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Hum Nirvana

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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!
 
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
 
Cheers - I didn't spot the schematic earlier when I checked out the page.

An externally hosted image should be here but it was not working when we last tested it.


One thing puzzles me though, (as you have hum nulling anyway) why is the middle tube at 270V on and driven by C7? Surely C7 can be deleted and the middle tube just moved up in voltage 350V to allow DC coupling there?
 
Globulator,
I only know the middle (V2) tube is the driver & the first (V1) tube is the gain. I don't have enough of a technical background to gamble that kind of change without much help. The designer went so far as to use only 1/2 each of two 6SN7's in each amp where I probably would have used one tube for both purposes.
Frank
 
Since I'm not one to leave "well enough" alone, I tried 0.01uF caps to grd. from the gain tubes heaters & the power tubes filaments without improvement.
While in the amp chassis, I noticed the AC power wiring from thr input socket to the switch, the fuse, the lamp & the outlet socket to the PS had reversed the power & the neutral wires. I corrected this for a slight improvement.
Adding the 0.01uF caps to the dual rectifier tubes in the PS
Total improvement was down to 0.3mV on the left channel but up to 0.7mV on the right channel.
Now, I'm bugged by the difference & I'm still searching. I almost wish I'd have left it alone at 0.6mV-right & 0.6mV-left.
I've tried switching tubes without change.
Frank
 
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!

Any chance of a schematic or photo...tried this and it made the hum worse :(
 
klanbach,
The chassis is so tight & crowded I'm unable to get a decent photo. If you've tried the same circuit, I suspect you may be having ground problems. I'm the last noob to give advice but delve into all of the expert's suggestions for better grounding. My HV power supply is on a different chassis & the LV power supplies are close to the filaments.
Good luck, hum chasing is not fun.
Frank
 
Frank, as a matter of interest, did you do a direct comparison between your final configuration, and where the 6V3 heater is not DC elevated, but rather taken to your 0V. I haven't tested the heater-cathode conduction resistance for 7N7s, so it would be interesting to see if it is relatively low, and hence would have a noticeable influence on the hum level.

Also, if you change the heater winding terminals around, then you may revert back to your prior 'balanced' hum levels for each channel.

Ciao, Tim
 
Last edited:
klanbach,
I finally had to use Vishay/Spectrol precision potentiometers 7/8 WW 100 ohms, 10 turn.
5%, linear taper,series 534,voltage rating 14.1 volts, 2 watts wirewound. With the 10 ohm resistors to each end from the wiper to reduce the sensitivity, it still required a deft touch. I did twist all wires & routed them away from transformers. Use separate star grounds for the signal & power sections then tie them together at the main ground. In dressing the wires (it's a tight fit) I found moving some a little bit made a difference.
I can forward a bunch of information I read while doing this amp. It may help or it may confuse.
Frank
 
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