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Thread: Help with Ampeg OT

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    Supporting Member Randall's Avatar
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    Help with Ampeg OT

    Dang. I did a bunch of work for a restoration of a '69 0r '70 Ampeg Jupiter B-22x, and upon the initial power up stages on my light bulb limiter, it misbehaved. I first powered it up with no tubes to tinker with the bias supply that I made from fixed to adjustable, I was curious as to why the voltages were lower than I expected, and took some time to reach the high and low adjusted levels. The bulb was not lit.

    When I put in just the rectifier tube in standy, the bulb lit slightly, but when I switched out of standby the bulb lit up and it blew a 4 amp fuse. Uh, oh. I started probing around and found short paths to ground where there shouldn't be, most notably 10 ohms from rectifier pin 8 to ground. At first I suspected the PT, but when I disconnected the B+ lead, I found the 10 ohms to ground on the OT center tap, as well as 10 ohms to the lifted secondary.

    So this OT is toast, but what to do now? I honestly don't really know much about spec'ing transformers, and could use a bit of guidance.

    https://ampeg.com/support/files/Sche...matics%201.pdf

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

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    What happens if you disconnect the OT CT from point A?

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    Supporting Member Randall's Avatar
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    Perhaps I was not clear, but this is the first thing I did. I can power up the amp with a rectifier in, with no other tubes, and I measure B+, bias and filament voltages, with bulb barely glowing.

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    Regarding OT specs, the 7027A datasheet suggests a ~70W OT with a primary impedance of 6k to 6.5k.

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    Sorry to sidetrack, but how the heck do you blow a 4A fuse using a lamp limiter? What is the wattage of your bulb?

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    Supporting Member Randall's Avatar
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    It did not blow on the limiter.

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    "Regarding OT specs, the 7027A datasheet suggests a ~70W OT with a primary impedance of 6k to 6.5k. "

    How specific do these specs have to be? I found a 50 -60 watt Magnetics Components Classic Tone transformer with a primary impedance of 4K, would this "work"?

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    How specific do these specs have to be? I found a 50 -60 watt Magnetics Components Classic Tone transformer with a primary impedance of 4K, would this "work"?
    OTs having specs of 50/60W and a primary impedance of 4k to 4.5K are very common for 2x 6L6 amps with a B+ of around 450V.
    I guess it would work but the lower impedance will cause excessive plate dissipation with signal. To compensate bias should be as cold as possible.

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    Turns out Fliptops has them for $140 +. Not cheap, but a way out of this.

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