Tweet
Hi All. I'm back here after a 13-year absence looking to fix a hum issue that has plagued my amp build since I stopped working on it back in 2008. At that time I only really noticed excessive hum after closing everything up, taking the amp out from under the 14 fluorescent tubes just above my workbench and hooking it up to a speaker with 100+db sensitivity (8 ohm Eminence Red Fang 12") in a quiet environment. But then the hum was very present and annoying.
I've finally found the time to dive back into my amp and have done some initial testing to get a feel for things as they now stand. My plan is to do whatever is necessary to get this amp quiet enough to be enjoyable.
The amp's configuration is Fender brownface and blackface preamp/effect circuits with a output stage of one SE 6V6 or two 6V6s in parallel (non push-pull), cathode biased. I'll skip most of the amp's details for now since the hum is apparently coming from some interaction between the power supply and the output stage.
Here is how the big components are positioned relative to each other. Preamp tubes at front left, 3 tremolo tubes in the middle, and reverb (tubes and transformer) at the right. Choke in the middle. Rectifier and 6v6's at the rear.
If I pull all tubes except the rectifier tube and the power tube(s), the hum is present and quite audible. Here is how it looks on my scope when reading across an 8 ohm output load resistor :
One 6V6 :
The overall cycle of what repeats is 120 hz (4 x 2ms/div) (vertical 5mv/div). If I play around with my heater balance pot, any additional ripple just affects (slightly) the regularity of this 120 hz repeat. It looks like the power supply wants to be pretty smooth (the flatter parts of the trace) but is interrupted by a 120 hz pulse in only one polarity. But I may be reading too much into that since I can get the waveform to change just by tapping lightly on my amp. The trace usually locks to the shape shown, but things aren't completely stable. I've heard of microphonic tubes, but it looks like my whole amp is microphonic to the slightest tap of a chopstick, especially on a tube but really anywhere except the power transformer. All this at finest resolution of my scope which is an old used Kikusui which has seen a lot of use.
The hum is quite a bit louder with two power tubes compared to one. And with the rest of the amp operational, everything signal-wise rides on top of this hum.
Two 6V6s :
When designing and building my amp I followed pretty much to the letter the "star ground" scheme described by Randall Aiken (Aiken amplification) in his article Grounding (https://www.aikenamps.com/index.php/grounding). Isolated jacks and all. Really more of a 'tree' scheme than a star. I also took note of Dai H.'s grounding info found in the thread http://ampgarage.com/forum/viewtopic.php?t=220. I just recently read Merlin Blencowe's "Grounding and Ground Schemes" (http://www.valvewizard.co.uk/Grounding.pdf) which seems to be the opposite of Aiken in that all grounds flow to the input jack end instead of the power supply end of things.
I'll be reconsidering the choices I made with respect to wiring and component layout, especially the grounding scheme, but was wondering if anything comes to mind from the shape of the hum waveform as seen on my scope. Especially that it only involves the power supply and output stage. I've found one very similar scope trace on the net but the accompanying discussion didn't clarify the situation producing such a trace. I've seen the big electromagnetic parts much closer together than mine on other amps. However, my first filter stage (2 x 20uf) is right under the power transformer and the little mixer board feeding the power amps is more or less under the choke. Chassis is pretty heavy gauge steel.
I've finally found the time to dive back into my amp and have done some initial testing to get a feel for things as they now stand. My plan is to do whatever is necessary to get this amp quiet enough to be enjoyable.
The amp's configuration is Fender brownface and blackface preamp/effect circuits with a output stage of one SE 6V6 or two 6V6s in parallel (non push-pull), cathode biased. I'll skip most of the amp's details for now since the hum is apparently coming from some interaction between the power supply and the output stage.
Here is how the big components are positioned relative to each other. Preamp tubes at front left, 3 tremolo tubes in the middle, and reverb (tubes and transformer) at the right. Choke in the middle. Rectifier and 6v6's at the rear.
If I pull all tubes except the rectifier tube and the power tube(s), the hum is present and quite audible. Here is how it looks on my scope when reading across an 8 ohm output load resistor :
One 6V6 :
The overall cycle of what repeats is 120 hz (4 x 2ms/div) (vertical 5mv/div). If I play around with my heater balance pot, any additional ripple just affects (slightly) the regularity of this 120 hz repeat. It looks like the power supply wants to be pretty smooth (the flatter parts of the trace) but is interrupted by a 120 hz pulse in only one polarity. But I may be reading too much into that since I can get the waveform to change just by tapping lightly on my amp. The trace usually locks to the shape shown, but things aren't completely stable. I've heard of microphonic tubes, but it looks like my whole amp is microphonic to the slightest tap of a chopstick, especially on a tube but really anywhere except the power transformer. All this at finest resolution of my scope which is an old used Kikusui which has seen a lot of use.
The hum is quite a bit louder with two power tubes compared to one. And with the rest of the amp operational, everything signal-wise rides on top of this hum.
Two 6V6s :
When designing and building my amp I followed pretty much to the letter the "star ground" scheme described by Randall Aiken (Aiken amplification) in his article Grounding (https://www.aikenamps.com/index.php/grounding). Isolated jacks and all. Really more of a 'tree' scheme than a star. I also took note of Dai H.'s grounding info found in the thread http://ampgarage.com/forum/viewtopic.php?t=220. I just recently read Merlin Blencowe's "Grounding and Ground Schemes" (http://www.valvewizard.co.uk/Grounding.pdf) which seems to be the opposite of Aiken in that all grounds flow to the input jack end instead of the power supply end of things.
I'll be reconsidering the choices I made with respect to wiring and component layout, especially the grounding scheme, but was wondering if anything comes to mind from the shape of the hum waveform as seen on my scope. Especially that it only involves the power supply and output stage. I've found one very similar scope trace on the net but the accompanying discussion didn't clarify the situation producing such a trace. I've seen the big electromagnetic parts much closer together than mine on other amps. However, my first filter stage (2 x 20uf) is right under the power transformer and the little mixer board feeding the power amps is more or less under the choke. Chassis is pretty heavy gauge steel.
Attached Files
Comment