AA270 Vibrolux Reverb OT Voltages vary

Discussion in 'Amp Tech Center' started by Mr Ridesglide, Dec 10, 2019.

  1. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Silver Supporter

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    Hey folks, what I have here tonight is a repeat amp to the house. Few years ago, a dear friend brought me his amp and a new cabinet he ordered from Mojotone. I put it in the new cabinet for him, cleaned all the pots and sockets and also replaced both the speakers as the two he had were not original, one 16 ohms and one 8 ohms, and the cones had several tears in them. Mind you this was a very hard working amp by a local legend and nobody could ever figure out how he ever had such a great clean sound. Me either - whew. Okay I put two Emi 1058's in it and the amp sounded fabulous. Never heard it sound so good.
    Okay round two - Friend has sadly passed, his estate sold the amp to a cool guitar player. Amp sounds good, then starts crackling after being on for a while. It took about 45 minutes for me to get it to happen, but it did. So I put the cable into the Normal input and it's not happening. I figure I'd open it up and start looking at stuff on the Vibrato channel. I take it out of the cabinet and set it on boards as is what I have to work with, make sure the speakers are plugged in and turn it on. Well for the love of ivy the amp starts sounding like a slo-motion techno phase shifter. No guitar signal passing at all.
    I pull V1, V3, V4, turn it on and I get nothing. So I put it out of mind for a day or two.
    Came back to it today and the darned thing works. No more odd slo mo BS. So I started to chopstick and it seems like I can duplicate the noise I heard while I was playing the amp when tapping on a few solder joints; especially where the 100K resister meets like 4 other components. Re-soldered that one and another by a capacitor that was microphonic in the reverb section. Fired it up and it seems like those areas of concern have been stopped.
    But ... I figured I would just check the bias. Amp has no 1R resistors so I do the transformer resistance method - Well to be honest I think it's biased way on the low side but..... the huge but here is that I can't get the CT of the OT to stay calm nor either one of the plate voltages. So it's really hard to know what is happening. 421 to 416 on the CT and 414 to 418 on the Plates.

    What can make the OT voltages float around like that?
     
  2. peteb

    peteb Friend of Leo's

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    The plate voltage won’t stay steady.

    Trace it back up stream.

    Take out the power tubes.

    Will the rectifier tube produce a steady voltage?

    If not, remove the rectifier tube. Will the PT provide steady voltages?
     
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  3. Wally

    Wally Telefied Ad Free Member

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    Same old question....how old are the electrolytics?
     
  4. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Silver Supporter

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    Thanks Pete. I’ll give that a go when I get home.
    Wally - they have been changed. All of them. When, however, I don’t know.
     
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  5. schmee

    schmee Poster Extraordinaire Silver Supporter

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    Some of the symptoms sound like a bad solder joint. Chop sticking often creates noise but that noise may not be where the problem actually is.
    -I had an old DR once that had some of those symptoms and the volume pot was shot. But the board was noisy when tapped.
    -Another amp had a wire loose on the heater wire at the pre tube socket. It was loose in a big glob of solder so looked fine with the naked eye, but with a magnifying glass I could see it wiggle in the glob. That amp exhibited noise all over the board, but the problem was on the socket.
    I'm just throwing things to see what sticks!
     
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  6. muchxs

    muchxs Doctor of Teleocity

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    Hush, hush. Keep it down, now. Bias varies.

    Disable the trem. See what happens.
     
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  7. King Fan

    King Fan Friend of Leo's

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    I've made *that* mistake... :)
     
  8. Bob Arbogast

    Bob Arbogast Tele-Afflicted Silver Supporter

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    I was thinking the same thing . . . the song, that is!
     
  9. peteb

    peteb Friend of Leo's

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    Good call Muchxs, it makes sense that the tremolo bias vary is causing the shifting plate conditions.



    That would be easy to verify without any tinkering.



    With the tremolo turned off or turned down, the bias voltage would not be steady like it should.
     
  10. Wally

    Wally Telefied Ad Free Member

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    As I understand it, this amp has an opto-isolator trem..not a bias vary. That said, I would lift the connection for that trem circuit input grid from the bias circuit and see if there is some sort of leakage between those two circuits.
     
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  11. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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  12. peteb

    peteb Friend of Leo's

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    I too was not considering that the VR is bias vary, but doesn't the bias tie in with the tremelo?





    vibrolux_reverb_aa270_schem.gif
     
  13. Wally

    Wally Telefied Ad Free Member

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    Well, I am not the one to e plain it...beyond my limited knowledge. robrob gives us a good explanation of the circuit.
    https://robrobinette.com/How_Spring_Reverb_Works.htm

    the explanation for how that negative bias voltage shuts the oscillation down is in the last paragraph.

    thanks again, Rob.
     
  14. peteb

    peteb Friend of Leo's

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    One key difference I see between the bias vary tremolo as seen on Princeton amps and the octo coupler tremolo of the blackface amps is that even though they both end in the bias circuit, the octo coupler tremolo takes the signal as an input. I don’t get it.


    Oddly, the bias vary is more highly regarded than the octo coupler, but the bias vary uses one triode and the octocoupler uses two. What’s the benefit of the octo coupler tremolo?




    I don’t know too much about tremolo, I don’t use it much.


    Howling for You
    Born on the Bayou



    That is about it.
     
  15. Wally

    Wally Telefied Ad Free Member

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    The op to-isolator is better suited for larger amps, and it does not impose limitations on the biasing of the power tubes. The only Fender 6L6 amps with bias vary trem circuits would be the 6G16 Vibroverb and the amps that were built on the 6G16 RI circuit...the CVR and the 68 Sf Vibrolux Reverb. I have set up a CVR that would not yield trem function if the power tubes were biased past 62% of max plate dissipation. Another CVR would run at 67% of MPD and still had a functional trem signal. Had I had those two amps in the same place at the same time, I would have traded power tubes to see if the difference followed the tubes.
    And...as I understand it it, the two circuits do not ‘end’ at the bias voltage. Rob’s analysis of the opto circuit says that the bias circuit connection disables the input of the LFO while the bias vary circuit’s output acts upon that bias voltage.
     
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  16. peteb

    peteb Friend of Leo's

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    You are right Wally, it is not bias vary and it tremelos by sending pre amp signal to ground.


    but it still has that connection to the bias voltage, so there is a possibility of unwanted interaction.




    A standard DC voltage check on a power tube would find this, if the bias voltage on the grid did not hold steady.





    now i'm kind of curious to watch the bias voltage vary on an analog voltage meter, and the plate voltage too.
     
  17. D'tar

    D'tar Friend of Leo's

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    without the tremolo pedal/input grounded, the negative bias voltage will hold the triode into cutoff and eliminate oscillation. If in doubt just pull the trem tube and recheck for voltage fluctuation.
     
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