Troubleshooting hum in 12b4 preamp.. finally almost done!!

[scott17]

Originally Posted by 240z4u

dumb laptop won't run it. Ill do it at work tomorrow.

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.

[240z4u]

Originally Posted by 240z4u

dumb laptop won't run it. Ill do it at work tomorrow.

Edit; I think its 120hz. It has a buzzy quality to it.

[240z4u]

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.

[scott17]

In case you haven't seen this, it's a great writeup. It pretty much sums up the recommended grounding scheme.

Star Grounding

Originally Posted by 240z4u

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.

[DF96]

Changing the volume with it shorted gave a linear rise in the loudness of the "buzz" which is a more accurate term I think.

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?

EDIT; For some reason I didn't use shielded wire from my volume pot to the tube socket. Ill rectify that immediately.

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.

[240z4u]

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

[DF96]

If the top of the volume pot was grounded, then maximum volume applies a ground to the grid wire. If the buzz is still there then something is wrong with the grounds. Did you ground the top of the pot to the same place as the bottom of the pot?

[240z4u]

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

[DF96]

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.

[240z4u]

I think I understand how to accomplish the grounding. I guess Ill give it a try, if it is safe and doesn't hum like crazy I am okay.