Hi,
I just starting up a new tube 100 watt guitar amp design/build. As I am having issues with hum and I want to carefully examine the the power supply.
The the design is a center tap to ground from the transformer and the HT through a bridge rectifier filtered by two 150 uf caps in series.
The loaded HT B+ voltage is 480 VDC. The ripple is 5.19 VAC RMS. Less than 2%.
I have next to no ripple past the B+ tap. I don't really think that small amount of ripple is causing issues but I am not sure.
Any ideas?
Thanks,
Billy
I just starting up a new tube 100 watt guitar amp design/build. As I am having issues with hum and I want to carefully examine the the power supply.
The the design is a center tap to ground from the transformer and the HT through a bridge rectifier filtered by two 150 uf caps in series.
The loaded HT B+ voltage is 480 VDC. The ripple is 5.19 VAC RMS. Less than 2%.
I have next to no ripple past the B+ tap. I don't really think that small amount of ripple is causing issues but I am not sure.
Any ideas?
Thanks,
Billy
That's very reasonable ripple, so instead look for grounding problems in your layout.
Can you post a photo?
That is fine, especially if all it goes to is the OT center tap. The push pull output stage will cancel it.
Hum is not generic, it comes from many places, and each place has its own cure.
Is it 60Hz or 120Hz - use your scope. 120Hz ripple is from power supply. It can be ripple currents in the ground wiring rather than just poor caps.
Many tube amp bias supplies are half wave - not usually a problem because very small current is used. But insufficient filtration there will result in 60Hz hum.
Isolate the problem. Pull the tubes one by one from the input end. Does the hum go away at any point? If it still hums with the phase inverter pulled, the output stage has the hum.
Does it hum with all the tubes removed? If so, the transformers are probably coupling. The power transformer and output transformer should be mounted with their laminations at a 90 degree angle. That is why amps like Fender Twin Reverbs have the power transformer mounted flat and the OT mounted vertically.
Do ANY controls affect the hum in ANY way at all? Level, tone, whatever. Controls that affect it are after its source. Controls with no change are either before the source or not in the signal path.
Hum is not generic, it comes from many places, and each place has its own cure.
Is it 60Hz or 120Hz - use your scope. 120Hz ripple is from power supply. It can be ripple currents in the ground wiring rather than just poor caps.
Many tube amp bias supplies are half wave - not usually a problem because very small current is used. But insufficient filtration there will result in 60Hz hum.
Isolate the problem. Pull the tubes one by one from the input end. Does the hum go away at any point? If it still hums with the phase inverter pulled, the output stage has the hum.
Does it hum with all the tubes removed? If so, the transformers are probably coupling. The power transformer and output transformer should be mounted with their laminations at a 90 degree angle. That is why amps like Fender Twin Reverbs have the power transformer mounted flat and the OT mounted vertically.
Do ANY controls affect the hum in ANY way at all? Level, tone, whatever. Controls that affect it are after its source. Controls with no change are either before the source or not in the signal path.
Thanks guys..I assumed as much on the ripple.
The hum is 60hz. It gets less as I remove preamp tubes and goes away completely when I remove the inverter tube.
The amp has six 12AX7 and one 12AT7 in the reverb. I ran the preamp heaters DC and the four output tubes AC. I did not twist the preamp heater wires and they lay beside the tube sockets.All the voltages on the tubes are very close to what I predicted they would be.
I will continue recording all the values involved with the power supply and check the cap on the bias.
Yes, the hum increases with the gain and volume controls. The hum is very loud at high gain and volume. There are a couple of other issues that are not normal. Every component I tap on in the preamp section is microphonic. I have never seen that in any amp. I assume something is really screwed up.
Having said that the amp actually plays and sounds more or less normal with clean looking signals on my scope except for loud hum and gain and volume control which is very strange.
I need to revise the schematic so someone else can read them because I made some changes to the power supply. I will post them tomorrow.
BTW, thanks for the help as I really need it on this one. I am in the deep end of the pool and in over my head...lol
Thanks,
Billy
The hum is 60hz. It gets less as I remove preamp tubes and goes away completely when I remove the inverter tube.
The amp has six 12AX7 and one 12AT7 in the reverb. I ran the preamp heaters DC and the four output tubes AC. I did not twist the preamp heater wires and they lay beside the tube sockets.All the voltages on the tubes are very close to what I predicted they would be.
I will continue recording all the values involved with the power supply and check the cap on the bias.
Yes, the hum increases with the gain and volume controls. The hum is very loud at high gain and volume. There are a couple of other issues that are not normal. Every component I tap on in the preamp section is microphonic. I have never seen that in any amp. I assume something is really screwed up.
Having said that the amp actually plays and sounds more or less normal with clean looking signals on my scope except for loud hum and gain and volume control which is very strange.
I need to revise the schematic so someone else can read them because I made some changes to the power supply. I will post them tomorrow.
BTW, thanks for the help as I really need it on this one. I am in the deep end of the pool and in over my head...lol
Thanks,
Billy
The microphonics are probably in one spot. Everything in there is physically linked. it is very hard to tap on something and not have the tap vibration spread to the rest of the board. Imagine a live mic on a stand. You tap the shaft, and hear it, you kick the base and hear it, you tap the mic holder and hear it, but when you get to the actual diaphragm area on the front, you REALLY hear it. I suspect if you look around you will find one thing that is extra sensitive.
The earliest gain control that controls hum level is after the source. Obviously any even later controls would also be after it. This isn't mysterious. If you put hum into the input jack - instead of guitar - the volume controls would turn it up and down.
If your bias supply is clean, then voltages are not likely to help find hum. Hum doesn;t happen because B+ is 300v instead of 330v or whatever.
Hum can easily come from more than one source. So if something reduces hum by half, then you fixed one source and have another left.
Is this more or less some commercial circuit? Like Twin Reverb? Your tube complement sounds like one. If it is close to a commercial model, we can use the schematic. Or post your working schematic.
One of the most common causes of hum is poor ground arrangement. The schematic won't show that. SO if the various circuit points are grounded at their sockets, or all on a star to one point, we need to know.
The earliest gain control that controls hum level is after the source. Obviously any even later controls would also be after it. This isn't mysterious. If you put hum into the input jack - instead of guitar - the volume controls would turn it up and down.
If your bias supply is clean, then voltages are not likely to help find hum. Hum doesn;t happen because B+ is 300v instead of 330v or whatever.
Hum can easily come from more than one source. So if something reduces hum by half, then you fixed one source and have another left.
Is this more or less some commercial circuit? Like Twin Reverb? Your tube complement sounds like one. If it is close to a commercial model, we can use the schematic. Or post your working schematic.
One of the most common causes of hum is poor ground arrangement. The schematic won't show that. SO if the various circuit points are grounded at their sockets, or all on a star to one point, we need to know.
Hi Guys,
I found the major hum issue. I had screw up the bias circuit. There is still more hum than I would like which after some investigation is coming from poor filtration on the DC heater supply. Well..that is at least one issue. The microphonics went away when I fix the bias issue.
This is basicly a EVH 5150 second channel combined with a old Bassman clean channel with a tube reverb and effects loop. There may be a need to build a different power supply that is better regulated.
I have schematics that we have drawn but they are not up to date with the changes we have made.
The whole thing is pretty experimental at the moment.
Thanks,
Billy
BTW....I would like to see a schematic of what "Galatic Grounding" actually is.
I found the major hum issue. I had screw up the bias circuit. There is still more hum than I would like which after some investigation is coming from poor filtration on the DC heater supply. Well..that is at least one issue. The microphonics went away when I fix the bias issue.
This is basicly a EVH 5150 second channel combined with a old Bassman clean channel with a tube reverb and effects loop. There may be a need to build a different power supply that is better regulated.
I have schematics that we have drawn but they are not up to date with the changes we have made.
The whole thing is pretty experimental at the moment.
Thanks,
Billy
BTW....I would like to see a schematic of what "Galatic Grounding" actually is.
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That's an individual's name for hierarchical grounding. Each circuit block has a local common,
which are then connected together. It's a better alternative to "star" grounding, which isn't even
physically possible in most cases. https://interferencetechnology.com/wp-content/uploads/2011/11/Figure10.jpg
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