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In Reply to: Instantaneous hum posted by breadhead on April 5, 2007 at 04:08:33:
I listened carefully to the amp and these are my observations. The moment I turn on power there is a slight hum from the speakers. Once the tubes start to conduct there is an increase in the volume but it now sounds more like a buzz than a hum. I also noted that as I turn the volume control up the noise volume decreases. There is a point when the volume control is turned up to about a 7 out of 10 that the noise almost disappears. Hope this info helps.
Thanks,
Jim
Follow Ups:
> The moment I turn on power there is a slight hum from the speakers.Does it do this with the EL84s removed? Elsewhere in this thread, you said you had oriented the OPTs 90 degrees from the power transformer. You might consider that it's sometimes difficult to tell how the windings are positioned on the core. If you're not sure, unbolt one OPT and rotate it while listening to that channel only. Don't forget to also tilt it over toward the chassis to cover all the possible planes of orientation. You'll find out quickly if it's contributing to the hum.
> Once the tubes start to conduct there is an increase in the volume but it now sounds more like a buzz than a hum. I also noted that as I turn the volume control up the noise volume decreases. There is a point when the volume control is turned up to about a 7 out of 10 that the noise almost disappears.
This has several possible causes. First, be sure the case of the volume pot is grounded. Second, be sure one side of the 12AX7 filament (or the transformer CT) is grounded. If that's not the problem, try another preamp tube; some are noisy and simply can't be used in this position. Other potential grounding problems can cause this but are too numerous to cover in a short post.
Good luck!
With the EL84's removed I still get the slight hum when I power on but it no longer changes to a buzz. The hum remains the same. Thanks for your help.
Jim
...the wiring.I suggest taking ALL the signal earth points to one place (star wiring) and all the powersupply earths to another point - and then link them with a solid single wire.
Thanks Allen for the advice. I am currently using a 12ga. bus for ground.
Jim
To be pretty direct - so what?Something somewhere is sharing powersupply AC current with signal circuits, and the most likely cause is the earth wiring.
For instance ,if you have several amps of 60Hz AC for the heaters sharing the same piece of 12ga. as the input circuit you'll have hum big time - because even if the DC resistance of the 12ga. is only milliohms you could still be generating many millivolts of 60Hz (and higher harmonics) which end upsuperimposed on the input.
And as it changes when the amp warms up and can be (partly) nulled out with a particular setting of the volume control, it means to me 100% the presence of an unexpected earth loop.
Keep hunting!
Hello Allen, I was looking at the schematic again and I noticed that the signal input is not referenced to ground in any way. Does this mean that it floats in regards to earth ground? I am going over all my grounds and I do have the input grounded to the 12ga bus.
Thanks,
Jim
In the schematic it's (should be) directly referenced to gnd because it's automatically assumed by any experienced tech that the signal input would go between the top and bottom of the input pot.The schematic drawer has just been sloppy, or trying to be smart. Connect the gnd end of the input RCA to the gnd buss.
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If you are using a big thick buss wire, can you give us the sequence that the grounds attach to it with reference to the schematic.ideally, the input RCA gnd, the bottom of the pot and the mess of stuff hanging off the cathode of the first triode should be right at one end - and the center tap of the two traffo windings RIGHT at the other end. Near to the taps should be the gnds of the PS filter caps, with the caps etc off the output tubes cathodes between these two ends.
If you haven't got it like that - and if grounding the input signal doesn't solve it - then it's rewire time!
And as someone else pointed out, there is typo in that circuit. IMO, if the PR merchant had the time to mention it in his blurb, he would have had time to fix the schematic - more sloppyness!
But a comment - it's a crazily complex circuit for a nubee, someone very recently posted in this forum basically the same circuit but WITHOUT all the added BS hung off the cathodes, and that's what I'd suggest for a early project.
the schematic shows no ground for the RCA input.
Jim
concerning grounding schemes in tube amps, star grounding, bus, etc. So what I am going to do is exactly as you suggest and make sure the grounding is done properly. This will TAKE A WEEK OR SO BECAUSE OF MY WORK SCHEDULE BUT i WILL POST RESULTS AS SOON AS i DISCOVER THE PROBLEM. Thanks for all of your help!! This is probably the sixth or seventh amp I have built and I thought I had proper grounding technique but I must have missed something for sure. This is the best way for me to learn though. I need to do things. This amp really does sound real good so I want to get rid of this HUMMMMM!!!
Jim
Makes sense to me. I will do as you say and keep looking for a ground loop. Thanks for your help.
Jim
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