IF transformer compatibility - UK Vintage Radio Repair and Restoration Discussion Forum

Biggles · 25th Aug 2018, 1:02 pm

I have a box of IFT's removed from various scrap sets over the years, and was just wondering about compatibility issues if re-using them for home brew projects etc. Just as an audio output transformer is matched to a particular valve impedance, I suspect that there may be compatibility issues with IFT's. I would imagine this would be the case when comparing a battery valve portable with a mains set. Surely the battery set's IFT would be designed to work at much smaller currents than it's mains operated brother, which would lend itself to designing a transformer with different characteristics. This has all come about because I am having difficulty achieving the correct IF response with a home built mains superhet. Basically the IF has two peaks, and is behaving as if it is overcoupled. I am adjusting for the outer peak response as usual. The overall effect is radio stations appearing twice on the dial. I wonder if I have chosen IFT's from a battery set in the spares box, and this is the problem. Ideas anyone?
Alan.

Radio Wrangler · 25th Aug 2018, 1:17 pm

An IFT operates as a 2-pole coupled resonator filter.

The impedance of the circuitry before or after it acting with the reactance of the winding (or of the resonator C - both reactances are equal at resonance) sets the Q of that resonator.

A twin-peaks response indicates that EITHER the coupling is too strong between stages, OR the Q of each stage is too high. or a bit of both, of course. You need to look at the bandwidth th know how to proceed. Reducing coupling narrows the filter, reducing Q broadens it.
Reducing coupling increases insertion loss, reducing Q reduces insertion loss.

David

turretslug · 25th Aug 2018, 2:15 pm

Some IFTs have anode and grid connections tapped down the winding to reduce the effect of changes of effective in/out capacitance under AGC action (the CR100 bristles with them!). Also, some IFTs were deliberately designed to have particularly high impedance at resonance to make best use of valves such as the 6K8 whose anode impedance was designed to be high so that even simple sets would have good selectivity. In other words, most RF-type valves can probably be made to work with most IFTs, but the outcome may not always be optimal.

Biggles · 25th Aug 2018, 8:24 pm

Thanks for the info. A bit of investigation is called for I think.
Alan.

25th Aug 2018, 1:02 pm	#1
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	IF transformer compatibility I have a box of IFT's removed from various scrap sets over the years, and was just wondering about compatibility issues if re-using them for home brew projects etc. Just as an audio output transformer is matched to a particular valve impedance, I suspect that there may be compatibility issues with IFT's. I would imagine this would be the case when comparing a battery valve portable with a mains set. Surely the battery set's IFT would be designed to work at much smaller currents than it's mains operated brother, which would lend itself to designing a transformer with different characteristics. This has all come about because I am having difficulty achieving the correct IF response with a home built mains superhet. Basically the IF has two peaks, and is behaving as if it is overcoupled. I am adjusting for the outer peak response as usual. The overall effect is radio stations appearing twice on the dial. I wonder if I have chosen IFT's from a battery set in the spares box, and this is the problem. Ideas anyone? Alan.

25th Aug 2018, 1:17 pm	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,902	Re: IF transformer compatibility An IFT operates as a 2-pole coupled resonator filter. The impedance of the circuitry before or after it acting with the reactance of the winding (or of the resonator C - both reactances are equal at resonance) sets the Q of that resonator. A twin-peaks response indicates that EITHER the coupling is too strong between stages, OR the Q of each stage is too high. or a bit of both, of course. You need to look at the bandwidth th know how to proceed. Reducing coupling narrows the filter, reducing Q broadens it. Reducing coupling increases insertion loss, reducing Q reduces insertion loss. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

25th Aug 2018, 2:15 pm	#3
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,400	Re: IF transformer compatibility Some IFTs have anode and grid connections tapped down the winding to reduce the effect of changes of effective in/out capacitance under AGC action (the CR100 bristles with them!). Also, some IFTs were deliberately designed to have particularly high impedance at resonance to make best use of valves such as the 6K8 whose anode impedance was designed to be high so that even simple sets would have good selectivity. In other words, most RF-type valves can probably be made to work with most IFTs, but the outcome may not always be optimal.

25th Aug 2018, 8:24 pm	#4
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: IF transformer compatibility Thanks for the info. A bit of investigation is called for I think. Alan.