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Thread: Super Reverb bias and tubes

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    Super Reverb bias and tubes

    I've got a real clean '69 Super Reverb in for a cap job and AC cord. It has what I am guessing are almost all the original tubes, including a pair of Sylvania 6L6CGs the owner really wants to keep. They test good on my Hickok, but are pulling 62 and 68mA, with the bias pot full tilt at -65v. I popped a spare used in and it was at 11mA. I started wondering how I might get a bias voltage increase to tame these tubes, but then realized that's a big increase I'd need, and I doubt that is going to happen.

    Is it strange that both tubes are drawing this much current and are relatively close to each other? And how does this happen, why do they draw so much more than I assume they did when they were young? Can they ever be good for anything again, or are they toast?

    https://www.thetubestore.com/lib/the...-Schematic.pdf

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    It's weird, because it WAS working fine.....

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    Are you sure it's not an AB568 or AA1069 circuit?
    It probably won't help spare the tubes, but there are some wonky things going on in that AB568. Either way, AA763 probably ain't it if it's original.

    And as much as the owner might wanna keep em, tubes are in sockets for a reason... They'll likely just keep on deteriorating no matter how far you go with it.

    Justin

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    Tube chart says AB763. I owned this very same model and year for decades, it's the drip edge model.

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    It's weird, because it WAS working fine.....

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    Are you measuring that -65V at the power tube grids? If so, I'd say those power tubes are done.
    You are sure of your idle current method/measurement?

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    Also being a 69, it could be either AB568 or AA1069. That AB763 is probably a leftover. And those AB568s had some strange stuff. A closeup of the power tube side of the chassis would narrow it down.

    Justin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    I'm not sure what we are trying to narrow down, I measure -65v at pin 5 with fresh grid resistors, and 68/62mA with a bias probe with 468v on pin 3 with the originals. And as I said, 11ma with new tubes in the same pot position. Customer is not interested in changing anything that isn't broken, so it doesn't much matter which exact circuit it is to me. I'm only looking to get it up and running, and get the tubes biased correctly, which should be happening today.

    I was mainly asking why do tubes draw this much after time, and isn't it curious they stayed relatively close?

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    I was mainly asking why do tubes draw this much after time
    Good question except for the after time part. For all we know they drew this hard from day one.

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    I was mainly asking why do tubes draw this much after time, and isn't it curious they stayed relatively close?
    You're right, this is very curious. A 6L6 that draws over 60mA of cathode current with a bias voltage of -65V must have extraordinarily high emission/transconductance. Typically emission reduces with life.

    So I would assume that this is/was a matched pair of very "hot" tubes since new.

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    Maybe they’re dumping a lot of grid current; what’s the Vdc across each 220k grid leak?
    Or maybe it’s oscillating; what happens to the power current if the tube in the phase splitter slot is pulled?

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    Well, today has been another learning experience. While I was in there measuring around, I discovered one of the 220K grid resistors was 454K, and the other was 241K, as well as the 82K/100K PI plate resistors being over 10% high when they call for 5%. I replaced them all and put in a new pair of JJ 6L6s and it biased right up nicely.

    After reading a couple of the latest posts, I was curious to put the originals back in, and darned if they didn't settle down, at least for a bit. I was able to dial one in to 32mA, but then it would rise to 42mA or more. I did make a mistake for a while of monitoring bias voltage, and seeing differences of a few volts from side to side, forgetting this effects current draw. I pulled that tube and put the other in the same socket to see if there was something there causing the fluctuation, and found it to be down around 12mA at the same bias pot setting. It did not seem to fluctuate much. I moved the probe to the other socket with the hot tube, and it was still up and down. This shows the tubes are indeed in need of replacement, but I can't reconcile this with my earlier findings. Maybe I made an error somewhere. What I did learn is, probing bias voltage one one side effects current draw on the other, though I'm not sure why.

    At any rate, the new JJ's are settled in nicely with no fluctuation or noise. So re-capped, new AC cord, put in the proper tubes in PI and tremolo positions, swapped a noisy V2 with V5 where it doesn't much matter, replaced a broken reverb cable plug, and new grid and screen resistors on the power sockets. A good cleaning and this one is out the door, and he got his money's worth on it!

    And for the record, I believe it is a AB763. Thanks gents!.

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    It's weird, because it WAS working fine.....

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    Maybe they’re dumping a lot of grid current; what’s the Vdc across each 220k grid leak?
    As long as grid to cathode voltage is negative, grid current is not likely. Normal grid leak current is a small negative (electron) current flowing out of the grid, which tends to make grid voltage even more negative.
    Maybe gassy tubes?

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    Last edited by Helmholtz; 05-18-2020 at 12:24 AM.
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    Tubes passed the gas test on my Hickok.

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