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Old 23rd April 2011, 04:36 PM   #11
rknize is offline rknize  United States
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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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Old 23rd April 2011, 06:49 PM   #12
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Hello Russ,

The spacing between the PT and the OPT's are sufficient I think. Here's some pics:


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Old 23rd April 2011, 07:55 PM   #13
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Yeah that looks fine. As a last resort, you could disconnect the B+ from the OPT and pull the output tubes to rule out the OPT.

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Old 29th May 2011, 07:27 PM   #14
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Was there ever any resolution to this? I'm curious, just how audible was 6mVAC?
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Old 29th May 2011, 09:52 PM   #15
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No! Not yet. 6 mV if you have a two way 100 dB (woofer and 112 dB HF)speakers is a lot.
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Old 30th May 2011, 12:06 AM   #16
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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.
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Old 31st May 2011, 07:12 AM   #17
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Quote:
Originally Posted by sprinter View Post
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.
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
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Old 31st May 2011, 07:52 AM   #18
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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.
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Old 1st June 2011, 12:04 AM   #19
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Quote:
Originally Posted by DigitalJunkie View Post
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.
Thank you. I appreciate your input!


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Old 1st June 2011, 01:55 AM   #20
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Quote:
Or just taking it off the chassis,and using some short jumper wires to connect it back to the amp temporarily
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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