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Bell Sound 2425 Repair and Restore -- On Hold

Discussion in 'Tube Audio' started by Spenser, Dec 16, 2017.

  1. Spenser

    Spenser AK Subscriber Subscriber

    Messages:
    382
    Location:
    Pittsburgh, PA, USA
    This is the initial post for documentation and seeking assistance repair and restore one Bell Sound 2425 by Matt McConnell aka Spenser.

    Comments and suggestions greatly appreciated.

    Information to begin:

    -This piece came to me missing the four (4) 12AX7 tubes.
    -Tubes all test OK. The four (4) 6V6GT outputs test weak.
    -Photofacts service manual is in my possession.
    -This piece does have the bolt-on, stereo MPX adapter as you can see; tube on it unable to test; did not have proper tube-tester-adapter.
    -Transformers do not smell or look damaged.
    -Bottom not yet removed. Going to do that next and see about replacing capacitors.
    -At the end is another Bell Sound 2425 thread an Audiokarma member kindly provided.

    Questions:

    1) Any recommendations on make and model 12AX7s justified at this stage for when we fire this up?
    2) I might see if I can borrow the 6V6GTs but at the same time any recommendations make and model justified at this stage, too, in case I decide to go ahead and buy a set of these, as well?
    3) Other comments and suggestions?

    Thanks!

    Bell Sound 1 10-21-17.jpg Bell Sound 5 10-21-17.jpg Bell Sound 9 12-16-17.jpg

    http://audiokarma.org/forums/index.php?threads/bell-model-2425-tube-receiver.644299/
     
    Last edited: Dec 17, 2017

     

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  2. Spenser

    Spenser AK Subscriber Subscriber

    Messages:
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    Location:
    Pittsburgh, PA, USA
    Still have have not taken off the bottom. Couple random things.

    First picture. Looks like a resistor poking up through hole in chasis. Peek-a-boo. Random.

    Second picture. Random wire hanging out in space coming from this other-bolt on.

    Bell Sound 10 12-17-17.jpg Bell Sound 11 12-17-17 .jpg
     
  3. gadget73

    gadget73 junk junkie Subscriber

    Messages:
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    Location:
    Southern NJ
    1) I like old production but I'm cheap so it ends up being a lot of whatever I find. Sylvania work nice and don't break the bank but I also have some Sovtek 12ax7lps that I can't say anything bad about. If this uses any tube heaters as the cathode resistor I'd be more inclined to make sure it has old production tubes in those spots at least just in case modern heaters are slightly different vs old ones.

    2) don't own any 6v6 gear

    3) cosmetic design on this is odd. The faceplate looks kind-of Japanese, innards are most definitely not. The add-on MPX thing looks very much like an afterthought with the wiring all over the place for it.

    peeping resistor is probably an alignment test point. Depending where it is exactly that might be the output of the IF strip or output of the detector. The other flying lead could be input to the IF strip.
     
  4. Spenser

    Spenser AK Subscriber Subscriber

    Messages:
    382
    Location:
    Pittsburgh, PA, USA
    This is great, Gadget 73. Many thanks!
     
  5. electronjohn

    electronjohn Plug it in & see!! Subscriber

    Messages:
    19,925
    Location:
    minnesota
    As far as 12AX7s go pretty much any set of four...even used (& known good) should suffice for initial testing. And, even though the 6V6s test weak they should be functional enough for testing.
     
  6. macyjrm

    macyjrm AK Subscriber Subscriber

    Messages:
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    Location:
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    The 1950’s at their very best including the Conelrad marks on the AM dial. Sweet unit!
     

     

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  7. Spenser

    Spenser AK Subscriber Subscriber

    Messages:
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    Location:
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    Thanks, ElectronJohn. I'm going to check around here for a used and known good set, while I shop for new, and for capacitors. I appeciate it very much.

    Thanks, Macyrjrm. Con-el-rad. Holy cow. I did not even know about it until your pointing this out. Thank you so much! Of course you know that the company that made it, Thompson Ramo Wooldridge (TRW), in those days was working on intercontinental ballistic and space travel rocket technology and manufacturing; and I think also was doing some Heathkit manufacturing. TRW was based in Cleveland while this Bell Sound Divison was out of Columbus.
     
  8. century tek

    century tek Super Member

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    The Evergreen State
    :cool:
     
    Last edited: Dec 18, 2017
  9. Spenser

    Spenser AK Subscriber Subscriber

    Messages:
    382
    Location:
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    Yes it seems the program was around for a dozen years, and then near the end, around the time of Bay of Pigs in 1961, a single, full scale test of the intended, information dissemination and radio-guidance-confounding system was performed.
     
  10. macyjrm

    macyjrm AK Subscriber Subscriber

    Messages:
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    Location:
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    I’m quite aware of TRW. I worked side by side with some great TRW folks overseas on a space related program while I was a Capt in the USAF. We were doing some really bleeding edge stuff.
     
  11. havols

    havols Active Member

    Messages:
    223
    Location:
    Manaus, Brazil
    Hi AK members
    I own a 2445 Bell/TRW Receiver with this same front panel model.
    These receiver and amplifier series (2420/2425 and 2440/2445) have been supplied in 2 different versions, and I believe they are all Made In Usa.
    One serie with the dark front panel (I believe this was the debut version) and another with the silver front panel, around 1963/1964.
    These Bell / TRW were somewhat expensive at the time, with similar prices to amp/ receivers by fisher and scott.
    Mine 2445 had power tap repaced with new ones, and rectifier now is with diode.
    Sounds great, and the phono preamp is very good.
    Regards
     

     

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  12. Spenser

    Spenser AK Subscriber Subscriber

    Messages:
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    Location:
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    Hey Havols. Thank you.

    Here is the latest on the one I have here.

    I removed bottom panel. Hmm. Several detached wires to ponder.

    I've marked with blue tape strips where someone has been in here and cut -- or somehow wires were otherwise detached -- at seven locations. I've marked separations with tiny strips of blue tape.

    Six are all originating up through the chassis from the main transformer and going to the rectifier tubes or can caps. The exception is the yellow wire which looks like it was attached to a can cap and is common to the four output tubes.

    Before seeing this evidence, I was simply planning on recapping; finding some 12ax7 tube; and firing her up (variac and DBT). Given what we are seeing now, perhaps it would be justified to test the main transformer, and then if transformer good, also test all resistors.

    What do you think?

    Bell Sound 12 12-22-17.jpg
     
    Last edited: Dec 22, 2017
  13. havols

    havols Active Member

    Messages:
    223
    Location:
    Manaus, Brazil
    Hey Spenser.
    First thing I would like to ask if you have experience working / fixing these old tube amp and receivers? I ask it because they provide 400VDC ... 500VDC ... being a fatal shock risk. Be careful about this, or consider send the receiver to competent service.
    Here is the schematic link of the Bell 2420, that i think is the same as the Bell 2425 amplifier section. You can start with this.
    You can track the cut wires, considering where they come from and where they go (using pin-tube-sockets, capacitors, resistors, etc.)
    Take it easy, and re check.
    Don't plugged it on variac yet. Take care.
    You know the Dim Bulb Test?
    Regards
     
  14. havols

    havols Active Member

    Messages:
    223
    Location:
    Manaus, Brazil
  15. Spenser

    Spenser AK Subscriber Subscriber

    Messages:
    382
    Location:
    Pittsburgh, PA, USA
    Hi Havols,

    Great job and thank you for asking safety questions! I apolgize for being so cavalier in communication of status and plans.

    Yes, this will be my first tube project. Will move methodically; take my time; follow protocols. Only three '70s semiconductor-type amplifiier repair-restores under my belt. I know we are talking serious difference in voltages plus where can lurk.

    Another AK member here in Pittsburgh (responsible for getting me into this sport), has cautioned me, as well, and is standing by to help as needed. I am also a relatively new member of local antique radio club; and a very nice amateur short wave radio club.

    Aside detail: I probaby would not really have fired this baby up without checking the resistors during recapping.

    Yes I do know the dim bulb test. Thank you. What a wonderful tool.

    The schmatic you provided is gorgeous. Thank you so much for sharing. Will compare as proceed. Attached is a picture of the Photofacts schematic I have for the Model 2425. Sorry for poor resolution.

    Something I failed to mention. Whoever was working on this before lost a bottom-panel-screw, and, reversed the two rectifier tubes. Small things I know, but also a clue, on competence and/or care. I am hoping what was going on here was a capacitor or resistor problem the person found or did not find and gave up on.


    Bell Sound 13 12-23-17.jpg

    Bell Sound 14 12-23-17.jpg
     
    Last edited: Dec 23, 2017
  16. gadget73

    gadget73 junk junkie Subscriber

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    can't quite read the later schematic but I do see the two rectifiers. What are they, EZ80 and EZ81? Those sort-of interchange but the EZ81 is rated fora little more current. The heater current is a bigger difference, the 81 wants more.

    honestly both of those seem a little lightweight for a stereo amp. An ez81 is only good for 100ma, I'd expect a set of four 6v6 tubes to idle at more current than that. Datasheet says 70ma per pair doing absolutely nothing at 250v, so 140ma for four. Plus you've got the rest of the tubes to power as well.
     

     

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  17. Spenser

    Spenser AK Subscriber Subscriber

    Messages:
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    Location:
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    Hey Gadget 73! Sorry the resolution poor. The already marginal-to-begin-with file -- I think is losing some integrity on downloading to the site.

    Yes, precisely: they are an EZ80and EZ81. I appreciate the observations on how things fit together and what these guys originally went with, compared to probable, real demands.

    I'm hopeful the transformers are good. I was really looking forward to digging into this thing.
     
  18. Spenser

    Spenser AK Subscriber Subscriber

    Messages:
    382
    Location:
    Pittsburgh, PA, USA
    Gadget 73,

    Could you suggest substition(s), or other possible remediation I might consider for if and when the time comes later?

    Did you see Havols's mentioned mods earlier. Reference below quote.

    Thanks!
     
    Last edited: Dec 23, 2017
  19. gadget73

    gadget73 junk junkie Subscriber

    Messages:
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    Location:
    Southern NJ
    really depends if the transformer windings can support more load. If the transformer can't give more than the tube can pass, changing the tube for a solid state rectifier won't get you anywhere. If the transformer can give more, than an SS diode with a suitable drop resistor to put the voltages back right might be a solution.
     
  20. steveh1155

    steveh1155 Member

    Messages:
    96
    Location:
    Indiana
    I had that same model a few years ago. Those 4 RCA output tubes are probably original as well as the 2 Mullard rectifiers. The 4 12AX7's would have been Telefunken (which explains why they are missing!). My dad bought one new when I was a kid and that's how it came from the factory. That multiplex unit on the back is original too but there would have been a shield on that tube. Nice sounding unit, but really cramped underneath which makes working on it a little tedious.
     
    Last edited: Dec 23, 2017

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