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Old 3rd January 2013, 02:00 AM   #1
tcqanh is offline tcqanh  Viet Nam
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Default DHT SE amp with ultrapath

I did build 2E22 SE amp with AC filament (because I like AC for filament ). Sound is so sweet and warm. Hum is > 13mVac, so I can hear them at my chair.

I intend to take an ultrapath with a Capacitor between power tube's cathode to B+ in oder to get lower hum AC and lower gain. There are some ultrapath schemas on the internet, but the tube is auto bias. Mine is fixed bias .

So :

- How can I calculate the value of this capacitor ?
- 2 these capacitors will be connected to the same point, B+. Is it OK ?

Thanks and regards,

QA.
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Last edited by tcqanh; 3rd January 2013 at 03:14 AM.
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Old 3rd January 2013, 08:29 AM   #2
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Hi!

ultrapath can only be used for hum reduction with auto bias (cathode bias).

I would never use ujtrapath for that purpose though. I only use it to 'shorten' the signal path in te output loop.

I would rather find the cause of the hum and fix it

Best regards

Thomas
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Old 3rd January 2013, 02:54 PM   #3
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DHP + 6.3V AC filament = > 13mVac hum
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Old 3rd January 2013, 08:12 PM   #4
Ketje is offline Ketje  Belgium
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AC for tubes is normal to indirectly heating cathodes.Direct heated cathodes needs DC to get rid of all the hum.On a PP the hum is commonmode and is cancelled in the OPT,but I suspect that even then you have to adjust the balance regularly.
Conclusion,use DC,
Mona
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Old 3rd January 2013, 09:14 PM   #5
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"AC for tubes is normal to indirectly heating cathodes."

That's OK for 2.5V filament but for a 6.3V tube you have to expect some hum.
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Old 4th January 2013, 01:38 AM   #6
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Hi,
Have you tried winding a filament transformer with bifilar wound
3.15V windings, connect them in series and use them to power the
filament? Sensing resistor is soldered to the junction of the two windings.
I once did this with my 3C24 and there was no audible hum a foot
from my 12" fullranger.
As always, YMMV.
Reza
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Old 4th January 2013, 03:13 AM   #7
tcqanh is offline tcqanh  Viet Nam
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Quote:
Originally Posted by rezaalamsyah View Post
Hi,
Have you tried winding a filament transformer with bifilar wound
3.15V windings, connect them in series and use them to power the
filament? Sensing resistor is soldered to the junction of the two windings.
I once did this with my 3C24 and there was no audible hum a foot
from my 12" fullranger.
As always, YMMV.
Reza
I use separate toroided transformer for 2E22 filament, so hum get a bit low. I will try CT transformer.

There are some ways of hum cancelation for DHT. I never try this way : http://www.dmitrynizh.com/dht-filament-hum-cancel.htm

Last edited by tcqanh; 4th January 2013 at 03:24 AM.
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