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Old 30th March 2009, 01:25 PM   #1
abacus is offline abacus  Croatia
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Unhappy Tubelab SE 300B low level hum, help

Hello

My Tubelab SE 300B amp have a low level hum, its barely to hear with my ears close to the speaker(5cm). It stays consistent regardless to the volume level. Volume pot case is grounded (blue Alps 100k), disconnecting the input wiring from the PC board no change. OPT James JS-6123HS, choke installed 6H, 200mA, changing PT orientation, no change. Tubes, Shuguang 300B-98B mash grid, rectifier Sovtek 5AR4.Any help about this problem?
And what grounding scheme will be the best to use, i used the first scheme from the Simple SE page.

Thanks.
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Old 30th March 2009, 04:55 PM   #2
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You can try to put non elektr. capacitor 1 uf/65-250v parallel to choke 6 H.
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Old 22nd April 2009, 04:19 PM   #3
abacus is offline abacus  Croatia
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I tried all possible combinations, cap across choke, separate filament transformer, 2 chokes in series, wiring and grounding combinations.
Nothing changed, hum is still there, anyone ?
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Old 22nd April 2009, 04:57 PM   #4
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Maybe you have magnetic coupling between your power transformer and your output transformers.
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Old 23rd April 2009, 01:14 AM   #5
rknize is offline rknize  United States
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What kind of chassis do you have? Did it always hum? My Tubelab SE on a bread board with no extra cap is dead quiet on my 98dB speakers. I am using a 10H choke and a 220uF final cap, though.
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Old 23rd April 2009, 02:58 AM   #6
kevinkr is offline kevinkr  United States
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Do you have ac or dc filament supply to the 300B?
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Old 23rd April 2009, 11:32 AM   #7
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Please, disconnect preapl. tubes /if there are interstages caps between stages/ and hear....
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Old 23rd April 2009, 10:02 PM   #8
chrish is offline chrish  Australia
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Just keep your ears more than 5cm from your speakers and there is no problem
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Old 23rd April 2009, 10:27 PM   #9
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Tubelab SE hum shouldn't be audible even at 5cm from the speaker. I've had a loud hum problem but that was due to OP tubes I used at the time. Once I changed them, it became silent.

abacus, do you have another pair of 300B tubes that you can try temporarily?
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Old 24th April 2009, 07:40 AM   #10
abacus is offline abacus  Croatia
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I have another pair of 300B tubes, but it is the same Shuguang 300b, i tried them with same result. Noisy tubes ? At the moment i don t have different 300B tubes to try. And moving PT away on long wires, nothing changes, hum is still there.
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