Still have a hum in my Express

[sliberty]

As I mentioned in my previous thread, I have a significant hum in my Express build (even though the amp sounds great, its annoying).

So the first thing I tried was to change a big portion of the grounding scheme in the amp. Before now, the grounds from the preamp board, tone stack and input jack were all grounded to the chassis by way of the ground bus wire running along behind all of the pots. I knew when I did this that it was a ground loop waiting to happen, but if it was good enough for Ken......why would I do something different?

So, today, I added a bus wire to the edge of the preamp board, and tied all of the above mentioned grounds to it, and then I tied it to the same ground point that the power tubes use for pins 1 & 8 (and the output jacks and the OT secondary ground). Unlike Ken's layout, I have a single ground point for both power tubes, and now it is also used for the preamp, tone stack and input jack as well. My other chassis grounds are similar to Ken's layout - one right near the mains cord for the AC ground and the two PT center taps, and the other for the filters and the bleeder resistors. So there are three places that the circuit connects to the chassis in total.

Still, there is a significant hum, which I believe is 120hZ.

The hum exists whether or not there is a guitar cable attached. It also is generally not effected by the volume control, although there is a spot near 1 on the dial when the hum seems to drop down a little (at zero it is the same as at 2 and above - 1 is like a notch). The hum's tonal character seems to change with the tone controls (probably a buzz component increasing and decreasing), but its level is about the same regardless of tone control settings.

If I pull the PI tube, the hum stops. Pulling V1 or V2 has no effect on the hum. My input lead and my volume control lead are both shielded cable with the shield connected only at one end (at the input and at the pot).

I have tried different 12AX7's in all positions.

I have read Paul Ruby's grounding recommendations from Mar 27th. I am not sure that I understand all of them. But even before I ask all sorts of questions about his recommendations, I wonder if they are really needed given that many other folks here have built an Express according to Ken's layout without a problem.

One major deviation in the layout is that I placed my OT in the center of the chassis instead of on the same side as the PT. This places it in proximity to the preamp board - is that a problem? I had little choice due to my salvaged chassis.

I am open to any and all suggestions.

You can see some pictures at http://music.sliberty.com/uniwreck/hum. You'll notice that I didn't remove the old ground bus yet, and there are 3 green wires still hanging off of it, but its not connected to the circuit anymore, so its not significant. I'll be removing them later.

Thanks,
Steve

[rhinson]

hello, here's what you need to do. ground only the plate and screen filter caps at your original grouding lug for them. also here ground: the hv. sec. center tap and the grounded ends of the 1 ohm resistors from the power tubes. take off the ground buss from the board and mount it on the chassis with chassis grounded end on the far preamp side (right around the vol. pot or so) and running all the way down to the presence control to an undgrounded 1 pos. terminal strip for support--use a big gauge wire like the 12ga stuff in your wall outlets. unhook each of the preamp filter caps and ground them all along the buss where their specific stage cathode is grounded---ie--ground the presence control grounds on the part of the buss right below it and it's filter cap there; the 2nd preamp tube's cathode ground goes right out to the buss from where it is on the board and it's filter cap goes right there as well--somewhere along the middle of the buss, and so on. follow this, and most of your hum should go away. rh

also do what phil said and line the bottom of your cab with aluminum--ie--roof flashing-----this will help with both hum and high pitched oscillations.

[sliberty]

rhinson,

Yee Ha!!!!!

You are so the man!

Thanks so much for the detailed and very helpful reply. I did most of what you suggested, and that nasty hum is virtually gone.

Here is what I did and what I didn't do:

- I moved the HV CT.
- I moved the filters for B+4 and B+5 to the ground bus near V1's cathodes.
- I moved the connection of the bus to chassis to be very close to the input end of the preamp board.

- I did not remove the bus from my board and make a new one as you described (just lazy I guess, after moving the bus once already today). Richt or wrong, it seemed to me to be equivalent to what I had (as long as I moved the chassis connection).
- I did not move the bias probe ground since these plugs are not currently connected yet.That may be the next step though.
- I did not line the cabinet mainly because my chassis has a metal bottom cover - so the lining would be a bit redundant I suppose.

Its 11pm now, so I can't really play the amp and really get a sense for how quiet it is, but in turning it on with no guitar connected, the difference was night and day.

Now its time for me to learn if you don't mind being the teacher. Why did this help? What exactly was inducing the hum the way I had it? And how is it that the Kelly photos and diagrams have resulted in quiet amps, but not mine?

Thanks again so very much!

Steve

[rhinson]

hey steve, cool man glad i was able to help. this is just good tube amp grounding technique---you'll always have minimal hum from grounding if you do those few simple things: always ground all the power section stuff (power tube cathodes, plate and screen cap grounds, hv sec ct, etc) on the far side of the amp where the power trannie is located and sum those grounds at single point--typically a lug trapped under on of the bolts that holds on the power trannie. in the preamp, do just what i outlined earlier: cathodes of stages and their associated filter cap get grounded at the same point----all the preamp grounds should be far away from the power side---they should grounded on the preamp side. i always use a buss like i wrote about but you don't have to---the buss on the pots is fine but it's messy and makes it hard to replace parts. you could also just use individual ground lugs for each of the summed preamp sections all along in front of your board. cool that you have a cover for the bottom of your amp---that'll help a bunch with high pitched oscillations that can occur in these types of amps.

as far as what you did, you just had some stuff summed that should be kept apart. the cardinal sin is grounding power caps with something involving the first or second stage--never do that. i've fixed several old gibson amps that had the plate filter cap grounded on the input jack lug------freaking hum city!! rh

[Rattler66]

Hey Rhinson............how would i implement your scheme, I am using a 40,20,20,20 JJ cap can (not individual caps).....thanks

[joe6v6]

I am using a 40,20,20,20 JJ cap can (not individual caps).....thanks

You wont be able to duplicate it exactly since the caps in the can all share a common ground, I think the key is to keep the pre-amp grounds away from the power grounds, I built a wreck using a 50-50 can and individual caps and all the grounds together & it works fine with very little noise.

JOE

[Rattler66]

thanks joe i will rewire today accordingly, this amp sounds incredible but for the hum