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Old 19th August 2012, 09:20 PM   #1
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Default SOS: First Build, Lots of Hum

Hello everybody.
I've just finished the construction of a 2A3 SET wit a SRPP 6SL7 gain stage. It's the Angela SRPP 2A3 (2A3 Tube Amplifier Construction Project).
I'm getting 40mVrms of hum at the speaker terminals, mainly 100Hz (mains freq is 50Hz), clearly audible even with low efficiency speakers. I've read a number af threads pointing to AC heaters. My amp has hum pots which are fully adjusted to one end to produce the less hum. The Power Supply (5UA4g) measures 1,4Vpp of ripple at B+ (measured at 288V instead of the 322 V of theoretical value, need to understand why).
For now I'm wondering how much of the 1.4Vpp ripple at B+ translates to the 40mVrms hum at the output.
Your comments will be greatly appreciated.
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Old 19th August 2012, 09:30 PM   #2
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A picture of your build, especially heater wiring, wold be very helpful.
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Old 19th August 2012, 11:42 PM   #3
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Thank you for your answer LinuksGuru. I rushed in the end and the build, as you see, got quite messy. I tried to twist the heather wiring though.
By the way, I made a mistake in my previous post. The B+ ripple is 2.24Vpp, twice the value at the simulations with PSUD II.

Click the image to open in full size.
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Old 20th August 2012, 02:26 AM   #4
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check to see how much current you are drawing from the supply, and as a whole, compare that to the size of ur power xfrmr and see if perhaps you are drawing more than the iron can supply?

also your link is NG. if you want to post pix, best to use the "Go Advanced" or "Post Reply" and then use the upload feature of the attachment manager...

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Old 20th August 2012, 02:58 AM   #5
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by memristor View Post
Hello everybody.
I've just finished the construction of a 2A3 SET wit a SRPP 6SL7 gain stage. It's the Angela SRPP 2A3 (2A3 Tube Amplifier Construction Project).
I'm getting 40mVrms of hum at the speaker terminals, mainly 100Hz (mains freq is 50Hz), clearly audible even with low efficiency speakers. I've read a number af threads pointing to AC heaters. My amp has hum pots which are fully adjusted to one end to produce the less hum. The Power Supply (5UA4g) measures 1,4Vpp of ripple at B+ (measured at 288V instead of the 322 V of theoretical value, need to understand why).
For now I'm wondering how much of the 1.4Vpp ripple at B+ translates to the 40mVrms hum at the output.
Your comments will be greatly appreciated.
If the hum is 120Hz it may not be from the heaters. Well it might be because that 6.3VAC will have lots of 120Hz on it.

The test to see if the hum is comoing from the B+ or the heaters is to get some batteries. AA cells will do and build a battery powered heater supply. It only has to run for 30 seconds, so as I said even AA cells will work. Four in series can supply an amp of 6V.

If the amp still hums with batteries on the heater then the hum id caused by the B+ poower supply. if the batteries fix the hum problem then you need to work on the heaters by

1) remove the ground connection from the hum balance pot and conect it to something that is at about 30V above ground (like a voltage divider) this "elevates" the heaters and work most of the time.

2) re wire the heater so as to eliminate "loops" in the wire. Go over the tube sockets, twist the wires 8 turns per inch (use a drill motor)

3) any signal in the amp that feeds a high impedance (like a tube grid) needs to go in a shielded cable with ONE (and only one) end of the shield tried to ground. Even so make sure no signal wire runs parallel to an AC wire

Lot of things to do but first use the battery test to determine the cause of the hum
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Old 20th August 2012, 07:14 AM   #6
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Hi!

The AC heaters are very unlikely the cause for that kind of hum.
Are the heaters referenced to ground as indicated in the schematic? A symmetric reference to ground would be better via two resistors.
Are you using a metal chassis? Is the chassis connected to ground? Connected to safety earth?

Check all these things first then:

Disconnect the amp from any source and short the inputs. Still humming?
If so, short the output tube grid to ground and see if it still hums.

Once you've done that, the results will give hints as to where to search further

Best regards

Thomas
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Old 20th August 2012, 08:38 AM   #7
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Quote:
Originally Posted by Vinylsavor View Post
Hi!
Check all these things first then:

Disconnect the amp from any source and short the inputs. Still humming?
If so, short the output tube grid to ground and see if it still hums.

Once you've done that, the results will give hints as to where to search further
Thomas is 100% right. Do this and post result, at least we could hunt down most likely source of hum.
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Old 20th August 2012, 03:59 PM   #8
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Default Lots of room for improvement

Thank you gentlemen for your valuable answers.
I'm attaching a hi res pic. I know it's not a clean build; I rushed thing at the end. I should replace specially the heater wiring for the 2A3s (red twisted cable). Will do that possibly today.
The enclosure is metallic, and it's connected to safety earth. Power supply 0V is also connected to safety earth. I know I could use some sort of ground loop switch but I wanted to keep it simple at the beggining. The OPT secondary is not referenced to earth. Should I do that?
Regarding grounding the grid of the 2A3 to ground, I'm wondering whether this could damage the 6SL7 by drawing too much AC current from there through the coupling cap. I prefer to sound silly by asking than to damage the 6SL7s or the caps: Should I remove the 6SL7s to make things safer?
ChrisA: As for elevating the 2A3 heater, please note that the hum pot is actually connected to the leg going to the RC auto-bias circuit so it actually gets around 40 Volts in there if I'm getting you right.
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Old 20th August 2012, 04:52 PM   #9
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Hi!

There is no pic in your post...
It is difficult to help if you do not give all the information.
Is the 6SL7 heater referenced to ground as per the schematic? This is something which is easily forgotten.
Does the amp hum without any source connected and the inputs shorted?
You can short the 2A3 grid without damage, just keep the input shorted, there will be no AC signal. If you are worried, remove the 6SL7

Thomas
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Old 20th August 2012, 05:09 PM   #10
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Default Elusive Attach

I'm sorry I failed to attach my picture before. I hope you can see it this time.
The 6SL7 heaters are elevated to 80Vdc so as to not exceed the maximun cathode to heater voltage. I'll check this again to be sure.
I've shorted the inputs and the hum it's still there. The FFT function at the scope shows a 50Hz and 100Hz component. The 100Hz component is twice the 50Hz one.
next step will be to ground the grids.
Thank you.
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