Delta Blues bias voltage problem

Discussion in 'Amp Tech Center' started by firemedic, Mar 5, 2013.

  1. firemedic

    firemedic Tele-Meister

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    I have a Peavey Delta Blues on the bench that redplates V6 & V7 EL84s. I have checked the 4 gridstoppers & 2 gridleaks that go to the power tubes, they all check out OK. I also replaced the PI coupling caps.

    After all that, my nominal pin voltages w/ no tubes are all good except the V6 & V7 grids still have -9.8v, where they should match V4/V5 at -16v.

    I don't see where that bias voltage is going, maybe someone else has an idea....
     
  2. muchxs

    muchxs Doctor of Teleocity

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    Are you sure your tubes are o.k.?
     
  3. firemedic

    firemedic Tele-Meister

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    This voltage is found with no tubes in.
     
  4. firemedic

    firemedic Tele-Meister

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    Bump.

    Here's one for ya: The bias voltage on all 4 output sockets is good at -16v, until I connect the OT secondary. Everything else is also connected. Again, no tubes. Calling all amp techs, this is kind of a doozy.
     
  5. muchxs

    muchxs Doctor of Teleocity

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    Leaky coupling cap between the PI plates and the EL84 grids.
     
  6. firemedic

    firemedic Tele-Meister

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    Nope. replaced those already. The voltage in question is nominal -16v on the grids x4, as I connect the various jumpers to the PCB board. HT, reverb, OT primary: still good. Plug in the OT secondary: oops. hey there, -9.8v on V6, V7.

    A shiny new nickel to the clever tech who figures this one out.

    It is always -9.81v, no more no less.
     
  7. muchxs

    muchxs Doctor of Teleocity

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    Ok.. What is the exact condition when the bias fault occurs?

    You're describing it as no tubes, connect the OT secondary. That means connect a speaker.

    The primary connects the plates to the power supply via the center tap.


    Your obvious suspects are R49 and C35. If R49 is actually 220k make sure R45 is, too.
     
  8. BiggerJohn

    BiggerJohn Friend of Leo's

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    Almost sounding like a problem with the sockets or the wiring.
     
  9. gtr_mann

    gtr_mann TDPRI Member

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    I looked at the schematic briefly , I would look at R66 in the low voltage supply area. It is hard to read the schematic I downloaded off the Internet, but it looks to me like it says 15 ohm 5 watt... This is a protection part. I would suspect it is way off value, probably was damaged when a output tube failed...
     
  10. firemedic

    firemedic Tele-Meister

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    R49, C35, R45, C32 are good. In fact I replaced both PI coupling caps as a matter of course.

    The speaker is not connected, no tubes are in so it's unnecessary. It doesn't matter anyway for the purpose at hand.

    R66 is good @15ohms. If it was messing up the bias voltage, it would do it on all sockets, regardless of whether the OT secondary was connected.

    I haven't had a chance to cut & jumper the PCB trace. But your brainstorming is much appreciated, guys. I'm not too proud to tell you a lot of this circuit is gibberish to me that I have no experience with. For instance, the low voltage supply is unfamiliar. The SS reverb & effects loop are Greek.
     
  11. firemedic

    firemedic Tele-Meister

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    Having disconnected the C35 coupling cap at the junction of R52, R55, & R49, the bias voltage is back where it's supposed to be.

    I have already replaced C35 with a Mallory, and in any case it comes off the cathodyne cathode which shouldn't have DC anyway with no tubes. That end of the cap reads 0.6v with the OT secondary connected, 0.3v with it disconnected.

    I get negligible DC voltage on the OT secondary when it is isolated.

    Somehow, a small positive voltage is being generated on the OT secondary post connection not feeding the NFB. When the OT secondary is plugged in to the PCB, this small voltage follows the OT winding to the post which does feed the NFB loop, thence somehow to the PI junction I have disconnected.

    I'm tired and quitting for now. Hot testing this board is extremely tedious and kinda jangles my nerves.
     
  12. firemedic

    firemedic Tele-Meister

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    All right everybody, here was the problem:

    A solder bridge from C35 to C54 in the V3a cathode. Looks like when the board was manufactured, C35 had a lead bent over just far enough to form an intermittent bridge with C54.

    When I replaced C35, the solder bridge looked like it was supposed to be there so I faithfully reproduced it, turning an intermittent connection into a solid one.

    I got rid of the solder bridge and now everything is gonna be OK. All nominal voltages are... nominal. Nice sounding amp. I hope I never see it again.

    Thanks for helping me work through this problem, everybody! Next up, a Carvin XV-112E. See upcoming thread.....
     
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