[Boatanchors] Collins Filter Questions

Bry Carling bcarling at cfl.rr.com
Tue Aug 20 16:52:18 EDT 2013

Thanks to everyone for the replies...

I did find this:

F455 FB 21 1019 6328
PN 526 9448 00
6 terminals, 2 studs, 2.1kHz -3dB Bandwidth

So it is indeed a 455 kHz SSB filter. 

They seem to go for asking prices around $85 to $120 on eBay without the crystals. 

and thanks to Michael Tauson for finding the following from another site - interesting material: 

Your F455FB-21 Mechanical Filter offers the same stability and steep-skirted selectivity 
characteristic of all Collins mechanical filters. It is a precision built component and should be 
treated with care. Do not apply excessive current through the transducer coils or subject the 
filter to excessive shock. Treat it as you would a vacuum tube or any piece of precision 

Each filter has 20 db points measured and marked to insure proper crystal frequencies for 
SSB applications.

Specifications: F455FB-21 Mechanical Filter
Center Frequency................................. 455 kc. nom.
Frequency Response
Bandwidth, 6 db attenuation.................. 2.1 kc nom.
Bandwidth, 60 db attenuation................. 5.3 kc max.
Passband Response Variation...................... 3 db max.
Resonating Capacity.............................. 130 pf +/- 5 pf
DC Voltage: 300 VDC max. potential between terminals and ground.
Source and Load Impedance
Parallel Resonance........................... 50 K ohms min.
Series Resonance............................. 1,500 ohms max.

Recommended Operating Parameters:
Signal Input Voltage: 0 to 2 volts RMS. Direct Current: 2 ma max. Shunt feed is 
recommended to eliminate DC current and potential from transducer coils. Direct current 
should be shunted around the transducer coils through a 10 mh choke or a suitable resistor. 
In no case should more than 2 ma of DC be permitted to flow through the filter coils.

In order to obtain full advantage from the mechanical filter, use a common ground connection 
and effective shielding between the input and output. Keep the leads from the filter terminals 
as short and direct as possible. If these precautions are not observed, the signals normally 
rejected by the filter may bypass the filter through inductive or capacitive coupling.

Vacuum Tube Applications. When the filter is used in vacuum tube circuits, it should be 
tuned to parallel resonance within the range of 125 to 135 pf. This value will include tube, 
stray and wiring capacity. It may consist of a fixed capacitor of 82 to 100 pf, shunted by a 
variable capacitor with a range of 5-48 or any similar arrangement that will permit peaking the 
filter for maximum output at its center frequency. This will compensate for the inherent 
capacity existing in the circuit. Both input and output must be resonated. The F455FB-21 is 
internally terminated; therefore, a high resistance termination (100K ohms or more) is 
preferred. Mechanical filters are normally used interstage; when the filter is used in I.F. 
circuits, it essentially replaces an I.F. transformer.

Transistor Application. For low impedance applications the mechanical filter should be tuned 
to series resonance; for high impedance requirements parallel resonance should be 

On 20 Aug 2013 at 9:30, Bry Carling wrote:

I have a Collins Mechanical Filter  F455-FB-21  (6W2) - it looks new.

Can anyone tell me what radio this was used in? What bandwidth is the filter?
I presume it is a 455 kHz filter.

Does anyone have any idea what this filter is worth? I have two crystals with it so presumably 
it i was used as an SSB generator, or they were used as BFO crystals.  453.58 kc. and 
456.43 kc.

Thanks for any helpful ideas.

Bry AF4K

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