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
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
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.
GoatGuy
GoatGuy
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.
GoatGuy
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/109570777566485968551/Amplifiers#5379198671502580578
https://picasaweb.google.com/109570777566485968551/Amplifiers#5379584404731452706
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!
https://picasaweb.google.com/109570777566485968551/Amplifiers#5379198671502580578
https://picasaweb.google.com/109570777566485968551/Amplifiers#5379584404731452706
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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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.
regards
Oscar
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.
regards
Oscar
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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/Scots_Guide/info/earth/door1/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
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.
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.
Still haven't got back to working on the amp, but do have some new observations.
1. The hum is reduced (but does not completely disappear) when a steel sheet is inserted between the power transformer and output transformer.
2. The hum is present with only the heaters powered on (no B+ to the tubes).
3. The output transformers are physically vibrating, but the power transformer is not. I think they still vibrated some while shielded by the steel plate, but don't remember.
What might cause my OPTs to vibrate like that? Ground loop, perhaps? Or just magnetic coupling?
1. The hum is reduced (but does not completely disappear) when a steel sheet is inserted between the power transformer and output transformer.
2. The hum is present with only the heaters powered on (no B+ to the tubes).
3. The output transformers are physically vibrating, but the power transformer is not. I think they still vibrated some while shielded by the steel plate, but don't remember.
What might cause my OPTs to vibrate like that? Ground loop, perhaps? Or just magnetic coupling?
Have you try turning the OT 90 deg. I don't mean going from vertical to horizontal. I mean literally rotate the OT. Magnetic field pattern is very directional, it's like inductors, two inductor in "T" has minimal coupling and two in parallel has max coupling. I hope you have the OT mounted in vertical position, then it will be very easy to rotate. try before you even drill new screw holes.
But my bet is on the grounding problem. that you have to show pictures of the wiring to know.
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I have already spent quite a bit of money to get this top plate made, so it isn't desirable to rebuild to move the power supply to a separate chassis. The component layout didn't hum in the breadboard stage - only in the final build. Differences between the two were only a couple ground connections...
The OPTs are already 90-degrees offset to the power transformer.
The OPTs are already 90-degrees offset to the power transformer.
I have attached a picture of the inside of the amp along with a schematic (connections shown as built). The layout of the top can be seen in my photo gallery post.
I checked for a ground loop today, but don't think there is one. Now pretty sure the hum is magnetically coupled or being transmitted through the top plate.
With only the heaters on (no B+ at all), there is 1mV AC ripple shown on the B+, but 16-24mV AC ripple across the output transformer primaries. This yields about 2mV AC ripple across the speakers, which is audible from about 4 inches away from the speakers. The cores of the output transformers also physically vibrate, but the power transformer does not appear to vibrate. These conditions appear even with no tubes in place (so no load on the power transformer).
I have not yet tried to mechanically isolate any of the transformers from the top plate. Does anyone see any other possible causes of my hum?
I checked for a ground loop today, but don't think there is one. Now pretty sure the hum is magnetically coupled or being transmitted through the top plate.
With only the heaters on (no B+ at all), there is 1mV AC ripple shown on the B+, but 16-24mV AC ripple across the output transformer primaries. This yields about 2mV AC ripple across the speakers, which is audible from about 4 inches away from the speakers. The cores of the output transformers also physically vibrate, but the power transformer does not appear to vibrate. These conditions appear even with no tubes in place (so no load on the power transformer).
I have not yet tried to mechanically isolate any of the transformers from the top plate. Does anyone see any other possible causes of my hum?
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