There would still be a voltage present..?? I think...maybe injecting a little noise into your b+? I dunno..
Onto the RCA plugs..
The one 300B amp we built....was humming pretty bad until we isolated the rca plugs from the chassis.
How can you have humming with the rectifier pulled?.. a ground loop in your system could cause that.
kmtang,
Yes, it still hums when there is nothing connected to the RCA's
Non of the sources I use have earth cables. The only earth comes through the RCA leads.
Yes, it still hums when there is nothing connected to the RCA's
Non of the sources I use have earth cables. The only earth comes through the RCA leads.
I have found an AC signal (4.1v) between the earth cable and the neutral cable on my mains supply.
Could this cause any problem?
Could this cause any problem?
BTW,
The RCA's are grounded to the chassis, but the screened cable to the volume pot is only grounded at the RCA end, so there's no loop there.
The RCA's are grounded to the chassis, but the screened cable to the volume pot is only grounded at the RCA end, so there's no loop there.
Can't be the earth/neutral signal.
I just disconnected the mains earth lead from the chassis and it still hums.
Looks like I might have to move some grounds then!
I just disconnected the mains earth lead from the chassis and it still hums.
Looks like I might have to move some grounds then!
Hi,
I had a similar problem and also what i did was to check ground connections. In the end it turned out to be the power trafo that was coupling noise to my OT. Just had to move it away a few centimeters and the hum was gone. But your layout seems correct so i don't know if this is your problem.
Al.
I had a similar problem and also what i did was to check ground connections. In the end it turned out to be the power trafo that was coupling noise to my OT. Just had to move it away a few centimeters and the hum was gone. But your layout seems correct so i don't know if this is your problem.
Al.
2a3 hum
I have built a few amps using both a star ground technique and a heavy ground bus grounded at the input. I have been able to get hum down to less than 2mv (using ac fil) in every case, however a few things I have noticed that will help reduce hum:
1. Play with polarity of the ht ac leads (a Gordon Rankin tip) - usually one direction will show a little less hum.
2. Try using an external fil trans for 2a3 to see if you are coupling back through the main xformer.
3. The polarity of primary winding on the output trans. I had the wrong color scheme for transformer leads on one amp - I was having trouble with hum - wired the output trans correctly and the hum dropped to about one third of its previous level.
4. Try a different 2a3 (probably a long shot).
I think it is a good idea to put a resistor from center tap to each leg of the hum put (22-47 ohm) so that the humpot functions as a very fine adjustment, often the difference between noisey and quiet is a very small movement of the pot.
Good luck and enjoy your amps.
Jim
I have built a few amps using both a star ground technique and a heavy ground bus grounded at the input. I have been able to get hum down to less than 2mv (using ac fil) in every case, however a few things I have noticed that will help reduce hum:
1. Play with polarity of the ht ac leads (a Gordon Rankin tip) - usually one direction will show a little less hum.
2. Try using an external fil trans for 2a3 to see if you are coupling back through the main xformer.
3. The polarity of primary winding on the output trans. I had the wrong color scheme for transformer leads on one amp - I was having trouble with hum - wired the output trans correctly and the hum dropped to about one third of its previous level.
4. Try a different 2a3 (probably a long shot).
I think it is a good idea to put a resistor from center tap to each leg of the hum put (22-47 ohm) so that the humpot functions as a very fine adjustment, often the difference between noisey and quiet is a very small movement of the pot.
Good luck and enjoy your amps.
Jim
bobhayes said:
Hmm (no pun, just thinking), if the rectifier and driver tubes are pulled out, then the only current flowing in the system is the current for the heater supplies. This assumes that the caps have discharged. In this case, somehow there is coupling between the power tranny or wiring and output trannies? I would think that a ground issue that puts noise directly into the RCA would be 50hz, no?
Sheldon
There are two possible paths for hum in this case and I have seen both of them. One is a magnetic coupling between the output transformers, and the power transformer. If you look at the amp in the picture on my web site: http://www.tubelab.com/index.html
you will see what NOT to do. This amplifier always hummed, the hum was noticeable when the amp was switched on and died down as the output tubes warmed up. It didn't bother me on my 87db speakers, but when I took the amp to a friends house with 97db speakers, it was bad. I have since rotated the power transformer 90 degrees and the hum is gone. The hum was audible with ALL of the tubes removed from the amp, but got louder with the tubes in.
The 833A amp prototype had a pretty loud hum that appeared after the output tube warmed up, WITHOUT ANY other power supply turned on! Thats right, ONLY the filament transformer to the 833A is powered up and hummmmmmmm. Moving the output and filament transformers around had no effect. Breaking the plate circuit anywhere caused the hum to stop. The hum balance pot had no effect (until the rest of the amp was powered up). Breaking the grid path had little effect. Reversing the filament leads made a slight improvement. The hum appeared to be mostly 60 Hz until the whole amp was powered up, and then there was obviously a strong 120 Hz component, this varied with the hum balance pot, but in no case could the amp be made quiet. The prototype has since been dismantled, but I decided that the final design would need DC filaments, all 10 Amps worth. It would seem that sufficient current was flowing through the tube with only filament voltage present. I later confirmed that the plate will go about 6 volts negative with respect to the filament on filament voltage alone (big filament 10 volts 10 amps).
http://www.tubelab.com/833SE.htm
Listen carefully to your amp, does the hum appear as soon as the amp is switched on, and change slightly as the tubes warm up (rectifier and driver tubes removed)? Does it hum at all with the output tubes removed? If so then I would suspect magnetic coupling.
Does the hum appear as the output tubes warm up, silent on initial switch on (rectifier and driver tubes removed)? 2A3's warm up quick, 2 or 3 seconds. If so is there any other source of voltage in the amp? If there is no other source of voltage, try to find a simple way of breaking the plate circuit path. If the hum goes away, it MUST be a current that is flowing through the tube. How do you fix this? I don't exactly know. I have used DC filaments. Just exactly why some amps and some tubes exhibit this problem worse than others, I don't know.
you will see what NOT to do. This amplifier always hummed, the hum was noticeable when the amp was switched on and died down as the output tubes warmed up. It didn't bother me on my 87db speakers, but when I took the amp to a friends house with 97db speakers, it was bad. I have since rotated the power transformer 90 degrees and the hum is gone. The hum was audible with ALL of the tubes removed from the amp, but got louder with the tubes in.
The 833A amp prototype had a pretty loud hum that appeared after the output tube warmed up, WITHOUT ANY other power supply turned on! Thats right, ONLY the filament transformer to the 833A is powered up and hummmmmmmm. Moving the output and filament transformers around had no effect. Breaking the plate circuit anywhere caused the hum to stop. The hum balance pot had no effect (until the rest of the amp was powered up). Breaking the grid path had little effect. Reversing the filament leads made a slight improvement. The hum appeared to be mostly 60 Hz until the whole amp was powered up, and then there was obviously a strong 120 Hz component, this varied with the hum balance pot, but in no case could the amp be made quiet. The prototype has since been dismantled, but I decided that the final design would need DC filaments, all 10 Amps worth. It would seem that sufficient current was flowing through the tube with only filament voltage present. I later confirmed that the plate will go about 6 volts negative with respect to the filament on filament voltage alone (big filament 10 volts 10 amps).
http://www.tubelab.com/833SE.htm
Listen carefully to your amp, does the hum appear as soon as the amp is switched on, and change slightly as the tubes warm up (rectifier and driver tubes removed)? Does it hum at all with the output tubes removed? If so then I would suspect magnetic coupling.
Does the hum appear as the output tubes warm up, silent on initial switch on (rectifier and driver tubes removed)? 2A3's warm up quick, 2 or 3 seconds. If so is there any other source of voltage in the amp? If there is no other source of voltage, try to find a simple way of breaking the plate circuit path. If the hum goes away, it MUST be a current that is flowing through the tube. How do you fix this? I don't exactly know. I have used DC filaments. Just exactly why some amps and some tubes exhibit this problem worse than others, I don't know.
If the amp sounded OK before connecting it to the TV, then I would strongly suspect magnetic coupling from the vertical deflection coils to the amp. I once made the mistake of trying to record audio off a TV set with a microphone, and got some nasty background noise from the vertical deflection coils to the mic coil.
Yuck!
Thanks for all the replies.
I've tried a few different things and have managed to get the hum down to 7.5mV, which seems to have made an improvement.
I swapped primary leads on the output tx's and the mains H.T. secondaries,
moved the main earthing point to the C.T. position of the H.T. secondaries, and re-arranged the heater wires. Not sure which of these made the greatest improvement (if any) but I think I'll have to live with the 8mV.
Moving the choke around made no difference and the mains Tx is about 6 inches away from the output Tx's, and I can't move it easily so I won't.
The mains Tx is isolated from the chassis with rubber feet to stop hum being transmitted into the chassis as the Sovteks are quite microphonic.
I can live with the residual hum as long as I don't use it for AV!
Thanks again everyone... Bob
P.s. At switch on the hum rises from zero, peaks after about one second then dies back to it's 7.5mV level. All the transformers and choke on this amp are C cores. Any further discussion welcomed!
I've tried a few different things and have managed to get the hum down to 7.5mV, which seems to have made an improvement.
I swapped primary leads on the output tx's and the mains H.T. secondaries,
moved the main earthing point to the C.T. position of the H.T. secondaries, and re-arranged the heater wires. Not sure which of these made the greatest improvement (if any) but I think I'll have to live with the 8mV.
Moving the choke around made no difference and the mains Tx is about 6 inches away from the output Tx's, and I can't move it easily so I won't.
The mains Tx is isolated from the chassis with rubber feet to stop hum being transmitted into the chassis as the Sovteks are quite microphonic.
I can live with the residual hum as long as I don't use it for AV!
Thanks again everyone... Bob
P.s. At switch on the hum rises from zero, peaks after about one second then dies back to it's 7.5mV level. All the transformers and choke on this amp are C cores. Any further discussion welcomed!
Hi Bob,
So far there have been a lot of suggestions..if possible it is probably easiest though to try to exclude one or 2 things...
First of all try DC on both the driver and the 2A3...? If that cures your hum...you know what is causing it. If not ...you have very likely excluded a probable cause.
And a further suggestion would be to remove the input rca from chassis contact! It's a very easy thing to do...connect the ground via a wire to any of your star earthing points.
Regards,
Bas
So far there have been a lot of suggestions..if possible it is probably easiest though to try to exclude one or 2 things...
First of all try DC on both the driver and the 2A3...? If that cures your hum...you know what is causing it. If not ...you have very likely excluded a probable cause.
And a further suggestion would be to remove the input rca from chassis contact! It's a very easy thing to do...connect the ground via a wire to any of your star earthing points.
Regards,
Bas
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