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HOHNER Orgaphon 45MH (1965) guitar tube amp

Discussion in 'Musical Instruments' started by Winters, Nov 20, 2018.

  1. Winters

    Winters Active Member

    Messages:
    236
    Location:
    Poland
    Hello dear AKers,

    Several months ago I became the proud owner of a 1965 Hohner Orgaphon 45MH guitar amplifier. Well, not exactly a guitar amplifier as this is a universal amp than can be used with pretty much every electric instrument. This things weighs a ton and screams crazily sophisticated German (over)engineering. It’s a 2x10” combo with floating (!) baffle and the circuitry divided into two separate chassis – power amp/power supply located in the bottom of the cabinet and the preamp section in the upper section. Oh, and it has four channels and ten inputs.

    The cabinet is made of a wooden frame with fiberboard walls.

    The tubes are: 3x ECC808 (which is a newer and improved version of the common ECC83), 2x ECC83, ECL86 (which is driving the reverb tank), EF86 and 2x PL84 in the power amp section. Yes, PL84s and not EL84s. This was a very common tube in European tube TV sets from the 1960s/1970s. The plate voltage is 420V, the grid voltage is half that (about 200V). Produces at least 25W of power. Funny thing is that Hohner never mentioned the actual power output of these amps.

    The poor thing did not have an easy life but that’s what I like about it. It kind of worked when I first got it. I replaced all of the electrolytic capacitors, put new tubes in it, cleaned everything and it worked beautifully for the first several months. Sounds very warm, has a great lush reverb and a nice optical tremolo/vibrato. And it has a separate vibrato for the reverb signal which creates a cool stuttering/delay effect.

    But now it has developed a problem which is hard for me to repair. All is great until I turn the treble pot on the 3rd channel. Turning this particular pot even a small bit introduces a rather peculiar noise. It is not your typical 50/60Hz hum but a strange buzz/whirring similar to a rough pronunciation of the letter “r”. The pot was cleaned, tested with an ohmmeter and is good. I tried poking at several connections in the area with a wooden stick to look for bad solder joints but it’s not that. I have resoldered all of the connections of that potentiometer. Still the same. I have checked the 2nF and 150pF capacitors that are connected to this pot and both are good.

    One thing I noticed is that when I desoldered the side of said potentiometer that goes to the ground through a 2nF capacitor, the buzz was there at all times, no matter whether I turned the knob or not.

    Best regards,
    Pete

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  2. Binkman

    Binkman AK Subscriber Subscriber

    Messages:
    6,756
    Location:
    Chapel Hill, NC
    Not a tube expert? but often replacing tubes seems normal. You could try putting in one of the old tubes. Might have some life in it (them) for a test. seems like you restored most of it.
     
  3. Winters

    Winters Active Member

    Messages:
    236
    Location:
    Poland
    Thanks for your reply. I tried replacing the new preamp tubes with the old ones. There is no difference :(

    I noticed that when I touch one of the soldering lugs of the "noisy" potentiometer while it is turned all the way down with a wooden stick it gives a classic 50/60Hz hum. Other potentiometers in the amp don't behave this way, they are totally silent. So it seems to me that this pot is somehow ungrounded. On the other hand, it is connected to ground through 2nF capacitor which tests good. All the connections between this pot and ground are good.

    I'm including a photo of the area. The "noisy" pot is marked.
    I'm also including a schematic of this amp.

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