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Output tube generates pitch tone
Output tube generates pitch tone
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Old 13th June 2018, 05:51 PM   #1
eddietanghf is offline eddietanghf  Hong Kong
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Default Output tube generates pitch tone

hi there

I am testing my 2nd DIY amp. 6SN7 drives KT66 at triode mode.

Gain looks fine when testing with a signal gen and a 8 ohm resistor load.
The problem is when I increase the input frequency from 1kHz to higher, the output tube generates significant pitch tone directly(there is no speaker connected). Tone varies with input frequency changes until 15kHz that my ears are not able to hear them anymore!

Do you guys have any idea!
thanks.

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Old 13th June 2018, 06:05 PM   #2
FullRangeMan is offline FullRangeMan  Brazil
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Inside the tube there is a partial vacuum that makes outside noise emission difficult.
It is not the choke or OPT winding/reel?
The OPT is encapsulated?
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Old 13th June 2018, 06:35 PM   #3
eddietanghf is offline eddietanghf  Hong Kong
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hi fullrangeman

ah, that makes sense and I could not identify the source of the noise tone actually, I just hear and not sure where they from!

choke and opt are all with full metal cap. the only thing is i left the "earth" pin open on opt, the hashimoto one. I am going to check...

cheers
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Old 13th June 2018, 07:55 PM   #4
Lingwendil is online now Lingwendil  United States
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Output tube generates pitch tone
Probably the output transformer. Try a zobel network across the primary or secondary to see if it tames it.
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Old 13th June 2018, 09:40 PM   #5
Wavebourn is offline Wavebourn  United States
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Output tube generates pitch tone
Zobel network won't help. It needs to be either tightened, or impregnated.
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Old 13th June 2018, 11:05 PM   #6
electrix is offline electrix  United States
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Positive feedback? Grounding issues?
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Old 14th June 2018, 12:20 AM   #7
kodabmx is offline kodabmx  Canada
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You ARE using a load without the speaker, no? If not it's probably magnetostriction in the OPT. Even with a dummy load I can hear 10kHz sine from the transformer if I crank it enough.
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