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

[240z4u]

Quick update, I cut out all the grounding and am doing a reassembly right now. I installed screened cable where it wasn't and took apart the umbilical and tightly twisted the heater wires. I also got the other 12vac transformer installed for the heater.

Now, Ill get this beast grounded! Is it okay if I tie all the grounds on the input/output terminals together then run a wire from there to the star setup? I hope so, because I am about to do it! lol

Evan

[DF96]

I would run screened cables from the inputs to the point where the inputs are selected. At this point the screens connect together. This means that the input wiring within the amp is merely an extension of the external wiring. The screen-joining-point should then be connected to the cathode circuit of the input stage, perhaps via the screen of the cable to the input grid circuit - you can do this with either star or bus grounding. The idea is that you don't have hum loops even inside an amplifier, and ultimately you want to apply your signal to g-k on the input stage.

[240z4u]

Alright sounds good, that is basically exactly what I did. I did as you guys said, added screened cable where it wasn't and re-grounded. The hum that varies with volume is gone. There is a small 60hz hum across the spectrum now though.

I star grounded everything together, and left the "safety ground" grounded to the top plate. I had my hum still, so I tied the "safety ground" to the star ground I made and the hum that varied with volume setting is gone.

I did meter out my heaters at 17vac!! Seems too high, I have kind of high line voltage at my house combined with the +/-5% variation on the trafo has put it pretty high. Is there a reasonable way to tame this? I assume the tubes will not live long happy lives with heater voltage being so high. I am also hoping this is the source of the minor hum I still have (which is definitely a 60hz hum).

Thanks a lot for your help guys.

[Doz]

17 Volts is way too high.

Calculate how much current the heaters are supposed to pull, then work out how many volts to drop across a series resistor in the supply to bring the voltage down to 12.6V. If your heater supply is centre tapped, you'll need two resistors, one in each leg to maintain balance.

Is you heater supply referenced to ground anywhere? This may be the cause of your slight hum. Inclusion of a "hum-dinger" pot may prove this, and may even solve your remaining hum... Or rectify it, regulate it!

[240z4u]

Actually, duh. I don't think I checked it with a load on the transformer. Ill check that right when I get home. The heaters do seem pretty bright though. Resistors I can handle, no problem.

Ill search 'hum-dinger".

I can't remember if the heater is referenced to ground honestly.

Evan

EDIT; Doz, could you either link me to a hum dinger explanation or explain it? I looked on here and didn't find anything.

[Doz]

Originally Posted by 240z4u

Actually, duh. I don't think I checked it with a load on the transformer. Ill check that right when I get home. The heaters do seem pretty bright though. Resistors I can handle, no problem.

Ill search 'hum-dinger".

I can't remember if the heater is referenced to ground honestly.

Evan

EDIT; Doz, could you either link me to a hum dinger explanation or explain it? I looked on here and didn't find anything.

A hum-dinger is a pot, with the two ends connected to the heater voltage, and the wiper connected to ground. A couple of hundred ohms should do it. Adjust the pot for minimum hum. Bear in mind it will draw current itself, and therefore will load your heater voltage, as well as dissipate power itself, so will need to be appropriately rated.

[Doz]

Originally Posted by Doz

A hum-dinger is a pot, with the two ends connected to the heater voltage, and the wiper connected to ground. A couple of hundred ohms should do it. Adjust the pot for minimum hum. Bear in mind it will draw current itself, and therefore will load your heater voltage, as well as dissipate power itself, so will need to be appropriately rated.

I should point out that it creates "an artificial centre-tap" (never did like the expression) and references the heaters to ground.

[240z4u]

Alright, thanks Doz. Eli mentioned earlier in this thread that I had to reference the heater to ground. I think I may have, but it was so long ago I can't remember!

Thanks.

[Doz]

Originally Posted by 240z4u

Alright, thanks Doz. Eli mentioned earlier in this thread that I had to reference the heater to ground. I think I may have, but it was so long ago I can't remember!

Thanks.

Easy to check, pull the tubes, and measure each side of your secondary heater winding to ground. Lowish resistance would mean you did !

I've "got away with" just grounding one side of my heater transformer before now, but I couldn't stick my hand on my heart and tell you that's a good engineering solution , and it certainly unbalances the heater circuit!

[DF96]

Some heater windings have a centre-tap, which is usually grounded. If you want to use a humdinger, then remove the ground from the winding centre tap, otherwise you have two centre-taps fighting each other.