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Old 19th April 2011, 10:07 PM   #1
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Default Hum On SSingle Ended

Hello guys,

I built amps before(most with AC on the filaments) and my hum figure is about 2.2 mV input all the way open. This one I still can't figure out why I am getting around 6+ mV at input shorted!

Any suggestions much appreciated.

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I tried moving the star ground point (seen from pic 2) to the point close to the IEC(left yellow alligator leads on pic 1 and 3), no change. I tried separating all the wires(AC in, HV and filament) without the tie wraps and still the same.

What can it be?


Thanks!

Abe

Last edited by amandarae; 19th April 2011 at 10:12 PM. Reason: change pic 4
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Old 19th April 2011, 10:30 PM   #2
rknize is online now rknize  United States
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Hum with the input shorted often means power supply noise. Is it a soft 60Hz hum or more of a 120Hz buzz? Two things I see in the pictures:

1) Your C2 seems very small (40uF). The design calls for at least 100uF here. If 40uF is all that you have, then that is the most likely culprit.

2) Your choke seems underrated. What tubes are you running?
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Old 20th April 2011, 12:07 AM   #3
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Quote:
Originally Posted by rknize View Post
Hum with the input shorted often means power supply noise. Is it a soft 60Hz hum or more of a 120Hz buzz? Two things I see in the pictures:

1) Your C2 seems very small (40uF). The design calls for at least 100uF here. If 40uF is all that you have, then that is the most likely culprit.

2) Your choke seems underrated. What tubes are you running?
Hi Russ,

1.) The 40uF is C1. C2 is at 120 uF. You are looking at the board underside.

2.) 6L6GC. I thought we are running the tubes on triode mode (~44mA x2) and the 12AT7( ~10mA x2). So that is around ~108mA no? Did I miss something here?

I should be able to get a 10H choke at 200mA rating. I think I have one lying around here somewhere(that's the beauty of being a DIYer, parts accumualtes like crazy in the storage!)

Thank you very much!

Abe
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Old 20th April 2011, 01:41 AM   #4
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1) Ah yes, OK. That looks fine. Looks like you have a motor run cap too.

2) It's a good choice for EL34s or 6L6s. It doesn't have much headroom if you want to run bigger tubes, though it may manage. I do overload mine a bit and it seems OK (Allied choke).

It looks like you have only one ground point at the RCA jacks, which is what you want. Just to eliminate that possibility, you can try lifting that safety ground temporarily to see if it has any effect. You are not trying to use CFB, correct?

You might also try lifting that cap from the power switch. Is that a ceramic cap? Is the top plate aluminum?
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Old 20th April 2011, 02:08 AM   #5
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Originally Posted by rknize View Post
1) Ah yes, OK. That looks fine. Looks like you have a motor run cap too.

2) It's a good choice for EL34s or 6L6s. It doesn't have much headroom if you want to run bigger tubes, though it may manage. I do overload mine a bit and it seems OK (Allied choke).

It looks like you have only one ground point at the RCA jacks, which is what you want. Just to eliminate that possibility, you can try lifting that safety ground temporarily to see if it has any effect. You are not trying to use CFB, correct?

You might also try lifting that cap from the power switch. Is that a ceramic cap? Is the top plate aluminum?
Thanks Russ!

Yes, top plate is aluminum. I will take the ceramic cap off the switch(1.2kV) and lift the ground point as you suggested.

I took the board out and will reflow/check solder joints especially the PSU section. Will report back! Thanks again!

Abe
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Old 20th April 2011, 08:26 PM   #6
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Update:

After re-flowing solder joints on the PSU circuit, and following Russ's suggestions on the previous post, same value for hum was measured(6.3 Vac 60 Hz).

Can any of the guys here be so kind and measure their Simple SE hum(at the speaker terminal) and let me know what they are getting especially those using Hammond transformers please? I already ordered new Hammond 374BX to try and see if there's a difference since the hum is 60 Hz (before rectification).

Any idea on how to check if I have a contaminated ground? I tried straight to the wall and with AC conditioner and the hum level stays the same.

This ongoing troubleshooting is a tough one for me and I do not want drilling the chassis so many times trying to locate the best ground point if possible. Please let me know if you have any idea that I can try.

Thank you very much, as always.

Abe
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Old 20th April 2011, 10:05 PM   #7
Michel is offline Michel  Canada
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Abe,

Can you please revove the 4 wires coming from the potentiometer that are connecting to the board and tell me if the hum is still there?

Michel
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Old 20th April 2011, 10:07 PM   #8
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Abe,

Can you please revove the 4 wires coming from the potentiometer that are connecting to the board and tell me if the hum is still there?

Michel
Will do! So you want me to "float" the board from its ground source?

Abe
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Old 20th April 2011, 10:12 PM   #9
Michel is offline Michel  Canada
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Since the thing is not in front of me I am working kind of blind here.
I am trying to see if there are conflicting grounds.
Is the PCB ground connection for the potentiometer still measure as grounded even when you remove the wires?
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Old 20th April 2011, 10:21 PM   #10
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Since the thing is not in front of me I am working kind of blind here.
I am trying to see if there are conflicting grounds.
Is the PCB ground connection for the potentiometer still measure as grounded even when you remove the wires?
Hi Michel,

Thanks for your response!

No. If the wires are disconnected, there is no PCB ground connection(checked by a meter set to ohms in "audible" short range). Hum level stayed the same.

What I do not get is that in normal operation, everything connected, moving the dual pot to "open" or "grounded" does not change the hum level.

regards,

Abe
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