[UntilThen]

1. Is the hum also audible from headphone out?

A good question. One that I want to know as well. I want to know how the whole chain is set up. Connections from source to preamp to amp and speakers.

 

[SonicTrance]

Hi again all,
Previously I have a Hum noise issue that was very audible from my speaker when the pre-amp is on.
I have tested various scenarios and tried many electric outlets in my house with no solutions avail.
If @SonicTrance or anyone here has a solution to this, please do let me know.
Cheers

Have you tried different tubes? I also want to know if this hum is present at the headphone XLR output as well.

 

[Capunk]

1. Is the hum also audible from headphone out?
2. Try to pull all conecting cables out -) RCA/XLR. Is it still humming?

1. Yes, low gain very minimal but presence from my hd800, high gain slightly more audibl.
2. Done this experiment, Oblivion disconnected from all sources except power cable, noise still came out from the headphone out.

Also done this,
I have replaced all the stock tubes, EL81 and 6AT6, also the XLR & power cable and placed the amp with no nearby electronics except the power outlet. The noise is still there

 

[Erwinatm]

1. Yes, low gain very minimal but presence from my hd800, high gain slightly more audibl.
2. Done this experiment, Oblivion disconnected from all sources except power cable, noise still came out from the headphone out.

Also done this,
I have replaced all the stock tubes, EL81 and 6AT6, also the XLR & power cable and placed the amp with no nearby electronics except the power outlet. The noise is still there

Could be ground loop, you can try :

- check your ground connection
- use power cable with no ground / floating. Try the floating ground cable also with your other eq : dac, streamer, cd etc . 1 equipment at a time OR
- plug it to different wall socket that has different MCB with the rest of your audio power source.

If it still there and loud, something is failing inside.

I see you have tried many options if the hum still persist then sonictrance is the man for you.

Hope this could help

 

[senseitedj]

If you are certain is a ground loop issue, and not an impedance or a tube-related problem, then getting this Ifi product might help.

Another fellow was having similar issue with his woo audio amp in another thread. This seems to have done the trick.

 

[baronbeehive]

1. Yes, low gain very minimal but presence from my hd800, high gain slightly more audibl.
2. Done this experiment, Oblivion disconnected from all sources except power cable, noise still came out from the headphone out.

Also done this,
I have replaced all the stock tubes, EL81 and 6AT6, also the XLR & power cable and placed the amp with no nearby electronics except the power outlet. The noise is still there

Is it OK with your power amp out of the chain, ie just Oblivion as power amp?

 

[Capunk]

Is it OK with your power amp out of the chain, ie just Oblivion as power amp?

Unfortunately i need that power amp to power my speakers, and i didn’t have the speaker out feature in my oblivion.
I’ll head back to the shop tomorrow and see if the problems still persist over there.

 

[BzzzzzT]

1. Yes, low gain very minimal but presence from my hd800, high gain slightly more audibl.
2. Done this experiment, Oblivion disconnected from all sources except power cable, noise still came out from the headphone out.

Also done this,
I have replaced all the stock tubes, EL81 and 6AT6, also the XLR & power cable and placed the amp with no nearby electronics except the power outlet. The noise is still there

As others have stated I could be a ground-loop. Do you have any audio equipment on the same circuit? I have also had this problem before. I have been using this brand of isolation transformers to lift the ground for several years. They also are better than a power conditioners IMO and I notice a slight improvement in sound.

UMI Isolation Transformers

 

[BzzzzzT]

I have seen writings (JACMusic) that tube coolers attached to the tube glass could affect the stability of the glass envelope because of different temperatures on the glass, leading to hairline cracks and eventual loss of vacuum. Don't have much experience with tube coolers so can't have an opinion. However, lots of old tubes have big paper stickers attached to them and they don't seem to do any harm. In the same vein I don't think that a little piece of scotch tape will damage the glass envelope.

I probably overreacted since the tube your using is a signal tube and the heat would be much lower than a power tube. The magnet could cause problems due to it being hotter under the magnet. But since this tube already has problems and the bulb temperature would be low, I would not think this would be too big a deal.

After reading Tomer's book, I would guess tube coolers could affect the glass in a negative way. The point where the metal meets the glass results in two different temperatures causing strain. This would cause differential cooling. Glass has a very low heat conductivity. A sharp heat gradient would develop at the point of contact with the colder metal and the warmer glass. This would cause strain. The worst thing for glass is bent pins Glass cold flows so it can develop strains that exceed it's elastic limit. Overtime the strain of something on the glass can cause cracks or failure. Glass can also develop degradation from really high voltage in certain situations, but this is not an issue with audio amps.

A really strong magnet could possibly cause the cathode sleeve to move as it is made of nickle to facilitate electrolysis with the barium oxide on the cathode to create emission. I don't think the anode or supports would move as it's fairly well secured and the heather would be unaffected as its made of tungsten with an aluminum oxide insulation covering the heater wires. I have no clue how the heater wires would be affected by a really strong magnet. I don't think there is much to worry about.

 

[BzzzzzT]

Would you mind describing how you do an interference test and how to interpret it?

This test was to check for electrostatic or electromagnetic interference. You just put a strong magnet in the vicinity close to the tube. If there is a change in amplitude in the superimposed signal that is being heard or seen on the scope, you would know that this is caused by this type of interference and not a tube defect like heater to cathode or heater to grid leakage for example. These tests were important because almost all the early electronics were designed by people without a formal education and many mistakes were often made in designs that could lead to interference. For example not having a grid stopper resistor to block radio frequencies or prevent oscillations. An example would be old Fender guitar amps. They did not have the college educated EEs that we have today. Many of the people who designed tube equipment during that time, were children of the great depression who lacked the means to pay for an education.

Robert Tomer the worlds foremost expert on tubes in service was also self-taught and did not receive a formal education. He talks about in his book how lots of the equipment used by Americans everyday was designed by servicemen who received their training during the war. This equipment often had design mistakes in their circuits.

As much of the equipment back in the day was designed by servicemen who got their electronics education from serving in the military during WW2 or through courses. It could have many unintentional errors or be very dangerous. Much of the US and the rest of the world was still being electrified. My family was an example of how most got an electronics education during that period. My Uncle Paul was a radioman during WW2 and received a bronze medal for heroism for repairing radio equipment when no others would, while taking direct fire from Nazi machine guns in France and saving thousands of lives as a result of bringing this equipment back online. He told me that electronics took his mind off the war, as it traumatized him. His skill from the service, later led him to become the chief railroad engineer for his region. My aunt's husband who we affectionately called Uncle Al, was the first TV and radio repairman in Arizona. Those old TVs and radios were crazy and he had to fix lots of design mistakes. He was often grumpy and short tempered from a lifetime of doing this. His favorite word was goddammit and he liked a stiff drink or two at the end of the day. I remember him teaching me to solder and my fondest memory of the time I spend with him was building an old battery operated Heathkit Code Oscillator under his guidance when I was 5. I would ride around on my trike doing some morse code he taught me, in an attempt to impress him. He was born in 1902 and was self-taught and got his first job as a telegraph operator.

 

[iFi audio]

If you are certain is a ground loop issue, and not an impedance or a tube-related problem, then getting this Ifi product might help.

Another fellow was having similar issue with his woo audio amp in another thread. This seems to have done the trick.

Thanks for mentioning our GND Defender. It's a simple device that will remove any noise related to ground loops, of course as long as they're the problem. But if a any buzz is heard on drivers of speakers or cans, it most likely is a ground loop

 

[mordy]

This test was to check for electrostatic or electromagnetic interference. You just put a strong magnet in the vicinity close to the tube. If there is a change in amplitude in the superimposed signal that is being heard or seen on the scope, you would know that this is caused by this type of interference and not a tube defect like heater to cathode or heater to grid leakage for example. These tests were important because almost all the early electronics were designed by people without a formal education and many mistakes were often made in designs that could lead to interference. For example not having a grid stopper resistor to block radio frequencies or prevent oscillations. An example would be old Fender guitar amps. They did not have the college educated EEs that we have today. Many of the people who designed tube equipment during that time, were children of the great depression who lacked the means to pay for an education.

Robert Tomer the worlds foremost expert on tubes in service was also self-taught and did not receive a formal education. He talks about in his book how lots of the equipment used by Americans everyday was designed by servicemen who received their training during the war. This equipment often had design mistakes in their circuits.

As much of the equipment back in the day was designed by servicemen who got their electronics education from serving in the military during WW2 or through courses. It could have many unintentional errors or be very dangerous. Much of the US and the rest of the world was still being electrified. My family was an example of how most got an electronics education during that period. My Uncle Paul was a radioman during WW2 and received a bronze medal for heroism for repairing radio equipment when no others would, while taking direct fire from Nazi machine guns in France and saving thousands of lives as a result of bringing this equipment back online. He told me that electronics took his mind off the war, as it traumatized him. His skill from the service, later led him to become the chief railroad engineer for his region. My aunt's husband who we affectionately called Uncle Al, was the first TV and radio repairman in Arizona. Those old TVs and radios were crazy and he had to fix lots of design mistakes. He was often grumpy and short tempered from a lifetime of doing this. His favorite word was goddammit and he liked a stiff drink or two at the end of the day. I remember him teaching me to solder and my fondest memory of the time I spend with him was building an old battery operated Heathkit Code Oscillator under his guidance when I was 5. I would ride around on my trike doing some morse code he taught me, in an attempt to impress him. He was born in 1902 and was self-taught and got his first job as a telegraph operator.

Thank you very much for your insightful answer and the historical perspective!

 

[SonicTrance]

Here's a Telemachus complete!
This one has the dual psu option, Yamamoto sockets, Mundorf caps, Khozmo attenuator, impedance switch, big VU meters and a nice (huge) old school looking volume knob!

 

[UntilThen]

What is the dual PSU option? Amp looks beautiful as usual.

230v. Is that bound for me?

 

[senseitedj]

Here's a Telemachus complete!
This one has the dual psu option, Yamamoto sockets, Mundorf caps, Khozmo attenuator, impedance switch, big VU meters and a nice (huge) old school looking volume knob!

very nice, what is the procedure to switch between the tube and SS PSU?

Do we have to turn the amp off and wait a few minutes for the tubes to power down? or could we just flip the PSU toggle switch when it is on? I am expecting that more likely the former is true.

Happy and excited for the lucky owner!