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Old 28th February 2011, 09:59 PM   #1
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Default Octaves/Ghost notes from Bogen K10. Caps?

I have a Bogen K10 that had been converted to a guitar amp in 2004. It absolutely rocks. It has started to sound octaves/ghost notes at mod/high volumes but doesn’t have any of the 60 cycle hum that typically signal cap issues. What else creates this?
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Old 28th February 2011, 10:38 PM   #2
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Might be microphonics of the valve.
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Old 1st March 2011, 10:15 AM   #3
DF96 is offline DF96  England
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Resistors? Especially carbon composition resistors.
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Old 7th March 2011, 11:29 PM   #4
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Already checked this...
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Old 7th March 2011, 11:30 PM   #5
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2 part question... How can I identify carbon comp resistors? If I have them, do I need to replace all ?
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Old 8th March 2011, 11:28 AM   #6
DF96 is offline DF96  England
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Carbon comp are likely to be lower tolerance (e.g. 10%) and larger than modern resistors. I'm not sure that it is easy to tell just by looking, as size and shape varies between brands. Possibly they will be nearer to a perfect cylinder in shape, with sharp corners. Film resistors tend to be a bit lumpy, with rounded ends and a waist in the middle.

The critical ones are those with high signal voltages. Bear in mind that some people deliberately fit carbon comp because they like the sound.
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Old 8th March 2011, 01:12 PM   #7
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If you want to make a frequency doubler, you can do it with diodes. You fullwave rectify the signal, and bingo, the resulting signal has twice the number of peaks. The energy at the octave is somewhat reduced, but it works.

Course you haven't got anything like that designed in there, but that's the kind of mechanism that might be producing the effect. Are you running any pedals?

OTOH I can make my guitar shriek when turned up just by hitting it right. In fact, if I had an amp that I could provoke to do that easily, I wouldn't be unhappy, because I have to work pretty hard to make it happen.

w
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Old 8th March 2011, 03:53 PM   #8
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Being a PP design the hum may not be an issue even tho the caps are acting up. So I'd look into the caps first, since that's a common cause. Perhaps you can simply parallel a known good cap using clips or something, just to quickly determine if the caps need replacement.
You said 'signal caps' , but I assume you meant power supply caps...?
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