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vintage Orange OR80 bias pot?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by tejastubes, Dec 22, 2009.

  1. tejastubes

    tejastubes Member

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    I just a got a vintage OR80. Where is the bias pot,,,there are two internal trim pots on the top of the board,,,does anybody know? The tubes are redplating slightly and I want to check the bias and how much current they are drawing.

    Also, if there are any other vintage OR80 owners, where do you typically bias your EL34's in at?

    Joshua
     
  2. Roadeye

    Roadeye Member

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  3. tejastubes

    tejastubes Member

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  4. Prairie Dawg

    Prairie Dawg Member

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    I don't know whether this will help but here's an orange schematic that shows two pots, one of which is a bias control and the other is a balance control for the filament wiring. It may be of use to you.

    http://users.telenet.be/orangefg/OFG_SCHEM/OR120schem_post74.jpg
     
  5. tejastubes

    tejastubes Member

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    Yes its helpful, but its a schem, not a layout, so I cant tell by looking at the circuit board which is the bias and which is the other. There is one in the center of the board, and one near the end.
     
  6. schmidlin

    schmidlin Member

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    One has 6.3 volts AC on it and the other has ~-40 volts DC. Frankly, you have me worried for your safety, perhaps you should find a tech. Not trying to be a jerk, but...I would also suspect the PI blocking caps.

    Also, those run very high voltages/current: you need QUALITY tubes.
     
  7. tejastubes

    tejastubes Member

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    I was actually thinking the same thing,,,I will probably send it to Jeff Bakos in Atlanta who works on all my vintage gear. I will use my amp bias tool to see where they are actually drawing, but from what I am reading online it may need a good thorough servicing to make sure everything is is kosher.

    Yeah the tubes the guy sent with it were ****,,,hammered below the 50% min acceptable gm, leakage also,,,I put a new pair of Siemens RFT el34 which actually drew less current on the maximatcher then the trash winged C's, but they were redplating slightly so I assume there is something with the components.
     
  8. Prairie Dawg

    Prairie Dawg Member

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    You could simply clip your multimeter on a filament lead and then take the other probe and poke around the two pots and see if you make a circuit-that way, you'll know which one it isn't.

    That seems simple enough, no?
     
  9. gldtp99

    gldtp99 Member

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    The trim pot near the center of the board is the bias adjustment pot------ the trim pot near the end of the board (by the diodes) is the heater balance (or "hum") pot.
    I wouldn't use RFT EL34's in this amp---at least until i was sure that the bias circuit was ok and the amp was stable, as a whole. I'm not sure the RFT's are robust enought for this amp.
    I had to go through a process of increasing screen grid circuit resistance in mine before the output section was stable. But mine came to me pretty butchered with a Fender Super Reverb OT and other suprises---now it sounds good and operates properly when cranked. I bought a transformer set from Matamp USA-- now Electric Amps, i believe--- so my OR80 has a factory replacement OT and choke.
    After getting this OR80 back in playing condition i built a copy (circuit-wise, not a cosmetic clone) and played around with mods to the tone stack......................gldtp99
     
  10. tejastubes

    tejastubes Member

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    Thanks,,,I decided to send the amp to my tech to have him go through it as there seems to be alot of discussion about the bias circuit etc.

    BTW, what kind of EL34 do you run in your Orange OR80?
     
  11. gldtp99

    gldtp99 Member

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    When i did the work on mine, i ended up with Shuguang EL34B's------ the ones i was getting then have held up well to the pounding that this OR80, the copy i built, and other builds give them----- the last batch i received has had a much higher failure rate.
    I have too many amps to run quality NOS EL34's in all of them but my OR80 does run an RCA 12ax7a in V1 and a Telefunken ECC83 in V2.
    If i had reasonably priced access to more NOS Mullard xf..'s then i would use them in an OR80-----i'd think that these would take the high plate, screen voltage and high current demands as well as any EL34 ever made---- probably better than most, besides giving superior tone..................gldtp99
     
  12. Prairie Dawg

    Prairie Dawg Member

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    You did right. The hardest thing to do is to know when to down tools and walk away.

    The path to true knowledge is that sometimes things can get worse before they get better, grasshopper.
     

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