[AMRadio] Incorporating AM Ceramic Filter In Tube Radio


Byron Tatum bjtatum1 at att.net
Sat Jan 3 13:33:25 EST 2015


Hello-
  Can anyone suggest the best way to incorporate a 455 KHz ceramic filter (impedance is 1500 ohms) into the IF amplifier chain of a tube receiver?
  I wish to add the AM bandwidth filter (NTK LF-D6) at the start of my 2 stage 6BA6 IF amplifier chain. There is a pair of conventional type IF transformers (each transformer has a pair of tuned circuits), the transformer pair lightly coupled with a 2.2 pF cap. 
  The first 455 KHz IF transformer of this pair has its primary feeding B+ to the second mixer plate circuit (6.0 Mhz to 455 KHz.) The secondary of this first 455 KHz IF has one end grounded and the other end connected to the 2.2 pF coupling cap. The other end of the coupling cap is attached to the primary of the second 455 Khz IF transformer; the other end of its primary winding being grounded. The secondary of this second 455 KHz IF transformer is directly connected to the grid of the 6BA6 first IF amplifier, with AVC feed (bypassed by capacitor) connected to other end of secondary.
  Can someone suggest the best manner in order to properly match the low impedance input/output of ceramic filter with the minimum of loss?
     Thanks, Byron WA5THJ


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