harmony electra transfomer + questions

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mojoworker, Feb 16, 2012.

  1. mojoworker

    mojoworker Member

    Messages:
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    Jan 22, 2012
    i bought an old harmony electra this week. it is the version with tube preamp and power stages. it has 3x12ax7 + 1x7199 (PI), 2x6V6 and a 5Y3 rectifier. despite pretty rough looks and caps that date to 1966 and 1970, it really sounds fantastic. i couldnt find a schem despite a pretty long internet search.

    the original speakers have no continuity, give no sound and have no type or impedance specifications that i can see. since they were wired parallel, i figured it was most likely a set of 2 16 ohm speakers, since the search function here in the forum suggested that 2 x 6V6 are usually matched with an 8 ohm impedance load.

    I have photos uploaded of the layout and OT to help with the OT question, which is a little convoluted:
    http://s1055.photobucket.com/albums/s519/langlieger/

    i have four questions:

    1) the tube layout plan in the cab shows alternate power amp tubes (7159s) and rectifier tube (5u4). (se photos) can these potentially be subbed in without changing the circuit? or is that a different version of the model?

    2) (see photos) the OT is attached to the speakers and to an 'extension speaker' output jack. between the speakers and the OT is a two eyelet strip. if you look at the wiring, there are 3 wires which come from the OT, two of these go to the extension jack and one to one of the eyelets. a fourth goes between the extension speaker jack and one of the eyelets. at first i assumed this was the common ground, but it is connected to the signal position of the jack. can anyone help me decipher the wires leaving the OT? and is there a way to determine the expected impedances (used singly or with an extra speaker) ?

    3) there are 2 power amp wires capped w/ no connection in the chassis - red/black and green/black. can anyone identify these? are they potentially inputs for say 220V?

    4) the tube sockets are so tight that i am afraid to break the bases when i attempt to remove the power tubes and rectifier. is there a safe way to get them out?

    any help or suggestions would be greatly appreciated -

    chris
     
  2. mojoworker

    mojoworker Member

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    Jan 22, 2012
    the fuse is a 250 v fuse? is that ok or a false replacement?
     
  3. guitarcapo

    guitarcapo Senior Member

    Messages:
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    Mar 16, 2010
    You can figure out speaker impedances by getting a cheap $10 meter from Radio Shack and measuring the DC resistance across the speaker. It will usually be a little off from the actual impedance because you are measuring resistance. For example a speaker reading 7.2 ohms is probably an "8 ohm" sppeaker.
     
  4. mojoworker

    mojoworker Member

    Messages:
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    Jan 22, 2012
    that was my first thought. unfortunately no signal goes through the speakers when i tried to measure them. i was hoping for help understanding the OT contacts, because it didn't seem as straightforward as the other ones i have dealt with in the past (see photo link above). i also don't know if there is a way to measure backwards to see what the impedance load should be?
     

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