Hello, I have a Jet city JCA100h 100w tube head amplifier that has some noise and a weird hum. The hum is present as soon as I put the standby on, the hum is very high (100Hz) and will gradually disappear until it is completely silent after about 10-15 minutes of normal use.
What I know:
With the master volume at 0 there is still some hum, and increase if I increase the master.
If I remove the PI 12ax7 the hum disappears, so the noise has to do with the preamp.
I verified that the heaters ac current is about 7v, above the recommended 6.3v. I'm using two 0.15r resistors in series to lower the voltage, but that didn't solve the hum problem.
What can cause this hum?
I've already checked all the solder joints, I can't find anything wrong, I'm thinking maybe to change all tube sockets to something with better quality, but I don't know if this will be the problem.
Sometimes I hear some noise like pops, it happens even without PI tube, but they are also amplified by the preamp. I heard it may have to do with charging the capacitors ... does that make any sense?
Some help please 🙂
What I know:
With the master volume at 0 there is still some hum, and increase if I increase the master.
If I remove the PI 12ax7 the hum disappears, so the noise has to do with the preamp.
I verified that the heaters ac current is about 7v, above the recommended 6.3v. I'm using two 0.15r resistors in series to lower the voltage, but that didn't solve the hum problem.
What can cause this hum?
I've already checked all the solder joints, I can't find anything wrong, I'm thinking maybe to change all tube sockets to something with better quality, but I don't know if this will be the problem.
Sometimes I hear some noise like pops, it happens even without PI tube, but they are also amplified by the preamp. I heard it may have to do with charging the capacitors ... does that make any sense?
Some help please 🙂
Hello from Greece! The 12ax7 tube is problematic! It has leak between cathode and filaments,when is cold, if goes hot,leak is lower,and the problem dissapear.Simply change the tube, and check.If the problem not dissapear,check all tubes of preamplifier!
Hi, thanks, this really makes sense, because the noise gets smaller when it gets hotter. I will test with new tubes, but I know that the problem is not just one tube, maybe it is just a bad design of these Chinese tubes that increase this problem ...
In China's defence, their valves are improving in quality and even Mullard had a 10% failure rate.
Mazda used to allow a 10% 'discount' when purchasing bulk valves, to cover for failures.
I have used and still do use, some Chinese branded ECC83s and have not had an issue.
If the heaters are over run then damage to the insulation may occur and become an issue.
Replace the ECC83, reduce the heater voltage and all will be fine.
Mazda used to allow a 10% 'discount' when purchasing bulk valves, to cover for failures.
I have used and still do use, some Chinese branded ECC83s and have not had an issue.
If the heaters are over run then damage to the insulation may occur and become an issue.
Replace the ECC83, reduce the heater voltage and all will be fine.
I was just referring to these tubes, I didn't mean that all Chinese tubes are poorly made.
There are tons of good Chinese equipment today.
The high voltage of the heaters I think is related to the AC voltage in Portugal, it should be about 230-240v but I have measured around 248-258v which probably makes the voltage rise in the secondary.
There are tons of good Chinese equipment today.
The high voltage of the heaters I think is related to the AC voltage in Portugal, it should be about 230-240v but I have measured around 248-258v which probably makes the voltage rise in the secondary.