1962 RMS Double-Talk Tube Intercom

Discussion in 'Tube Audio' started by xXSHADoW93Xx, Jul 14, 2016.

  1. xXSHADoW93Xx

    xXSHADoW93Xx Active Member

    Messages:
    118
    Location:
    Flushing, New York
    Picked this little guy up for a dirt cheap $10

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    Replacing the electrolytic capacitor with spare Panasonic 47uf/200v ones i had left over from a previous project.
    [​IMG]

    it's a 3-tube unit! The 4th glow is the pilot light which lights up after the tubes light up. Cold chassis (thankfully).
    [​IMG]

    After figuring out which were the input terminals and testing for stray voltages, wired in a 3.5mm jack cable, works like a charm as an external speaker.

    [​IMG]
     
    Last edited: Jul 14, 2016
    random_noise and Retrovert like this.
  2. cademan

    cademan Addicted Member

    Messages:
    8,442
    Cold chassis? How is the chassis isolated? Just curious!

    Tell me if I'm close on guessing the tube compliment........35W4, 50C5 and 12AV6?
     
  3. xXSHADoW93Xx

    xXSHADoW93Xx Active Member

    Messages:
    118
    Location:
    Flushing, New York
    Maybe it has something to do with the twin transformers present on the chassis. I'm wondering myself how the chassis isn't even hot, regardless of polarity.

    As for the tubes, you had all but the last one correct:
    [​IMG]
     
    Retrovert likes this.
  4. cademan

    cademan Addicted Member

    Messages:
    8,442
    12AT6 is like a 12AV6 except with different MU. My tube site isn't working at the moment and I can't remember off hand which is which, but one is a medium MU and one is a high MU like the 12AX7 and 12AU7 tubes.

    Series strings are usually hot chassis designs where one side of the mains is usually connected directly to the chassis unless it has some sort of capacitor isolating it from the chassis at the plug. One transformer is the speaker output, but I believe that the other one is an impedance matching transformer for when you switch to talk, it makes the speaker like a microphone, but without a schematic, it could be a type of isolation transformer. Look and see if the mains connects directly to the other transformer.
     
    Last edited: Jul 14, 2016
  5. Retrovert

    Retrovert AK Subscriber Subscriber

    Messages:
    2,041
    Location:
    New York City, NY
    I was writing this when cademan posted.

    He's correct. I can't speak for that unit, but I have a different one. These tube intercoms generally used a hot chassis AND a death cap. Woo-hoo, two chances to win!

    The transformers:
    (1) speaker output transformer.
    (2) uses speaker as a microphone.

    There is no isolation transformer. Would have cost money. Heaters are wired in series, and yes, the heater voltages are probably exceeded.

    Let me find the schematic.
     
  6. xXSHADoW93Xx

    xXSHADoW93Xx Active Member

    Messages:
    118
    Location:
    Flushing, New York
    When i was testing for continuity, i wasn't getting any continuity between the chassis and either blade of the plug. Was expecting it to be a hot chassis though.
     
  7. Retrovert

    Retrovert AK Subscriber Subscriber

    Messages:
    2,041
    Location:
    New York City, NY
    12AV6 µ = 100
    12AT6 µ = 70
     
  8. Retrovert

    Retrovert AK Subscriber Subscriber

    Messages:
    2,041
    Location:
    New York City, NY

    Attached Files:

  9. Retrovert

    Retrovert AK Subscriber Subscriber

    Messages:
    2,041
    Location:
    New York City, NY
    I was going to add a power transformer for safety.

    The cost of a small transformer with a heater supply, rectifier, and a few caps is not expensive compared to a burial.
     
  10. xXSHADoW93Xx

    xXSHADoW93Xx Active Member

    Messages:
    118
    Location:
    Flushing, New York
    Here's some better pics of the guts in this thing:

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
    [​IMG]

    Using my multimeter, getting NO continuity between the chassis and either prong of the plug
    [​IMG]

    [​IMG]
     
  11. Retrovert

    Retrovert AK Subscriber Subscriber

    Messages:
    2,041
    Location:
    New York City, NY
    Probably because the death cap failed as an open. Or it's miswired.

    I can't tell you how many schematics I looked at for tube intercoms, and they all have the death cap for noise reduction.

    Check out the circuit and you'll find it. I suspect it is the giant flat ceramic capacitor on the right. One end appears to run to the chassis in the first photo.

    Again, these intercoms typically (a) do not use a transformer and (b) use string heaters, both of which can be easily and inexpensively remedied for safety and longer tube lifespan. With a deathcap installed every time you touch the transmit knob, the volume control, or anything else on it that is connected to the chassis you are potentially touching AC mains voltage.

    BTW: some metal polish will make that chassis look like new.
     
  12. knockbill

    knockbill Addicted Member

    Messages:
    6,447
    Location:
    SE PA
    Yep, Thats a hot chassis amp,,, teh tubes give it away, along with the Bakelite box to keep hands off... Here's a cheap transformer... Triad N68X , if you want to isolate teh chassis...
     
  13. xXSHADoW93Xx

    xXSHADoW93Xx Active Member

    Messages:
    118
    Location:
    Flushing, New York
    Actually, it runs from one of the tubes to the volume pot. Will be ordering a Triad N68X to isolate the chassis.
     
  14. Omegaman

    Omegaman Ultra Super Member Subscriber

    Messages:
    2,780
    Location:
    Scottsdale, Arizona
    I have this one. A FlexTone
    I always wanted to make a guitar amp out of it

    FlexiphoneA.jpg
     
    Retrovert likes this.
  15. xXSHADoW93Xx

    xXSHADoW93Xx Active Member

    Messages:
    118
    Location:
    Flushing, New York
  16. ktppp

    ktppp New Member

    Messages:
    2
    Hey all -- going to attempt to isolate this unit and install a three-prong power cord. Shadow, could you walk me through the method by which you isolated this box? Did you need to install a full wave rectifier?
     
  17. battradio

    battradio Electron trainer Subscriber

    Messages:
    5,513
    Location:
    Near ST. Louis MO
    That unit most likely has an input and output transfomer , not a power transfomer . don't ground it until you're , sure . you will ne a transfomer to isolate the chassis .

    I reminds me of an early talk a phone intercom
     
  18. Retrovert

    Retrovert AK Subscriber Subscriber

    Messages:
    2,041
    Location:
    New York City, NY
    Truth.

    If you examine the schematic I posted, which is typical, these units use the series-string heater configuration which was standard for radios and televisions. I expect you'll find two tubes with 50 V heaters and two at 12 V.

    B+ uses a halfwave solid-state diode, usually selenium.

    No power transformer is present.

    The two transformers present are:
    (1) an output transformer to drive the speaker.
    (2) an input transformer to use the speaker as a microphone.
     
  19. ktppp

    ktppp New Member

    Messages:
    2
    Thanks all -- by "isolate this unit" I meant install a power transformer. Any information about how anyone's done so would be extremely helpful. Using a Triad N68x.
     
  20. knockbill

    knockbill Addicted Member

    Messages:
    6,447
    Location:
    SE PA
    Connect N68X primary to plug and secondary to line cord of intercom...
     

Share This Page