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Thread: B-52 ATX100 Low Power Output

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    B-52 ATX100 Low Power Output

    Hi All,

    I've got a B-52 ATX100 tube head I'm working on. It previously had a shorted output tube and some scorches on the PCB, which I've fixed. Amp is stable and sounds pretty good, but has very low output volume. It's quieter than a Champ and should be 100W.

    This schematic is as close as I could find. The ATX is very similar, minus the tube rectifier and the flyback diodes (D6A and D7A) and a few other components.
    B52 AT100 (1).pdf

    Sending out of the FX send works just fine, there's a ton of gain on tap there. Returning into the power amp sounds the same as running in the main input. I figure it has to be the stages around V11-2, V5, or one of the 5881's. Tried with multiple cabs and on all impedence settings.

    All the tubes are fine. I've checked them all more than once for shorts, gain, and emissions. The 5881's are brand new, as is V5. Continuity checks seem okay, as do resistor and cap values.

    Here's the voltages:
    B+: 482VDC
    B+2: 478VDC
    5881 Filament: 7.3VAC
    Bias Current: 35mA/tube

    V11-2
    Plate/Anode: 300.5VDC
    Grid: 1.4mV
    Cathode: 2.34VDC
    Filament: 6.7VAC

    V5-1
    Plate/Anode: 307.4VDC
    Grid: 32.09VDC
    Cathode: 51.7VDC
    Filament: 6.7VAC

    V5-2
    Plate/Anode: 285.5VDC
    Grid: 34.76VDC
    Cathode: 53.2VDC
    Filament: 6.7VAC

    I've checked the OT according to RG Keen's method here, and haven't found any issues: Output Transformers

    Are there any tests I may have missed? And is the imbalance in plate voltages between the two halves of V5 a concern? Thanks in advance.

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    What bias voltage is on the 5881's?

    When you put your meter lead on the output tube input grids, pin 5, you hear a fairly loud crackle or pop?

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    Quote Originally Posted by drewl View Post
    What bias voltage is on the 5881's?

    When you put your meter lead on the output tube input grids, pin 5, you hear a fairly loud crackle or pop?
    Thanks for the response!

    I get -50.5V on all of the 5881's, pin 5.

    No sound when I probe pin 5 at all. Dead quiet.

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    Be sure to check the speaker impedance switch as well as the connections to the speaker jacks. Bad or shorted connections can cause problems.

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    All check out for continuity. Just found a new symptom. With a test tone running into the amp, if the Master volume is raised above ~3/4 of the way up, the connection from the OT primary to the PCB starts to arc. I'm guessing this is probably the issue?

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    Here's some photos of the damage. Is there just enough of a conductive path between the traces for an arc to occur when the signal is strong enough? That other trace leads to the second half of the OT primary.

    Click image for larger version. 

Name:	B-52 Crap1.jpg 
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ID:	37469Click image for larger version. 

Name:	B-52 Crap2.jpg 
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Size:	2.12 MB 
ID:	37470

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    Sometimes arcing on the primary side happens because the secondary side is not getting connected properly to it's load.
    Check very carefully the things 52Bill mentioned in post #4.

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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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