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.
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
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.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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:
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?
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
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?
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
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
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
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
Based on your response to George in the previous thread, I don't think your problem is in the input or with the amp itself. If you get 60 Hz hum immediately after power on before the tubes warm up, then it is being induced in the OPTs directly somehow. The only possibilities are B+ or magnetic induction. How close is the power transformer to the OPTs?
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Hello Russ,
The spacing between the PT and the OPT's are sufficient I think. Here's some pics:
Abe
The spacing between the PT and the OPT's are sufficient I think. Here's some pics:
An externally hosted image should be here but it was not working when we last tested it.
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Abe
Okay.
I'm just thinking out loud here and I'm sure you've tried pretty much everything by now. Sorry if my ideas are old stuff, but I have to ask if you have replaced the power xfmr or maybe tried shielding it or the OPT's. Also, I noticed that your OPT's are under covers, so I can't tell the core orientation. Are you sure the OPT cores are 90 deg from that of the pwr xfmr?
For troubleshooting purposes, try disconnecting the OPT's primaries from the circuit altogether. Simple to do and If the hum is still there, it's got to be 60 hz getting into the OPT's externally, probably from the power transformer.
Have you replaced the tubes? I'm thinking filament noise from a defective tube. Is the noise on both channels equally?
Did I read that you get the hum before the filaments even warm up? If so, then the following ideas probably won't apply, but here goes anyway:
Hum can often be induced into the hi impedance stage if signal leads are run too close to B+ lines or other PS lines. Some guitar amps have this problem right out of the box and people have remedied it by replacing any hi impedance signal lines with small shielded coax even if it means cutting PCB traces to get the line off the board and shielding them. Usually that's 120 hz noise, though, from the B+. Are you sure it's 60 hz rather than 120? That will tell you a lot about the source. Have you scoped it?
Sounds like your speakers are unusually efficient. Has anyone actually measured the ripple from a known normally working amp as you asked? I'm just wondering if 6 mv, even though it sounds high, may not be unusual for this design and it just gets buried in less efficient speakers. Those are really hot speakers, especially the mid ones.
Gotta tell you, though, hum is usually a ground loop problem somewhere. That can be tough sometimes, but it sounds like that doesn't apply here.
FWIW. Good luck. I hate hum.
I'm just thinking out loud here and I'm sure you've tried pretty much everything by now. Sorry if my ideas are old stuff, but I have to ask if you have replaced the power xfmr or maybe tried shielding it or the OPT's. Also, I noticed that your OPT's are under covers, so I can't tell the core orientation. Are you sure the OPT cores are 90 deg from that of the pwr xfmr?
For troubleshooting purposes, try disconnecting the OPT's primaries from the circuit altogether. Simple to do and If the hum is still there, it's got to be 60 hz getting into the OPT's externally, probably from the power transformer.
Have you replaced the tubes? I'm thinking filament noise from a defective tube. Is the noise on both channels equally?
Did I read that you get the hum before the filaments even warm up? If so, then the following ideas probably won't apply, but here goes anyway:
Hum can often be induced into the hi impedance stage if signal leads are run too close to B+ lines or other PS lines. Some guitar amps have this problem right out of the box and people have remedied it by replacing any hi impedance signal lines with small shielded coax even if it means cutting PCB traces to get the line off the board and shielding them. Usually that's 120 hz noise, though, from the B+. Are you sure it's 60 hz rather than 120? That will tell you a lot about the source. Have you scoped it?
Sounds like your speakers are unusually efficient. Has anyone actually measured the ripple from a known normally working amp as you asked? I'm just wondering if 6 mv, even though it sounds high, may not be unusual for this design and it just gets buried in less efficient speakers. Those are really hot speakers, especially the mid ones.
Gotta tell you, though, hum is usually a ground loop problem somewhere. That can be tough sometimes, but it sounds like that doesn't apply here.
FWIW. Good luck. I hate hum.
Thank you!
I have tried most of your recommendations but I will try it again, one by one. I just don't have enough free time to tackle this problem I have but contemplating on doing it the first chance the opportunity presents itself.
Yes. I now have two Hammond 374BX and two sets of output transformers, a pair of 200 and 250 mA chokes and a new top plate in case I have to redo the lay out totally.
regards,
Abe
You might try turning the power transformer 90degreees (relative to the OPT's) so that the magnetic field radiating from the 'edge' of the core isn't facing them.
Or just taking it off the chassis,and using some short jumper wires to connect it back to the amp temporarily. Thus you can get some physical distance between the power transformer,and the output transformers/amp circuitry.
Or just taking it off the chassis,and using some short jumper wires to connect it back to the amp temporarily. Thus you can get some physical distance between the power transformer,and the output transformers/amp circuitry.
That would be my suggestion also. Since you have an extra power transformer, leave it on the chassis but disconnect it. Hook the second unit up to the board and carefully power it up. Remember you are dealing with 750 volts, loose wires can fry you. Lay the transformer on its side behind the amp and test for hum. Power off move the transformer, or roll it over 90 degrees and test again.
60 Hz hum that appears before the tubes are hot almost has to be transformer coupling. Spacing between the transformers helps but the field at 60 Hz can reach quite a ways. I would still expect the channel with the OPT closer to the power transformer to have more hum.
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