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Old 30th September 2014, 03:15 PM   #1
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Default New Amp With Hum...

I just finished my 6C33C SET and powered it up to discover a 60Hz hum in both speakers. The output transformers are about an inch from the power transformer, so since placing a steel sheet between the transformers reduces the hum, it looks like the hum is magnetically induced. Strange thing is that I tested for that very situation before building the amp. I even had the transformers touching at one point during the breadboarding process and had no hum at all. But, now the finished amp hums...

The hum is not bad at all, but it is annoying to have hum present in my finished, carefully thought out amp when the very rough breadboarded amp didn't hum at all. Does anyone have a suggestion to fix the issue? Rebuilding the amp is not an option since I already spent quite a bit to have FrontPanel Express build this top plate. Could a different choice of output transformer hum less?

Thanks all
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Old 30th September 2014, 03:42 PM   #2
tomlang is offline tomlang  United States
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Perhaps somehow induction is coming via the chassis? What about isolating the OPT's from chassis ground with grommets?
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Old 30th September 2014, 03:45 PM   #3
GoatGuy is offline GoatGuy  United States
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If stainless steel sheet reduces hum, find a relatively thick (2+ millimeter) pure copper plate. Much, much higher conductivity, so the ferrimagnetic shielding effect will be proportionately more. Or, if thick copper isn't in the cards, try a really thick aluminum plate. 5 mm minimum thickness. If the situation clears up, then have some nice small thick aluminum plates anodized a pretty purple or something. Coyly mounted on brass posts, and the steampunk look can be maintained. Questions about the purpose are simple enough: antiferromagnetic eddy shields.

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Old 30th September 2014, 04:04 PM   #4
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Unbolt the mains transformer and rotate until the hum stops. Make up a plate to cover the old holes.
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Old 30th September 2014, 04:35 PM   #5
GoatGuy is offline GoatGuy  United States
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Quote:
Originally Posted by JonSnell Electronic View Post
Unbolt the mains transformer and rotate until the hum stops. Make up a plate to cover the old holes.
That works too. Seems to me that what it means is not so much that the transformers are coupling to each other, but that the magnetic field of the transformer is coupling to the front-end tube (or something like this). Rotating would fix though. And you're right making a separate plate is easy enough.

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Old 30th September 2014, 05:04 PM   #6
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1. IMO, maybe You have mistake in construction of placing of Your amp? I made two similar amps and my decisions was to separate PS from amp.
https://picasaweb.google.com/1095707...98671502580578
https://picasaweb.google.com/1095707...84404731452706
Transformers are big and 1 inch distance is very small.
Better make new inclosure only for PS.
2. To avoid hummm also use big chocks in PS, for plates 6S33S, with core about 12-15 sq. cm /about 20 Hn and maybe more/ and separate chocks for pretubes.
Better make separate PSs for every channel. If Your PT is common, You can make two PS filters after it with their chocks.
For big dynamic, last electr. caps in schem of PS must be in amp inclosure /up, on the paces of PT, that You can remove in new PS box, for ex/, its nominal must be about 600-1000 uF per channel.
Don't forget, SET 6S33S is very serious project. Big issues are in PS.
Regards!
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Last edited by azazello; 30th September 2014 at 05:18 PM.
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Old 30th September 2014, 05:31 PM   #7
osscar is offline osscar  Latvia
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hi,

maybe if you put the screens on input tube it will help? I know - it's not good for tube cooling...
my 6c33c se don't hum and mine is also relatively compact - as i mentioned i used 2 toroids for PSU - i mount them vertically ( 6.3V for heaters, for 6c33c 6x470uf caps +1H + 100R for each channel and 2x470uf CRC for frontend each channel - fast mur diode bridges) - distance between input tube and toroids - 10cm or 4 inches or so.
I hope that you will solve your hum problem. And it would be interesting to see a picture of your SE.

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Last edited by osscar; 30th September 2014 at 05:57 PM.
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Old 1st October 2014, 09:40 AM   #8
cyliew is offline cyliew  Malaysia
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Good day,

Since you mentioned it has no hum during bread-boarding, is it possible to have a ground loop or something related to the ground path when put into chassis? Or could it be the internal wire placement that has coupled noise to each other?

Cheers,
Liew
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Old 1st October 2014, 11:29 AM   #9
Celsius is offline Celsius  Spain
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Quote:
Originally Posted by needtubes View Post
I just finished my 6C33C SET and powered it up to discover a 60Hz hum in both speakers. The output transformers are about an inch from the power transformer, so since placing a steel sheet between the transformers reduces the hum, it looks like the hum is magnetically induced.

Strange thing is that I tested for that very situation before building the amp. I even had the transformers touching at one point during the breadboarding process and had no hum at all. But, now the finished amp hums...


Thanks all
I am not sure that the problem is the situation, the 6c33 is very silent tube.
My last amp was a 6c33 Chinese and I don listen any hum, and the PSU and OPT very near too.

Check from the RCA input to POT and tube, all lines must be with screen to ground
https://www.st-andrews.ac.uk/~www_pa...or1/cross1.gif

and try to separate this line to the AC lines heater, main switch on/off line.....

in my 6c33 I dont use DC to any filament, only grounded by two resistor.

but check the line in one point you will find the problem, pot ground, screen line... show the schematic to try to see better.
Good luck
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Old 3rd October 2014, 12:29 AM   #10
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Sorry I disappeared after posting... Got very busy at work.

I will try to post a picture or two along with a schematic tomorrow. The amp has a main power switch which turns on the heaters, and then the high-voltage center tap is switched on separately. I get hum with only the heaters on, no B+ to the tubes at all. I have also tried removing the tubes entirely and switching on heaters only and still had a hum. I have been doubting that the issue is a ground loop, though, since the steel plate I placed between the transformers reduced the hum.

It is still quite odd to me that the breadboarded amp, with heater wires running untwisted over signal wires and transformers placed right next to each other, had no hum, while the finished amp does... I agree that this fact could point to a ground loop, but that doesn't seem agree with my steel shield findings...

Will try to do some more testing tomorrow.
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