The Drake TR-6 and TR4 Series AM Filter Up Date 

By: Jeff Covelli WA8SAJ  /


Filter in Review

    I wrote an article on the TR-4 series A.M. filter in August of 2001 ER #147 using the 9MHz filter #2311 from INRAD, International Radio, PO Box 2110, Aptos CA 95001 (831) 462-5511. The  #2311 is an experimental 6 kHz wide 9 MHz filter rated at 500 ohms impedance and I thought at the time this was close to the correct impedance and it is not. This might explain the fact there is plenty of ‘ripple’ across the pass-band when using a sweep generator or tuning across a carrier with the A.M. filter switched in line. The folks at INRAD at the time did not have a ‘stock’ filter for the TR-4 series, so I just used what they had and it worked fine. As you will read on, the new TR-6 / TR-4 A.M. filter is much better, as seen in the picture of both A.M. filters (Figure 1)


Figure 1 showing both old #2311 and new #1906 A.M. filter.


New Filter

    Well, since I wrote the article, there have been many folks wanting a 6 kHz wide A.M. filter for the Drake TR-6. Now your going to ask, what has this got to do with a TR-4? The TR-6 is really a TR-4 with a 6-meter converter built in and having the same I.F. frequency of 9 MHz. The only difference is the TR-6 uses a 50 ohm impedance for the filters and the TR-4 uses a 120 ohm impedance. The new INRAD TR-6 / TR-4 A.M. filter is priced at $115.

I have to thank Gary (W8PU) for sending in his TR-6 A.M. filter to INRAD about a year ago to see if they could manufacture one for the TR-6 and they did. This filter is a ‘plug-in’ type and works very well in the TR-6. Bill Frost (WD8DFP) who is service manager at Drake helped with the technical info for the new filter. By the way, Bill Frost just retired from the R.L. Drake Company after 43 years of great service to the ham-radio community!

 I have installed the new A.M. filter in a few TR-6’s and they work great. 6-meter A.M. is alive and well around the country on 50.4 MHZ most evenings and the TR-6 certainly works well at about 100 watts peak output and now the new A.M. filter makes it much easier to copy on receive. The new A.M. filter #1906 is rated at 6 kHz wide, 6-pole with a 50 ohm impedance and is a plug in type using four pins, two for grounding and two for the in/out leads and has a low profile design to fit in the tight space for the TR-6.


Adapting For The TR-4 Series

    I wanted to take my Drake TR-4CW and pull out the old filter I used back in August 2001 and try to see if I could use this new TR-6 filter and make it work. I used the sweep generator to see how much different the new filter #1906 (Figure 3) is compared to the old #2311(Figure 2). Certainly there is less ‘ripple’ across the pass-band for the new 50-ohm filter, since it is much closer to the 120 ohms needed for the TR-4CW. Remember the TR-4CW has mounting for the extra filter and I am only covering this model for that reason. The TR-4CW has a 500 Hz CW filter and I used the CW position to mount the A.M. filter. The plain TR-4 can be done using your own mounting and switching in and out.


Figure 2 shows the old #2311 sweep at 9 MHz.       Figure 3 shows the new #1906 seep at 9 MHz.


    Mounting for the TR-4CW needs to have a screw stud that is normally mounted on the filter, but using the TR-6 A.M.  filter there is no screw stud. All filters need a good ground connection for the filter to work properly, so I put a screw in place and mounted two ground lugs for the grounding (Figure 4). To mount the filter in place I used double sticky tape to mount it and afterwards the ground connections using #20 buss wire will hold it even more. The new filter is slightly longer in length and can fit in the space with no problem (Figure 5).


Figure 4 shows mounting of new #1906 A.M. filter.     Figure 5 shows installed new #1906 A.M. filter.


            The new TR-6 AM filter is much better when tuning across a signal and the same bandwidth is noted, just no more ‘ripple’ as with the old A.M. filter. Wow, now A.M. is much better sounding when using the new filter. I did not change anything else from the previous article I wrote in August of 2001, so the TR-4CW makes a great rig for operating A.M. in a compact package, just keep lots of air on the finals.

Figure 6 shows the front panel with the CW filter covered and A.M. label installed for the TR-4CW.


Here is a diagram of the TR-4CW wiring with the AM filter replacing the CW filter.