Original 300BPP hum fixed

[Steve Grimshaw]

Hi all
Well well, this is very interesting. I have just changed the position of the speaker post earth wire on the earth bus bar from being the last wire on the bar to being the first immediately after the resistor. I have not made any other alterations at all, the hum on the right speaker is now completely inaudible from the listening position, I have to put my ear within 12 inches of the cone to hear it. Hum on the left speaker is no different, just about audible from the listening position. I guess it must be something further upstream causing this on the left channel.

Fascinating how just changing priority on the bus bar has altered the hum. I would not have believed it if I hadn't tried this out! Many thanks for posting on this Richard. Next step is to rout everything back to 0v on power supply board as Richard has done.

Steve Grimshaw

[Richard]

Hi Steve,

I'm following and am interested to see how you end up.

It seems you, Greg, and some of the others may have installed different earth routes to the original layout. I've borrowed Greg's pic again and drawn the original on it.

My L channel always hummed more than the R. In the original, the mains wiring runs back to front straight through the middle of that channel and I put it down to that, but now think it wasn't an issue.

L hum is now as low as the R with regard to humbuckers etc. (The humbuckers travel nearly 90 degrees between just audible hum at the listening seat one way to the other. Plonking the pot visually in the middle of this arc is enough to enable silence from the heaters, not critical at all. Both small valves are DC heated which will lower signal induced hum.)

Looking at the original layout the real reason, earthing, may be clear. I think that if both channels had equal wiring/resistance they'd hum equally. Then, taking them all to a better "stiiffer" or "lower" ground point would lower the hum of them all. That seems to be borne out in the pink wiring on the previous pic.

Rich

[NealG]

I've now wired the 300B earth point back to the PSU and.....it made no difference to the hum levels at all! However, changing the 6072A does! Try that Steve before you go re-routing all the earths

[Ianm2]

I have some rather serious hum problems with a 5881 amp I have gotten.

Its pcb, quite complex and is going to be a gyet to solve. first try the grounding methinks, and new valves.

any other suggestions?

[Steve Grimshaw]

Hi all
Well, I rewired the earth back to the power supply 0v as per Richard's original diagram a couple of days ago and as Neal says it has made no difference at all to hum. The hum pot adjustment is still just as fine as it was before too. The original earthing I had on my amp was pretty much as in the picture on Richard's last post following the green wires. The mains earth and transformer screen were earthed straight to the chassis though, not floating with the signal earth. The only thing which has made any difference to the hum on my amp has been altering the order of the wires on the earth bar so that the loudspeaker post earths are in first position. This has all but eliminated audible hum from the right channel but done nothing to the left channel. I was thinking of re-routing the 300b heater wiring as they are running close to the chassis at the moment, perhaps not a good idea :-). I will see if I can suspend them in mid-air inside the casing leaving just the signal wire up against the chassis.

Neal, I am not sure what you mean by changing the 6072A. Do you mean that you changed a noisy valve for a duplicate one which cured the hum?

All very interesting stuff!

Steve Grimshaw

[NealG]

Steve, I was a bit cheeky in saying I'd got the hum down to 1.2~1.5mv in an earlier post, well, I did but for the left channel only, the RH channel measured approx. 3mv....

After I changed the 300B earth as I said above it made no difference. So having been 'here' before I changed the GEC JAN 6072 I've been using for an Electro-Harmonix 6072 and the hum changed to 1.8mv LH and 2.2mV RH after tweaking the pots.

The only way I can hear this hum is to put my ear up close to the 'speaker.

Can you post up what the mV reading is for each channel? If it's close to 2 or 3mV then I doubt if you will appreciably improve it without changing or selecting valves....

Edit: Steve, leave the heater wiring hard up against the chassis, you'll get more problems if they are suspended in mid air.

[JDA99]

Well it worked for me. I have spent years listening to this amp with feedback applied to reduce the hum to tolerable levels.
I cannot quote voltages, but with feedback applied there is now no hum whatsoever, and I can listen with zero feedback.

Thanks Richard.

JDA

[Steve Grimshaw]

Measured hum is 1.8mv on the right channel, 2.5mv left hand measured after 20 minutes. This is using a EH 6072. I swapped in a spare EH 6072, but no difference. To be honest, the hum has never really bothered me, it is not audible at all on the right channel from the listening position, and only just audible on the left channel if you listen hard for it. If you heard the amp in my system I doubt whether you would think of it as humming. I decided to try out Richard's new earthing arrangement more out of interest to see what would happen, just as an experiment really. I was especially interested to hear of the reduced sensitivity in the hum pots which Richard found, as mine were (still are) critical to within 2mm either way. I have little idea why re-routing the earthing has not worked in the same way in my amp though one idea that does occur to me is that maybe my soldering is not of the same high standard as one would naturally expect from Richard I would have to hang my head in shame if anyone of you were to look at the rats nest of wires that I have in my amp. I expect everyone elses wiring is laid out in orderly rows with perfect spacing and symmetry betwee all the components and soldered joints that look like they have been done by robot

Ok Neal, I will leave the heater wiring where it is.Thanks for that. I was just thinking that the chassis might be picking up some hum from the heater wires.

Steve Grimshaw

[Richard]

Hi All,

Interesting findings! It seems some of us have the original wiring and some the "clone gang" style which may be better. Perhaps that "better" is good enough and going further may not show any benefit.

Especially if the hum is from another source such as a noisy 6072 which may have developed over a period of time. I find the US ones such as GE very quiet.

Don't neglect the connection wire from the earthiest point(s) on the input pcb (= on or very near the electro cap neg leads) to the cap neg pad near 0v on the psu pcb (see pink wires), as the input will be particularly sensitive to noise.

Rich