Hi all..
I have built the 5f1 champ, It has a hum that gets louder when you turn it up, then disappears when full volume. I have grounded as follows ground buss all preamp from filter cap to input jacks these jacks are lifted then I have a heavy wire going to a star ground that has the remaining caps, filament CT and HT CT. This star ground point is a transformer lug. the mains earth is on a different lug.
I have done a lot of reading but cant work out what to do next.
Thanks
Luke
I have built the 5f1 champ, It has a hum that gets louder when you turn it up, then disappears when full volume. I have grounded as follows ground buss all preamp from filter cap to input jacks these jacks are lifted then I have a heavy wire going to a star ground that has the remaining caps, filament CT and HT CT. This star ground point is a transformer lug. the mains earth is on a different lug.
I have done a lot of reading but cant work out what to do next.
Thanks
Luke
There are MANY sources of hum, not just grounding.
Isolate the problem.
Is the hum 50/60Hz or is it 100/120Hz? (Are your mains running 50Hz or 60Hz?) the lower frequency hums come from things like grounding issues, radiated hum, heater coupling, etc. The 100/120Hz hum is power supply ripple hum, which can comne from poor filtration or indeed improper grounding of filter caps. KNowing whwich you have helps us.
Hum is not generic, in other words each source of hum has its own cures, and each cure only helps its particular type of hum. For example, if your heaters are causing hum, increasing the filter caps in the power supply will not help.
Leakage in the output transformer can cause hum. Pull the 6V6. The amp should go utterly silent. Did it?
Put the 6V6 back in, but pull the 12AX7. Hum still or not?
Have you tried other tubes? Any tube can cause hum.
Are you sure the output transformer is wired in proper phase? Disconnect the negative feedback and see if the hum drops. I refer to the 22k resistor from the speaker connection back to the preamp tube. Take its wire off the speaker jack. ANy help?
Want to get fancy? Disconnect the heater wires from the 12AX7 socket, and power the tube heater from a battery for test. ANy difference in hum?
It is normal for a simple single ended amp like this to hum a little. Without the hum cancelling attributes of a push pull final stage, B+ ripple leaks through. But check your B+ for ripple, either scope it or set your meter to AC volts and check the B+ at each filter cap. How much ripple is there at each of the three points?
It is possible your amp is oscillating at RF. The audible result of that is often just hum.
The all the way up stops the hum thing suggest a couple things. You could have more than one source of hum working here. One is controlled by the volume pot. The other not. And the two hums are out of phase (opposite polarity). As you turn up the one, the other one can cancel it due to the phasing. Hey, it happens.
SO those are some ideas. It may be ground related, but don;t overlook all the other possibilities.
Isolate the problem.
Is the hum 50/60Hz or is it 100/120Hz? (Are your mains running 50Hz or 60Hz?) the lower frequency hums come from things like grounding issues, radiated hum, heater coupling, etc. The 100/120Hz hum is power supply ripple hum, which can comne from poor filtration or indeed improper grounding of filter caps. KNowing whwich you have helps us.
Hum is not generic, in other words each source of hum has its own cures, and each cure only helps its particular type of hum. For example, if your heaters are causing hum, increasing the filter caps in the power supply will not help.
Leakage in the output transformer can cause hum. Pull the 6V6. The amp should go utterly silent. Did it?
Put the 6V6 back in, but pull the 12AX7. Hum still or not?
Have you tried other tubes? Any tube can cause hum.
Are you sure the output transformer is wired in proper phase? Disconnect the negative feedback and see if the hum drops. I refer to the 22k resistor from the speaker connection back to the preamp tube. Take its wire off the speaker jack. ANy help?
Want to get fancy? Disconnect the heater wires from the 12AX7 socket, and power the tube heater from a battery for test. ANy difference in hum?
It is normal for a simple single ended amp like this to hum a little. Without the hum cancelling attributes of a push pull final stage, B+ ripple leaks through. But check your B+ for ripple, either scope it or set your meter to AC volts and check the B+ at each filter cap. How much ripple is there at each of the three points?
It is possible your amp is oscillating at RF. The audible result of that is often just hum.
The all the way up stops the hum thing suggest a couple things. You could have more than one source of hum working here. One is controlled by the volume pot. The other not. And the two hums are out of phase (opposite polarity). As you turn up the one, the other one can cancel it due to the phasing. Hey, it happens.
SO those are some ideas. It may be ground related, but don;t overlook all the other possibilities.
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