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Old 9th October 2012, 02:47 AM   #1
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Default My humming home made tube amp

Good evening everyone! Iím brand new to the forum and of course, Iím here because I need some troubleshooting help. My background in electronics is funny, I can solder, read schematics, make circuit boards, even re-cap an entire Seeburg Jukebox amp from the 1950ís. Knowing why each part is being replaced?? Havenít a clue. I can have an amp that hums like mad or doesnít work and after replacing all the old, leaking caps with new caps and replacing all the tubes with new tubes, it sounds amazing and doesnít make a sound when turned on. What was actually wrong with it, again, havenít a clue. So, hereís where Iím at. I built a sweet sounding tube amp about 20 years ago and it was as quite as can be and was so warm sounding. Here we are 20 years later and it has two issues that annoy me.

Issue 1: Regardless of the volume setting (independent volume controls are used), it has a low hum after the tubes warm up. Even with the inputs disconnected, itís there. Also, I never turned on the amp unless a pair of speakers were attached.

Issue 2: The amp has a secondary hum that is directly effected with the volume setting. No input hooked up but the higher the volume, the louder this new hum gets.

What gives!? The plug is a two prong plug, possibly a grounding issue? Please help or give opinions on why it may be doing this. All caps were new at the time and soldered well, point to point. Tubes are 4-Sovtek Russian el84ís (Each pair matched), 2- Yugaslavin smooth plate pinched base 12AX7 and one vintage low noise pre-amp 7025 Great Britin tube. Transformers are older units, not new but was always kept cool inside a closet when not in use. Any suggestions or am I being over picky or did I just forget that the hum was always there??
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Old 9th October 2012, 03:27 AM   #2
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A schematic would help.
Is it a push-pull amp?
tube rectification or solid state?
what kind of power supply did you build, caps, chokes, etc?
is the heater supply AC and if so is it center tapped, heater wires twisted?

Tube amplifiers will always have some level of hum, unless some SMPS is used (which gives the "hum" beyond audible range). Most of the time hum problems are related to heater supply and wiring (60hz hum) and power supply filtering (120hz buzz).
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Old 9th October 2012, 03:58 AM   #3
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Yikes! I don't have a schematic. But let me try to find one. The main setup was from a Baldwin organ.
It is a push-pull amp, tube rectification, the heaters are AC and twisted. From what I remember they are center tapped but not certain. I didn't install any chokes but used capacitors where the old ones were. I also scaled down the amp on a smaller chassis to save room. I was thinking about some filters, may try those. I will also try to find a schematic also. In reality, the hum isn't very loud, when music is playing, you almost don't notice it but it is there.
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Old 9th October 2012, 04:31 AM   #4
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I just noticed that it is "two prong plug" - it's a problem. Get your chassis grounded ASAP!
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Old 9th October 2012, 04:39 AM   #5
AJT is offline AJT  Philippines
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you can elevate the flaments to about 25% of B+ using voltage deviders...

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Old 9th October 2012, 04:50 AM   #6
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I will get a better cord with a ground pin and ground the chassis to it. In addition, what would the chances of one if the 12AX7's being bad or "loose"? Could I power up the amp with no volume and no line signal and remove one 12AX7 at a time without hurting the amp? Just thinking out loud. The last amp I rebuilt is a Seeburg MRA6-L4 and she's as quite as a sleeping baby but really blows the roof off when the needle hits the acetate. Only problem now is that the mad scientist who taught me how to repair these things is no longer around. I guess he should have taught me theory before repair.....
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Old 9th October 2012, 04:59 AM   #7
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I need to pop open the bottom and take a picture of it. Something tells me I may have added a pair of resistors in series across the heater power. But your saying give it more power? That would be lowering the resistance of the pair of resistors right? Like I said, it's been awhile and forgot a lot since high school electronics class.
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Old 9th October 2012, 10:31 AM   #8
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Default humming issues

Quote:
Originally Posted by MrWindUp View Post
Good evening everyone! Iím brand new to the forum and of course, Iím here because I need some troubleshooting help. My background in electronics is funny, I can solder, read schematics, make circuit boards, even re-cap an entire Seeburg Jukebox amp from the 1950ís. Knowing why each part is being replaced?? Havenít a clue. I can have an amp that hums like mad or doesnít work and after replacing all the old, leaking caps with new caps and replacing all the tubes with new tubes, it sounds amazing and doesnít make a sound when turned on. What was actually wrong with it, again, havenít a clue. So, hereís where Iím at. I built a sweet sounding tube amp about 20 years ago and it was as quite as can be and was so warm sounding. Here we are 20 years later and it has two issues that annoy me.

Issue 1: Regardless of the volume setting (independent volume controls are used), it has a low hum after the tubes warm up. Even with the inputs disconnected, itís there. Also, I never turned on the amp unless a pair of speakers were attached.

Issue 2: The amp has a secondary hum that is directly effected with the volume setting. No input hooked up but the higher the volume, the louder this new hum gets.

What gives!? The plug is a two prong plug, possibly a grounding issue? Please help or give opinions on why it may be doing this. All caps were new at the time and soldered well, point to point. Tubes are 4-Sovtek Russian el84ís (Each pair matched), 2- Yugaslavin smooth plate pinched base 12AX7 and one vintage low noise pre-amp 7025 Great Britin tube. Transformers are older units, not new but was always kept cool inside a closet when not in use. Any suggestions or am I being over picky or did I just forget that the hum was always there??
yea man change that plug first man that could easily be source of your humming issues
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Old 9th October 2012, 12:01 PM   #9
jrenkin is offline jrenkin  United States
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If it used to be silent, look into what else may be on the house power circuit, like dimmers, compact fluorescent lights and such. Those can add humming, but will probably go away if you update the plug, too.
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Old 9th October 2012, 08:22 PM   #10
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Could easily be PSU ripple.
20 years is long enough for an electrolytic to fail..
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