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Old 21st October 2012, 05:01 PM   #1
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Default Bogen CHB-35a Troubleshooting: Distortion, Power Tubes Very Hot

Hi, I've got a pair of Bogen Challenger CHB-35a amps. I bought them as an easy educational platform.

One of them works fine and one does not. The "good" one amplifies a line-level signal through its Aux2 with a decent amount of clarity. The "bad" one has a lot of distortion that has a character that I scientifically refer to as "trashy". Also, the power tubes on the "bad" one get extremely hot very quickly, much hotter than on the "good" one.

I swapped all the tubes from the "good" amp to the "bad" and the problem persists. I recapped everything on the "bad" amp except the small ceramics and the capacitors in the can (I decided to listen for changes before doing anything else) and it sounds pretty much the same as it did before.

Because I'm using Aux2, the signal is going from the input to the Master pot to the 12AX7 (Tone Controls) to the 6C4 Phase Splitter tube to the 7868 power tubes. The 6EU7 is not involved at all. Here is the schematic:

http://makearadio.com/schematics/ima...n-chb35a-6.jpg

What would be the next step? Maybe something jumps out at someone as the likely culprit?

I'm a beginner at tube amps, but I have basic electronics/safety/troubleshooting knowledge, and I'm here to learn. Thanks for any assistance.

rs

Last edited by ryansupak; 21st October 2012 at 05:16 PM.
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Old 21st October 2012, 05:59 PM   #2
jane is offline jane  Norway
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Measure the voltages at the pins on the output tubes.. Do you have negative voltage at pin 2/6?
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Old 21st October 2012, 06:55 PM   #3
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jane: Thanks for replying.

I have -22VDC on V4 and -20.5VDC on V5. Looks very close to the -22.5 that the schematic describes. Those should be close enough, right?

rs
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Old 21st October 2012, 07:12 PM   #4
jane is offline jane  Norway
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And the other pin readings on the output tubes are OK?
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Old 21st October 2012, 07:19 PM   #5
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Yes, all the readings on the Output Tubes are within 10% of what the schematic specifies.

rs
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Old 21st October 2012, 08:09 PM   #6
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The voltage readings to the 12AX7 average about 35% too high, and to the 6C4 about 45% too high.

My novice's guess: the 10MFD capacitor in the can supplying 260VDC (denoted by the dotted box on the schematic) needs replacing. Does this seem like a reasonable guess?

Thanks,
rs
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Old 22nd October 2012, 03:34 AM   #7
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Replacing all of the caps put the voltages back to where they needed to be and made the amp sound good...but I'm not exactly sure why.

rs
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