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  • #9077
    Profile photo of Chris Wysong
    Chris Wysong
    CVRS Member

    I’m restoring a Majestic 6M1331. There are spots for 3 6F7M tubes. Since these tubes are about impossible to find, what are my substitution options? I see on the JustRadios website that there are no substitutes. I’ve seen another document that the 6P7G is a substitute but everything has to be rewired. On another board I’ve seen the 6B8G talked about as a substitute with only 2 wires needing a swap.
    Can anyone help me understand what my options are here, what tubes I can use, and what wiring changes are needed?
    Any help is greatly appreciated.
    Thanks
    Chris

    #9078
    Profile photo of Rick D
    Rick D
    CVRS Member

    How many 6F7M tubes do you need I do have one NOS 6F7M tube I also have a 6F7S
    that has a sp adopter and works fine in the 6F7M tube spot ,have used it on my 21-10
    Rogers Majestic.I did some checking on Rogers tubes some years ago ,I will do some
    checking to find the info,but I have a note that tells me you can swap tube base wires
    to use the 6B8 tube,I wish I could find the web page ,it tells you to swap wire on tube base
    #4 and #6,the wire on #4 goes to #6 tube base pin and the wire on #6 goes to #4 tube
    base pin,I did send a copy of the web page to member Dan Walker out west if he reads
    this post and still has the web site address maybe he could post it for us.Some of these
    Rogers tubes are very very hard to find I searched a long time to get my spares,I never
    tried the 6B8 tube switch because I found a NOS 6F7M tube,if you do let us know
    how it works out ,all the very best Rick

    #9079
    Profile photo of Chris Wysong
    Chris Wysong
    CVRS Member

    Rick, thanks for getting back to me. I would like to have 3 6F7M tubes. Feel free to email me at 30sradiocollector at gmail dot com and we can discuss details on the tube!

    I have the link to a 6F7M/6B8 discussion. It’s here.

    http://antiqueradios.com/forums/viewtopic.php?p=1029773

    If I can’t get 6F7M tubes I figured I would buy some octal tube converters and try both the 6B8 and the 6P7G. I don’t want to make the changes to the tube sockets so that when someone else gets the radio they won’t walk into any issues. But I would prefer the 6F7M tubes!

    Thanks for your help on this!
    Chris

    #9114
    Profile photo of Rick D
    Rick D
    CVRS Member

    Hello Chris
    I don,t think you will find 3 of the 6F7M tubes so you might want
    to change the tube base to take the 6B8 tube,I don,t mind selling the
    6F7S tube with the sp. adapter but I don,t think I want to part with both
    tubes,if you do change the base please let me know how you make out,
    one day I may pull the chassis out of my cabinet and change the base on mine
    just to try it,all the very best Rick

    #9162
    Profile photo of Chris Wysong
    Chris Wysong
    CVRS Member

    First update on the 6B8:
    I don’t want to change the socket wiring for the 6B8 on the radio since someone may come along at another time with a 6F7M tube and insert it, not knowing what has been done. I bought an octal socket converter and a 6B8G tube. I did the crossover of pins 4 and 6. All the other pins, including pin 1 are straight through. Looking at the 6R1331/6M1331 schematic, it works perfectly for the 1st IF Amp/Aux AVC Diode tube. It works perfectly for the Suppressor IF Amp tube. Putting it in the 2nd IF Amp/AVC Diode tube the volume starts out fine and then slowly drops to nothing.

    I repeated the tests several times and let the radio play for several minutes on each test. All tests were consistent with what I described above.
    One detail change I will be making is to increase the grid cap wire length about an inch to properly fit the new height with the tube converter in place.

    I just ordered a 6P7G tube and I’ll test that next. I’ll let you know what I find.

    Hope this helps. YMMV.
    Chris

    #9183
    Profile photo of Chris Wysong
    Chris Wysong
    CVRS Member

    I just received some 6P7G tubes today, and using the octal socket converter I wired it up. It works perfectly in all 3 positions, so I feel I have a good solid replacement for the 6F7M tube. I had the radio playing for several minutes on each test.

    Chris

    #9186
    Profile photo of Dan Walker
    Dan Walker
    CVRS Member

    Hi Chris and Rick.
    Dan Walker in Calgary here.. It is good to find out that there is a tube that can be substituted for the 6F7M tube.
    Chris where did you get the octal socket converter from.?. I should order a few for some of my Rogers radios.
    Dan in Calgary

    #9187
    Profile photo of Chris Wysong
    Chris Wysong
    CVRS Member

    Hi Dan,
    I purchased the converters from a company in Maryland. Here is the address for the converter.

    http://www.angela.com/octal8pinbottomtooctal8pintoptubeadapterkit.aspx

    I needed 3 for the 6M1331 and those are done. One of the 6F7M tubes in the 6R1331 is going out now, so I’ve ordered 3 more and am also ordering a set of tubes. I’ll use these for both radios and put the 6F7M tubes on display. I’ll be extending the grid cap leads to accomodate the new height.

    When you get your converter and tube would you test it out as much as you can and let us all know what you find? I’d like to see more testing done, just to make sure.

    Hope this helps.
    Chris

    #9189

    you can purchase a 6F7 for $5.00 USD at this web page : http://www.vacuumtubes.net/Prices%206C%20to%206LY8.htm

    #9190
    Profile photo of Chris Wysong
    Chris Wysong
    CVRS Member

    Markku, thanks for chiming in on this. The 6F7M is an 8 pin base and the 6F7 is a 7 pin base. That’s why we’ve been looking at the 6P7G. Someone could simply rewire under the chassis and nobody would ever know. I’m using the converter so that if someone puts in a 6F7M without looking at what’s there they wouldn’t be badly surprised.

    Chris

    #9191
    Profile photo of Dan Walker
    Dan Walker
    CVRS Member

    Markku:
    That does sound very good but did you notice that the tube they listed is a 6FM7 ”not ”
    a 6F7M.
    If you google the 6FM7 you will see that it is not even close to the 6F7M.
    We all make mistakes, but it was a good try,
    Dan in Calgary

    • This reply was modified 3 months, 3 weeks ago by Profile photo of Dan Walker Dan Walker.
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