Cosmic Blue Hot Rod

Discussion in '"Boat Anchor" & Classic Equipment' started by WB5WSV, May 16, 2017.

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  1. WB5WSV

    WB5WSV Ham Member QRZ Page

    My National NC-155 was my first piece of real amateur gear. I have had a ball with that set since 1975, and later bought another rather abused 155 and ended up converting it to a crystal controlled 2nd LO in order to get it working (post on this is on QRZ).

    I bought the "Cadillac" version of the 155, the NC-270, a while back. It was badly corroded, among other things. As I worked on it and looked for info, I read some old reviews that said that the 6M band on the 270 and 155 was pretty deaf. I wondered if there was a way to improve the performance of the receiver and studied the ARRL handbook for suitable RF amp tubes that might do a better job than the standard 6BZ6 used in both the 155 and 270.

    One tube that has the same pin out as the 6BZ6 but has higher transconductance is the 6GM6. Older and wiser heads advised me that was the tube to use rather than the alternatives, and also suggested that replacing the 6BE6 1st and 2nd converter tubes with the 6BY6 could both improve performance and lower noise.

    I tried the 6GM6 and two 6BY6's in the NC-270 and that really woke up the receiver. Of course, finding that someone had disconnected some resistors between the audio amp and the IF led to an even bigger improvement!

    This morning I tried a 6GM6 and one 6BY6 in my 2nd NC-155 and the improvement was quite obvious. Next I'll try another 6BY6 in the 2nd converter and see how that works.

    So, if you want to hot rod a Cosmic Blue receiver, I recommend trying the 6GM6 and 6BY6.


    KA0HCP likes this.
  2. KM1H

    KM1H Ham Member QRZ Page

    Ive used the 6GM6 since my 1965 75A4 purchase and then seriously modded and still in use. I use that and the 6BY6 in many of my and customer radios. For octal tubes the 6SG7 and 6SB7Y are my favorites.

    National Radio 1963-69
    WB5WSV likes this.
  3. WB1E

    WB1E Ham Member QRZ Page

    I have a NC-190 that I've had forever, and has spent some minor time on the bench. It uses the 6BZ6 for V1 & the 6BE6 for V2&3. Without referring to my RCA manual, is the 6GM6 an alternative at all?

    Carl, a National Radio Vet? Very cool!

  4. WB5WSV

    WB5WSV Ham Member QRZ Page


    Yes, I have an NC-190 also. I have not yet tried a 6GM6 instead of the 6BZ6 RF tube or 6BY6's in place of the 6BE6's but it should work just as it did in the NC-270 and in one of my NC-155's. By the way, I did not seem to need to realign the receiver after putting in the new tubes; there were little or no changes required.

    Also, I have not looked at the NC-190 relative to another aspect. The NC-270 has some resistors that running at or beyond their rated wattage. I found no such problems in the NC-155, but I don't know about the NC-190.

    By the way, it was Carl that confirmed to me that the 6GM6 was a good choice to replace the 6BZ6 and that suggested the 6BY6 as a replacement for the 6BE6.

    Good luck!

    WB1E likes this.
  5. KM1H

    KM1H Ham Member QRZ Page

    The 6GM6 was a later tube for TV IF amps in the 40 mHz amp. The 6BZ6 was an earlier version for the same purpose.

    Ive found over the past 50 years that they are a direct swap in most radios that use a 6BA6 or later comparable remote or semi remote cutoff pentode as long as the suppressor grid terminal is connected to the cathode. Some tubes swap pins 2 and 7 but if they both go to the same place it is OK.

    In some cases the RF gain control needs to be backed off if the radio is overloaded with strong signals which was pretty common anyway in that era.

    In very few cases the 6GM6 will oscillate on the highest bands but that is an easy cure.

    Connect a 10 Ohm 1/2 W resistor with very short leads at the grid pin. Connect a 100 Ohm 1/2W resistor right at the screen pin, Add a .005 to .01 uF disc cap to the far side of the resistor to ground. Leave the original screen bypass alone unless it is a paper cap such as in early runs of the NC-183D, HRO-60, various Collins, etc.

    The 6BY6 was developed as a FM radio mixer/converter and will replace any 6BE6 that does not have AGC applied which is seldom seen.

    The pair will really wake up a dead radio such as the SP-600, various in the HQ series and many other brands and models.
    It also wakes up a R-390A on the higher bands.

    Yep I started there in 63 after leaving USN active duty where I was an ET. Worked as Service Dept Tech and then Service Manager, and then in R&D. Im 76 and there arent many of us National vets left.

    Been a ham since 55.

    WB1E likes this.

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