Calculating IFT Padder Capacitor Values

[Old Sparky]

When replacing a failed IFT, in this case in a Romac valved radio, can any similar (i.e. 450-470Khz) IF T which will physically fit be used without modification, or would different padder caps. be needed, and, if so, how would the values needed be calculated, given that the inductance of the coils in the 'new' IFT isn't known, and an inductance bridge isn't available with which to measure this.  Obviously an IFT suitable for valve radios would be used, but do ther factors, such as the valve(s) used, affect this.  Personally, I'd have thought that, as long as the cores in the Transformer could be adjusted to bring it to resonance, the replacement IFT could be used unmodified, but I stand to be corrected.  I should add that I haven't seen the radio in question, but have been asked about this by it's owner, who is currently abroad, so I don't even know why the IFTs need replacing.

[Murphyv310]

Hi.
You would be best using an IF Transformer from a radio with the same valve line up TBH. Using a TX from a set with different valves will no doubt give a different resonant frequency due to differing inter-electrode capacitances. Layout too can have an effect on how the transformer will behave.
Without suitable test equipment I'd be inclined to suck it and see, try peaking up the IF and if acceptable leave it. You might finding having a "poke" with a low value cap will guide you if you need more or less capacitance.

[Terry]

A pound to a penny that any IFT you find will have the tuning capacitors inside the can!

So, provided the nominal IF matches the set you are putting it in to, there shouldn't be any problems.

True: layout and other considerations can lead to de-tuning, as Trevor says, but that's what the adjustable cores - or trimmers in very old IFTs - are for!

[Kalee20]

Ideally a transformer from the same type of set, for reasons already given.

Sometimes the IFT which drives the detector has a different turns ratio than the one between frequency changer and IF amplifier.

It might be important too, that not only can the cores tune the thing to the right IF, but also that they are in approximately the right position. Because not only does moving a core affect the inductance of the coil that it's in, it affects the coupling to the other coil, and that determines IF bandwidth and gain. IFT's with trimmer capacitors rather than core slugs obviously are immune to this effect!

You also need to be sure that you have enough inductance, with your parallel capacitance, to tune to the IF. Otherwise, you might find that you get a peak in response when the core is right in the middle of a coil, further screwing reducing the signal - but all that's happened is you got nearly on tune and then started to go off tune in the same direction, rather than passing right through resonance.

These are most of the pitfalls. But in practice, have a play - a bit like usual big a mains transformer as a single-end output transformer, you may not be optimum, but you'll still find yourself with something useable!

[Mike Watterson]

The electrode capacitances not a huge issue for AM IFTs. Most important is similar output impedance of valves, so HT Anode and G2 volts and battery or mains more important than EF41 or EF89.

Thusly Transistor / Mains Valve / Battery Valve (esp 45V HT) are not interchangeable. Transistor IFTs are a very different anyway.

It's very rare for the capacitors not to be in the can.

[Old Sparky]

Thanks for all the help and advice, foks. As I said, I haven't even seen the radio in question, and may not do for some time, but, unless I'm mistaken, the consensus of opinion is that, as long as IFT(s) designed for use with valved radios are used, peaking the cores at the correct frequency (bearing in mind the caveats mentioned by Trevor and Kalee 20 ) should be OK. Romac radios, IIRC wre made in the late 1940s/early 1950s, or at least the only one I've ever come across was. This used miniature all-glass valves. To the best of my knowledge IFTs made in those years had built-in padder caps., and adjustment was via dust cores (or ferrite cores),. I've only seen the type which is tuned by trimmer caps. in pre-war radios. However, I'm prepared to be wrong on several counts!

[Old Sparky]

Following an email from the radio's owner, I now know that it's a Romac 126, and that the IFTs are 20 x 20 x 30mm, with the 1st. one being single tuned, with a coil having DC resistance 9 ohms, whilst the 2nd. IFT is double tuned with 10 & 7 ohm coils respectively. This info. is from the service sheet for the Romac.

[pwdrive]

Not the usual type of arrangement, the mixer is capacitor coupled to the IF amp. the IF amp's transformer appears to be single tuned.

Lawrence.

[Old Sparky]

Would it be possible to use a conventional double-tuned IFT in the 1st. IF position , by only connecting one coil? I realise that the unused coil may (will?) have an effect on the capacitance/inductance of the transformer, and I don't know whether one or both the IFTs need replacing, but a single coil IFT may be difficult to source, unless one can be salvaged from a scrap set. When Denco IF Transformers were available, it wouldn't have been a problem, but it's a different story now!

[pwdrive]

Would it be possible to use a conventional double-tuned IFT in the 1st. IF position , by only connecting one coil?  I realise that the unused coil may (will?) have an effect on the capacitance/inductance of the transformer, and I don't know whether one or both the IFTs need replacing, but a single coil IFT may be difficult to source, unless one can be salvaged from a scrap set.  When Denco IF Transformers were available, it wouldn't have been a problem, but it's a different story now!

Don't see why not, so far as I can make out the target inductance needs to be approx. 930uH, You could try removing the unused winding on a double tuned replacement to avoid any possible stray resonance effects.

Lawrence.