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Old 29th July 2011, 03:58 PM   #1
dobias is offline dobias  United States
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Default Hum Nirvana

At long last, I've managed to achieve my hum nirvana with 0.6mV of hum.
I've replaced the single turn pots (& 10 ohm resistors to the wipers) with ten turn pots. I tried them without the resistors & found they were still 'knife edge'
in operation. Adding the recommended 12 ohm resistors from each end to the wiper made all of the difference in searching for the minimum hum setting.
I had already connected the 2.5vac CTs in series with 10uF caps to the wipers.
The combination of that & 6.3vac, elevated on top of 50vdc,to the gains & drivers have given me the results I seeked.... all this without anything more than Shuguang 2A3C & Sylvania 7N7 tubes.

PS: It sounds great!
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Old 29th July 2011, 04:09 PM   #2
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Err.....

Wot the 'ell are you on about!--What are you talking about??

What device, What 'pots' and what caps?

Any schemes??
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Old 29th July 2011, 04:48 PM   #3
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It was the pots with the 2.5vac CTs in series with 10uF caps to the wipers - obviously!

I had this amp too, where the wires went to the base pin of a tube, and the capacitor was just about level with the other resistor, hum is a funny thing but by adjusting the pot I was able to achieve a B grade in geography.
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Old 29th July 2011, 05:00 PM   #4
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aka...guess what I'm thinking about......
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Old 29th July 2011, 05:26 PM   #5
dobias is offline dobias  United States
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Alastair E,
I started with a custom version of the Welborne Moondog 2A3 dual monoblocks:
Welborne Labs 2A3 Moondog Monoblocks
I used a PS on a separate chassis from the stereo amp on the other chassis.
Since this was my first venture into amp building, I struggled, without success, to get rid of the hum.
Eventually, the 6.3vdc was changed to 6.3vac on top of 50vdc of B+.
I tried elevating the 2.5vac without success.
Adding resistors to the single turn humpots did little to eliminate the 'knife edge' adjustment.
Replacing the original cheap 2.5vac transformers to a better quality with CTs helped.
By guess & by golly, I found I needed caps between the 2.5vac CTs & the humpot wipers.
Changing to ten turn humpots, I thought I wouldn't need resistors to ground. There was still the 'knife edge' adjustment until I added 12 ohm resistors from the ends of the pots to the wipers. Finally, I was able to reduce the hum to it's present 0.6uV.
I have nothing special in tubes, just Shuguang 2A3Cs & Sylvania 7N7s.
Frank
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Old 29th July 2011, 10:34 PM   #6
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Quote:
Originally Posted by dobias View Post
Replacing the original cheap 2.5vac transformers to a better quality with CTs helped.
What is a CT?
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Old 29th July 2011, 10:58 PM   #7
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Central Tap...maybe?
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Old 30th July 2011, 01:52 AM   #8
dobias is offline dobias  United States
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CT = center tap of a transformer. In this case, the center tap of the 6.3vac or the two 2.5vac filament transformers.
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Old 30th July 2011, 11:08 AM   #9
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Ah I see.

Is this about trimming the filament supply to a DHT tube?
Schematics would be interest, a picture tells a 1000 words etc!

BTW congrats for nulling out the hum!
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Old 30th July 2011, 12:06 PM   #10
dobias is offline dobias  United States
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Globulator,
go to: home.earthlink.net/%7Eivol/moondog_2A3/
& see the original schematic, then you can follow the changes I made.
Frank
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