Hi,
I have a tube preamp that I've built 20 years ago. It recently developed a weird hum/ noise behavior on the right channel, after turning on for a while (sometimes, 5 mins, sometimes 2 hours after on). The noise is varied based on volume. I took it to the company that sold the kit but they also can't figure it out. They said that it is normal for tube since it is sensitive..... (they tried swapping the tubes but can't replicate the problem...)
I usually do not listen loud however I turn on volume quite a bit to about 12 o clock since I connect a dac with remote to it and use dac's remote for main volume control. It is quite annoying when the noise kicks in.
This is what I have done so far for diagnostic;
1. Check chassis ground and connect chassis ground to house's ground. Originally, the preamp is 2-prong so I change it to 3-prong.
2. There are three boards, source selector board -> balance & volume -> preamp board. I tried shorting signal to ground at the output of balance+volume board, the noise is gone.
3. I tried disconnect the preamp board from all inputs, the hum/noise is very loud. Then, I tried shorting signal to ground at the input of preamp board, the noise is gone.
Please suggest what else should I check.
Thanks,
AP
I have a tube preamp that I've built 20 years ago. It recently developed a weird hum/ noise behavior on the right channel, after turning on for a while (sometimes, 5 mins, sometimes 2 hours after on). The noise is varied based on volume. I took it to the company that sold the kit but they also can't figure it out. They said that it is normal for tube since it is sensitive..... (they tried swapping the tubes but can't replicate the problem...)
I usually do not listen loud however I turn on volume quite a bit to about 12 o clock since I connect a dac with remote to it and use dac's remote for main volume control. It is quite annoying when the noise kicks in.
This is what I have done so far for diagnostic;
1. Check chassis ground and connect chassis ground to house's ground. Originally, the preamp is 2-prong so I change it to 3-prong.
2. There are three boards, source selector board -> balance & volume -> preamp board. I tried shorting signal to ground at the output of balance+volume board, the noise is gone.
3. I tried disconnect the preamp board from all inputs, the hum/noise is very loud. Then, I tried shorting signal to ground at the input of preamp board, the noise is gone.
Please suggest what else should I check.
Thanks,
AP
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The fact that the problem vanishes when you short the input of the noisy channel might indicate something is wrong with the grid leak.
What if you short the input of the balance+volume board?
Have you tried swapping the tubes between channels?
Have you tried swapping the tubes between channels?
3 prong is good and safe but a direct connection of Audio GND to PE is not. I take you also connected Audio GD to chassis, then there is a high chance for ground loops. Do the 10 Ohm or diode bridge trick to refer to PE. If chassis is not connected to Audio GND then a direct connection of the chassis to PE of course is OK.
Thanks. Could you elaborate more of this? You meant my grid is leaked to ground?.. If solder is oxidized, I can just resolder, or maybe try to clean the tube connection?
Is there a way to confirm the issue before I start messing around?
It is single tube. I did try to short input of the balance volume board and it is not quiet. So, I thought it is the problem, but when I disconnect the volume-balance board, the noise is still there.
I need to recheck that I have successfully separate the audio ground from chassis ground. I believe I did, but need recheck.
The Magnet VL-3 is a hybrid vacuum tube stereo line-amplifier. It uses JFET at the input stage
and vacuum tube at the output stage. This combination helps in long life and better music quality. The
VL-3 has CD, Tuner, Video, Tape and Aux inputs. The output stage uses a cathode follower vacuum
tube which makes a low output impedance. It has a three-part regulated power supply which reduces
the modulation distortion, the circuit uses polypropylene capacitor, REL - cap Wonder cap. Monster
cable and 1 % metal film restorers.
ELECTRONICS Australia, May 1994
Where does the hum come from? From JFET or output tube? Are you able to check at JFET output when the hum appeared?
and vacuum tube at the output stage. This combination helps in long life and better music quality. The
VL-3 has CD, Tuner, Video, Tape and Aux inputs. The output stage uses a cathode follower vacuum
tube which makes a low output impedance. It has a three-part regulated power supply which reduces
the modulation distortion, the circuit uses polypropylene capacitor, REL - cap Wonder cap. Monster
cable and 1 % metal film restorers.
ELECTRONICS Australia, May 1994
Where does the hum come from? From JFET or output tube? Are you able to check at JFET output when the hum appeared?
To answer your question, locate the grid leak resistor look at the copper side and resolder both ends. The problem has to be there somewhere because as you said the noise disappeared when you shorted the input.
I think the operation is quite innocent so don't be afraid of 'messing around' - very little can go wrong.
If you have a dvm check the value; somewhere from 100k to 470k. Problem is that if it's broken it can still measure ok the moment both parts are somewhat pressed together. This can easily be checked by putting on the amp and knock on it with a screwdriver. Your speaker will certainly react.
I think the operation is quite innocent so don't be afraid of 'messing around' - very little can go wrong.
If you have a dvm check the value; somewhere from 100k to 470k. Problem is that if it's broken it can still measure ok the moment both parts are somewhat pressed together. This can easily be checked by putting on the amp and knock on it with a screwdriver. Your speaker will certainly react.
I haven't have chance to dig deep in this yet (was having fun with building a chip amp). However, I retest some settings and this is the result
1. I swap power amps to another one. Noise remains, so it must be the preamp.
2. I tried to locate the grid leak R but not sure yet. All resistors around the tube are measured as it should (in circuit).
3. The noise is raised when input is open circuit (source disconnected). There are 4 inputs in this amp, all behave the same.
4. Noise varies by volume. However, when go to max, noise disappear (only when source connected, if source is disconnected, the noise is max).
5. There is quiet hum.
6. There were cross talk between different inputs when more than one source is connected.
I guess it might be also grounding problem as well?
Also, when I check this, I kinda ashamed of my wiring back then. It is quite messy. Maybe I should redo all the wiring, remove tape mon and remove all other inputs (now I use only one input from DAC). Also, I should separate signal ground and earth...
What do you think?
1. I swap power amps to another one. Noise remains, so it must be the preamp.
2. I tried to locate the grid leak R but not sure yet. All resistors around the tube are measured as it should (in circuit).
3. The noise is raised when input is open circuit (source disconnected). There are 4 inputs in this amp, all behave the same.
4. Noise varies by volume. However, when go to max, noise disappear (only when source connected, if source is disconnected, the noise is max).
5. There is quiet hum.
6. There were cross talk between different inputs when more than one source is connected.
I guess it might be also grounding problem as well?
Also, when I check this, I kinda ashamed of my wiring back then. It is quite messy. Maybe I should redo all the wiring, remove tape mon and remove all other inputs (now I use only one input from DAC). Also, I should separate signal ground and earth...
What do you think?
From the circuit description I just read it appears there is a fet at the input although I haven't seen it on the board. So you should read gate resistor instead of grid leak. I don't know what the value of the input resistor is, presumably somewhere from 47k- 470k. Possibly the underside of the gate resistor of the bad channel is not grounded. You can check this by connecting one probe of your meter with the "hot"side of one of your inputs and the other probe to ground. If the meter shows a very high value you have found the cause.
Another possibility is something is wrong with the input fet itself.
Another possibility is something is wrong with the input fet itself.
I measure the grid leak resistor by removing the tube and measure grid (pin 2, 7) to ground resistance, which read 5.6k... They are the big resistors next to tube. I try knocking it, there's no change in noise/hiss level.
Just a note on the pin numbers...
They are numbered from the underside side of the socket, clockwise from the space. If you are on the 'top' of the socket you have to go anti-clockwise from the space. So the pin you have the red meter lead on is actually pin 3. (The cathode resistors.)
They are numbered from the underside side of the socket, clockwise from the space. If you are on the 'top' of the socket you have to go anti-clockwise from the space. So the pin you have the red meter lead on is actually pin 3. (The cathode resistors.)
I'm asking the Magnet company to give me one. I've lost the schematic over the year...
In the mean time, I experiment by replacing the short shielded cable between the volume/balance board to pre-amp board input. When I changed to loose twisted pair, the noise is very loud since low volume. The twisted wire reacts to my hand, as when I move closer, the noise intensifies. When I changed to a different shielded cable, the noise is reduced to similar level as before.
Is this oscillation? or maybe bad grounding? I don't have scope to verify this.