth itself, and connect your variable resistor (about 50k or so) across the reaction coil, while disconnecting the connection which goes to the anode. The wiper of the variable resistor connects to a small cap (500pf) which is connected to the anode of the detector. Thus the resistor controls the amount of feedback going into the coil from zero.

12th Nov 2005, 10:04 pm	#6
taipai_royal Pentode Join Date: Apr 2003 Location: Didcot UK Posts: 121	Re: TRF reaction control Thanks Quantum, I will take your advice and try the seconed option. Regards Graham.