If hum is varying with volume setting then it is probably being picked up on the input or the grid wiring. Hum from heater leakage or HT/B+ would stay the same. Are you using screened cable?
Incidentally, you ought to put a 1M from the wiper to ground. This will protect against the wiper coming off the track.
Incidentally, you ought to put a 1M from the wiper to ground. This will protect against the wiper coming off the track.
Just a thought, if heater leakage was in close proximity to the input tubes, would not the hum vary with the input level?
Also I agree, make sure you are using screened (shielded) cable for all low-level input signals.
Also I agree, make sure you are using screened (shielded) cable for all low-level input signals.
Leakage is internal, from heater to cathode. This would not vary with volume. Adding a cathode decoupling cap would check this: less hum with cap means heater leakage or HT/B+ ripple, more hum with cap means something else.
Capacitive coupling to the input becomes part of the signal, so varies with volume. Capacitive coupling to the grid circuit basically injects a hum current, so the voltage developed depends on the volume setting - probably highest at mid-volume.
What happens to the hum when the input (i.e. top of volume pot) is shorted to ground? If it goes away, whatever the volume setting, then it is input pickup. If hum is still present but disappears at min and max volume then it is being picked up on the grid wiring.
Capacitive coupling to the input becomes part of the signal, so varies with volume. Capacitive coupling to the grid circuit basically injects a hum current, so the voltage developed depends on the volume setting - probably highest at mid-volume.
What happens to the hum when the input (i.e. top of volume pot) is shorted to ground? If it goes away, whatever the volume setting, then it is input pickup. If hum is still present but disappears at min and max volume then it is being picked up on the grid wiring.
Okay, when I short the input to ground there is no change. If I touch a probe to the input signal I get another "different" sounding hum. I think the hum may actually be 120hz. I wish I had a signal generator so I could know for sure what I was hearing.
Changing the volume with it shorted gave a linear rise in the loudness of the "buzz" which is a more accurate term I think.
Anybody know of an online signal generator where I could hear 120hz in the flesh?
Changing the volume with it shorted gave a linear rise in the loudness of the "buzz" which is a more accurate term I think.
Anybody know of an online signal generator where I could hear 120hz in the flesh?
You could try this one: Introduction
It includes a signal generator.
It's what I use except for critical analysis, for that I have an HP function generator.
Alright guys, made a hint of progress. I shorted the input to the tube to ground and the hum disappeared. I am not sure what I did last time. I went ahead and cut out my ratsnest of ground wiring (which you can't see in the photos) and am going to do a nice clean job grounding along with twisting my heater wires tight.
My question comes to grounding. Should I just star ground the whole preamp to the same point? Or should I tie my signal grounds to the output ground, and keep all the PSU ground points tied to the "safety ground". I am sorry if I really just don't know the terminology but I am trying to make sure I do this the best way possible this time. Also where do I ground the shield of the signal wires, with the signal ground? Or at the same point the PSU is grounded?
Thanks guys.
Evan
EDIT; For some reason I didn't use shielded wire from my volume pot to the tube socket. Ill rectify that immediately.
My question comes to grounding. Should I just star ground the whole preamp to the same point? Or should I tie my signal grounds to the output ground, and keep all the PSU ground points tied to the "safety ground". I am sorry if I really just don't know the terminology but I am trying to make sure I do this the best way possible this time. Also where do I ground the shield of the signal wires, with the signal ground? Or at the same point the PSU is grounded?
Thanks guys.
Evan
EDIT; For some reason I didn't use shielded wire from my volume pot to the tube socket. Ill rectify that immediately.
In case you haven't seen this, it's a great writeup. It pretty much sums up the recommended grounding scheme.
Star Grounding
Star Grounding
Just to clarify - you went all the way up in volume and the buzz continued to rise? I would expect it to reduce again as you approach max volume, or was it too loud to risk it?
That could be it. It is the place where screened cable is most needed. You can sometimes get away without screening the cable from the input socket to the top of the volume pot (although I don't recommend it), as the signal level is high and the impedance is low. From the slider the opposite is true - small signal, high impedance.
Yes, the rise with hum was linear to maximum volume. The second I grounded the unshielded wire it was silent even at full volume. I trimmed out that length of wire last night and Ill install shielded in its place.
Ill go ahead and read that article on grounding, seems to me that even if it's not the problem I would like to do it correctly.
Evan
Ill go ahead and read that article on grounding, seems to me that even if it's not the problem I would like to do it correctly.
Evan
Good question, I don't remember. It is entire possible that I didn't. Ill check that out when I get home. The link above is good on the grounding methods, and seems as if I am to star ground the entire system to the chassis top plate. This includes the safety ground from what I have gathered.
Evan
Evan
Yes, that link to star grounding is helpful. I personally prefer bus grounding, but either can work OK if done carefully. The main points of any grounding scheme are to remember that even the thickest wire has some resistance and inductance, watch where your currents go, and don't forget the safety ground.
My amp (modified 5-20) has two busbars: one for input and LTP phase splitter, the other for output cathodes and grid leaks (both shared between channels). The point where they join together is effectively the star point. This has a connection to safety ground/chassis (through a fat wirewound resistor), and the negative feed from the PSU. So charging pulses are kept well away from the signals, and output currents are kept away from the inputs. The speaker outputs are grounded to the input bus via the feedback twisted pairs - very little current goes along here, and the feedback must be referenced to the input. The input sockets are isolated from the chassis, as they are grounded at the input bus.
My amp (modified 5-20) has two busbars: one for input and LTP phase splitter, the other for output cathodes and grid leaks (both shared between channels). The point where they join together is effectively the star point. This has a connection to safety ground/chassis (through a fat wirewound resistor), and the negative feed from the PSU. So charging pulses are kept well away from the signals, and output currents are kept away from the inputs. The speaker outputs are grounded to the input bus via the feedback twisted pairs - very little current goes along here, and the feedback must be referenced to the input. The input sockets are isolated from the chassis, as they are grounded at the input bus.
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