I was going to build one Alembic F-2B clone, but I was too lazy to build the enclosure, find the appropriate transformer, etc. and most of my DIY components are in another province. So, I decided to buy an assembled one from a local manufacturer.
It is terribly noisy. It has 3 noises: 1) very loud “ungrounded” noise, like touching the amplifier input with the finger, 2) 50Hz hum, 3) music from a radio station when the vol pot is at zero. It is unplayable
I returned it to the manufacturer, but he doesn’t want to assume any responsibility and he sent it back to me with a video showing how he uses the unit at his house without noise. He claims that it is my broken cables or my fault. It is not my cables!! I have triple checked continuity and used different cables that perfectly work without any noise at all. I had a discussion with him where he showed lack of professionalism.
The ground seems to be wired properly. I have verified continuity at all points. I do not observe ground loops. The ground is connected to the chassis through the input jack, and the output jack ground is not wired to the PCB.
Here is the circuit with the actual voltages that I have measured.
And the power supply. It runs with an external 12VAC 800mA adaptor. The transformer seems like a standard 220/12VAC transformer connected backwards. The capacitors are 22uF while the original circuit has 47uF. That could explain some hum but not the ungrounded noise.
I have disconnected the wire that feeds the grid of the second triode, to find which stage has the problem, and the noise is still there.
I have measured the output with the oscilloscope, with no signal at the input. I get 5.4mV, 48mV and 327mV with the Vol pot at 20%, 50% and 100%.
I have already checked ground continuity at all points many times. Any suggestion of what other test to conduct?
It is terribly noisy. It has 3 noises: 1) very loud “ungrounded” noise, like touching the amplifier input with the finger, 2) 50Hz hum, 3) music from a radio station when the vol pot is at zero. It is unplayable
I returned it to the manufacturer, but he doesn’t want to assume any responsibility and he sent it back to me with a video showing how he uses the unit at his house without noise. He claims that it is my broken cables or my fault. It is not my cables!! I have triple checked continuity and used different cables that perfectly work without any noise at all. I had a discussion with him where he showed lack of professionalism.
The ground seems to be wired properly. I have verified continuity at all points. I do not observe ground loops. The ground is connected to the chassis through the input jack, and the output jack ground is not wired to the PCB.
Here is the circuit with the actual voltages that I have measured.
And the power supply. It runs with an external 12VAC 800mA adaptor. The transformer seems like a standard 220/12VAC transformer connected backwards. The capacitors are 22uF while the original circuit has 47uF. That could explain some hum but not the ungrounded noise.
I have disconnected the wire that feeds the grid of the second triode, to find which stage has the problem, and the noise is still there.
I have measured the output with the oscilloscope, with no signal at the input. I get 5.4mV, 48mV and 327mV with the Vol pot at 20%, 50% and 100%.
I have already checked ground continuity at all points many times. Any suggestion of what other test to conduct?
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I'm not in to instrument amplifiers but it seems that all potmeters are in a high impedance circuit and should be shielded from any noise source (like the power supply). For starters you could simply solder a groundwire to each potmeter case. Next you should use heavily shielded (as close to 100% as you can find) signal wire. Miniature building asks for measures!
The power supply looks fine as does the schematic.
50HZ hum is an earth issue, not smoothing as smoothing/power supply is generally 100HZ.
Ensure there is a good connection between all the ground points and they all return to one point.
As it doesn't work properly, I would insist it is repaired by the maker.
50HZ hum is an earth issue, not smoothing as smoothing/power supply is generally 100HZ.
Ensure there is a good connection between all the ground points and they all return to one point.
As it doesn't work properly, I would insist it is repaired by the maker.
Have you tried to shield/separate the transformer of the tube?
That transformer could cause the hum you see it is certainly a bit too close to the tube
That transformer could cause the hum you see it is certainly a bit too close to the tube
The potentiometer cases are covered with plastic, and they are connected to ground through the chassis. Is it worth to remove that plastic and add extra wire soldered to the cases?
I am asking because I disconnected the circuit at the volume pot output, leaving only the second triode, and it was still noisy.
Only this was left:
I will try to find a metal sheet to insert between the transformer and the tube.
Thank you for all the suggestions! 🙂
The maker circuitfx.com is very stubborn and has no concern at all for customer care. He said that he could only return 70% of what I paid ($100). So I will try to fix it myself
I am asking because I disconnected the circuit at the volume pot output, leaving only the second triode, and it was still noisy.
Only this was left:
I will try to find a metal sheet to insert between the transformer and the tube.
Thank you for all the suggestions! 🙂
The maker circuitfx.com is very stubborn and has no concern at all for customer care. He said that he could only return 70% of what I paid ($100). So I will try to fix it myself
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Have you tried to shield/separate the transformer of the tube?
That transformer could cause the hum you see it is certainly a bit too close to the tube
That transformer could cause the hum you see it is certainly a bit too close to the tube
That preamp circuit is practically identical to the normal channel of the Fender AA864 Bassman, with the addition of the 10K Mid variable resistor instead of the 6.8K fixed resistor.
I wonder if following the original Fender layout grounding scheme for that amp would help? Just Google "AA864 Layout."
[I recognized it because I'm currently working on an AA864 build.]
I wonder if following the original Fender layout grounding scheme for that amp would help? Just Google "AA864 Layout."
[I recognized it because I'm currently working on an AA864 build.]
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Looks like a common issue as with any MI project. You are wiring signal ground from the input and output jacks to the board via shielded cable. You have unisolated jacks which short the ring of both jacks to a metal enclosure. It's a ground loop.
EDIT: Oops, just read with your post that it's not a ground loop, the output jack lead is actually snipped.
Still, that lack of shielding could be the issue in hum pickup. Suggest you investigate with a resistor to break the loop.
EDIT: Oops, just read with your post that it's not a ground loop, the output jack lead is actually snipped.
Still, that lack of shielding could be the issue in hum pickup. Suggest you investigate with a resistor to break the loop.
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You are right, sir! I've solved the problem with aluminum foil
I will add a proper tube shield when I get one
Thank you!
Yes, it is identical, with the addition of the Mid Tone Control. I've found that it is also similar to the Deluxe Reverb AA763
You can also reduce the stray magnetic field of the transformer by winding a one-turn closed loop copper foil around it, same width as the coil former. The winding should be over the iron core, not inside. The thicker the better, but you need to use a powerful soldering iron to close the loop with some overlap.You are right, sir! I've solved the problem with aluminum foil