Amature Express Hum

[BillyBob]

Hello,
I finished my first Express build recently. Its up and running and sounds good.!

I used the A1b circuit, in a steel fender Bassman chassis. I used an old Fender Bassman PT and a new Hammond 1650 OT. All other components are new. I have attached some photos below.

But Ive got some hum, sounds like power hum (60 cycle) and Ive tried everything that I could think of to eliminate it with no luck. There is an O-scope photo of the hum attached beow. Scope is attached to speaker output jack.

The hum comes in when audio developes/tubes power up. It goes away immediately when amp is switched off or standby.

Ive rolled alot of tubes, and pulling tubes one by one I noted that the hum goes away when pulling the PI or power tube(s). (Power tubes always biased at 60-70%).

I checked all the filter caps-OK. I paralelled in extra caps just to see effect-no change. Ive tried moving the transformers all over the place also-no effect.


NOTE: I have arranged a Zener diode (not shown in photo) on the CT of the PT to lower high B+ from Fender transformer to Express levels. Now I plan to go through the Express Voltage Check form on this site and see what results I get.

Ive built and rebuilt the amp a few times checking everything.
Im at a loss for direction..

Regards,

Bill

[BillyBob]

Hello,

The amp is fixed.!
I re-routed the two grounds from the filter cap board--problem solved.
O-scope is looking much better now. Hum is gone.

My first Express build sounds really nice.


Thanks for the help.

Regards,

Bill

[EaNot]

I re-routed the two grounds from the filter cap board--problem solved.

What kind of grounds? And where are moved?
Thanks!

[BillyBob]

Hello,
I built my Express on a Fender Bassman chassis.
I used the Express schematic (for filter cap values and resistors) but kept the Fender style filter board layout on top of the chassis.

The Fender filter cap layout that I used to fix the hum problem produces two ground wires that leave the cap board and come down into the chassis:

1) One ground wire coming off the first two filter caps which are in series. That wire runs to a lug on the PT.

2) A second ground wire coming off the end of the other remaining 4 filter caps (paralelle grounded to eachother under the board) and running directly (short run) to the input jack area, which is very far away from the PT.


Initaly, I had all the filter caps grounded together up under the cap board, with only one ground wire running to the PT lug......which caused hum.

Lesson learned.
Star Ground is where I was headed when I stubled accross that solution.


Amp sounds great by the way.!


Regards,
Bill

[martin manning]

1) One ground wire coming off the first two filter caps which are in series. That wire runs to a lug on the PT.

This is most important. There is a lot of ripple current flowing in the first filter, so connecting its ground directly to the PT CT keeps that ripple off the chassis and out of the preamp stages.

[EaNot]

BillyBob, many thanks! Traditional solution. But it is instructive (for me).

[RJ Guitars]

1) One ground wire coming off the first two filter caps which are in series. That wire runs to a lug on the PT.

This is most important. There is a lot of ripple current flowing in the first filter, so connecting its ground directly to the PT CT keeps that ripple off the chassis and out of the preamp stages.

I concur...

[BillyBob]

Hello,
I was wondering, what might be a less traditional solution?
What's another method of grounding the filter cap arrangement?

Bill

[RJ Guitars]

This is worth reading... http://www.aikenamps.com/index.php/grounding

[DaveWell]

Find this on EL34 World

[M Fowler]

I isolate the input jack from chassis and run ground wire to chassis ground tap near the input jack that all the preamp circuit is grounded to, pots, board and preamp filter caps.

The PI is grounded with the PT CT

If using isolated speaker jacks then that ground wire goes to PT CT chassis ground.

The NFB circuit in the Express circuit causes problems as well.

Mark

[DaveWell]

Is a Shielded wire for the NFB is a good Idea ?

[Bob S]

I have used shielded wire for this before. Not sure it was necessary.
I also grounded the presence pot to its own tie point on the chassis - close to the pot & that helped tremendously with hum.

[BillyBob]

Hello,
My Express build is very quiet now.
As far as hiss its quieter than some of my other (factory built) high gain amps.

Following someone's tip I did use shielded wire for V1 and V2. Otherwise no other shielded wire.

Through trial and error I did manage to make hum and general "noise floor" low.

Bill

[DaveWell]

Ok, I find this article from Premier Guitar.


Rule 1: Separate inputs from outputs as far as possible
Inputs are sensitive points that take in delicate, small signals. Outputs are places that have very strong, powerful signals. If you put an input next to an output, it shouldn’t surprise you that some of the output can bleed over into the input and cause oscillation, hum or noise. The bigger the output is compared to the input – the greater the gain difference between the two – the more likely oscillation is unless you are very careful – today’s monstrous high-gain amplifiers make this a real concern.


Rule 2: Keep wires short and direct from place to place
There are wires to inputs, and any point on the wire leading to the input is just as sensitive. Likewise the wires from big-signal places are just as “loud” electronically, so the placement of wires matters, too. Long, looping wires are invitations for crosstalk, RF interference and oscillation. One thing that makes true point-to-point layouts so effective is that the component’s leads are typically short, keeping the distance between parts small and discouraging the use of long wires simply to make things look neat.


Rule 3: Place AC power as far away from the inputs as possible
Wires from the AC power coming into an amp contain the amp’s biggest signals. That’s why you tend to see power transformers as far away from the input jack and circuits on the chassis as is reasonable.


Rule 4: Don’t mix reference, shield, and “sewer” grounds
Grounding deserves an article all its own, but here are some quick tips. There are three kinds of grounds: reference grounds that serve as a source of zero volts that some signal swings around, shield grounds that fence out interfering signals, and what I call “sewer grounds.” Sewer grounds are what drain the electricity back to the power supply after it’s been used by the circuit elements. If you cross-connect reference grounds with sewer grounds, some of the “sewage” – the burbles of used electricity – will show up in your signal.


Rule 5: Keep the output transformer separated from the power transformer and from the inputs
Another thing to watch for is magnetic interference and coupling. A tube amp will always have at least two, and often more, sources of magnetic radiation. The power transformer is a big one, belching out lots of power-line hum. In severe cases, the output transformer can produce hum in the speakers when the power transformer is near it, even without any tubes in the amp! Tubes only make matters worse. The power supply inductor can both pick up power line hum and emit power supply ripple hum.

There are many more layout rules that need consideration if you’re doing a design from the ground up, but you can get a long way with just these basic principles. The same rules apply in effects – place the input leads too close to the output buffer and it just may oscillate uncontrollably.