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I constructed a wide band Hi Fi TRF tuner for MW AM that sounds great. It used an old 365 pF air spaced 3 gang variable capacitor that had equal sections. Selectivity was marginal at the top of the band despite using cascode JFET amplifier circuits that preserve existing tuned circuit Q.

Almost all vintage MW AM car radios use pearmability tuning for reasons of size and resistance to vibration.

  • How would a TRF radio go with slug tuning?
  • Would you need more slugs to get the same performance?
  • How would tracking go if one attempted to use the existing osc coil in a common 3 slug setup?
  • Would the gain be higher at the bottom end of the band?
  • How do the tolerances of inductive slugs compare to 3 gang variable caps?
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  • \$\begingroup\$ There are no general guidelines, especially given that this is long-abandoned technology. You're going to have to make your own measurements. Do you have a good LCR meter or network analyzer? \$\endgroup\$
    – Dave Tweed
    May 11, 2018 at 13:20
  • \$\begingroup\$ @ Dave Tweed .I may have to measure coil Q at several spots across the MW band .If I can get a Q of say 100 at say 1.5 MHz then things will be good .The AM sound quality is much better than a normal car radio so I think that it is worth trying .I am hoping that somebody has tried something like this before . \$\endgroup\$
    – Autistic
    May 11, 2018 at 13:28

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Selectivity was marginal at the top of the band despite using cascode JFET amplifier circuits that preserve existing tuned circuit Q.

That is untrue - if you only alter C (or L for that matter) the Q of the circuit will vary with the square root of the ratio of L to C (or C to L depending on what type of tuned circuit you used). Q will definitely not remain constant across the band of frequencies.

How would a TRF radio go with slug tuning?

With difficulty I would estimate - keeping them all ganged to produce the same filter frequency would be a problem.

How would tracking go if one attempted to use the existing osc coil in a common 3 slug setup?

TRF receivers DO NOT use an oscillator coil (or an oscillator): -

enter image description here

They just use banks of tuned filters (operated in parallel) as per the above diagram.

Would the gain be higher at the bottom end of the band?

The selectivity will be greater at higher frequencies and less at lower frequencies.

How do the tolerances of inductive slugs compare to 3 gang variable caps?

Well, for most "vintage MW AM car radios" the slug was mainly used to retune an oscillator because the radio was invariably a superheterodyne type with a fixed intermediate frequency of 455 kHz. This means the tolerance is immaterial but the mechanical movement that pushed and locked the slug was the weakest link (usually).

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  • \$\begingroup\$ @ Andy aka +1 good to see an answer .Where I live in marshlands CHCH NZ there are listenable music AM stations that do give better coverage than FM .When I drive within about 2 Km of the 1503 KHz fox sport mast I can hear interference when I am trying to listen to 1593 KHz radio coast . These are both local stations .This means that I do not want to be any worse off when it comes to selectivity .Most old car radios only have 3 slugs one of which is the Osc which does have much less inductance .2 slugs would not be enough but coils of equal inductance should be easy to track . \$\endgroup\$
    – Autistic
    May 12, 2018 at 7:17

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