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Re: Power on, standby off: hummmmmmm ???

7/28/2000 4:51 AM
Richie{~}==:::	Re: Power on, standby off: hummmmmmm ??? I think Bruce is right.. is this transformer hum?? sounds like something is wired wrong or shorted.. or something center tapped that isn't supposed to be..or the wrong wire put somewhere..PT was checked and ok?? Something is weird here. If you do find the problem i think we all would want to know what it was. Richie[~]==:::

7/28/2000 1:09 PM
R.G.	As Bruce said, there's a very limited set of things it can be. (a) OT pickup from PT or filament wiring (b) Standby wired incorrectly, so the power amps is still running, delivering hum, but the preamp disabled (c) Physical hum from the transformer, whether normal magnetostriction or from some oddity about the wiring making this especially bad. I would debug it this way: (1) determine whether the hum comes from the speakers or the amplifier chassis by disconnecting the speakers. If you still hear hum, it's acoustic from the PT/chassis. (2)If speakers pull out all the tubes. If it still hums, it's either gotta be magnetic induction to the OT, and therefore correctable only by changes to placement and/or shielding, or an AC leakage to the OT by a quirk of incorrect wiring. (3) remove the tubes, and temporarily unsolder the CT line and one filament wire (on each filament winding) from the PT. If it still hums, it's magnetic induction/pickup, and you gotta do placement/wire routing changes. If it doesn't still hum, the OT is being fed AC through the power wiring somehow; fix the wiring. You can tell which wiring is causing it by which winding makes it start when you reconnect it. IF it's the B+ wire that restarts it, you have to check for leaky caps, leaky inductor, or wiring short on the B+ line. (4) if chassis, pull all the tubes; nothing can be amplifying, so any hum that remains has to be mechanical. There are two possibilities: "normal" magnetostriction and bum wiring making the PT shake harder than normal. (5) To separate out bum wiring, check the no-load power transformer current drain, either by measuring the AC line current or doing the AC-bulb-in-the-AC-line trick. With no tubes in, (and rectifier disconnected if solid state) the power used should be very small. A 25W bulb should be dim, not bright. Note that you could have a very leaky filter cap or inductor with a short to chassis causing high current drain on the B+, and therefore high current and magnetostriction on the PT, leading to mechanical hum acoustically amplified by the chassis, so you do have to separate out excess loading on the

7/28/2000 1:09 PM

R.G.

As Bruce said, there's a very limited set of things it can be.

(a) OT pickup from PT or filament wiring
(b) Standby wired incorrectly, so the power amps is still running, delivering hum, but the preamp disabled
(c) Physical hum from the transformer, whether normal magnetostriction or from some oddity about the wiring making this especially bad.

I would debug it this way:
(1) determine whether the hum comes from the speakers or the amplifier chassis by disconnecting the speakers. If you still hear hum, it's acoustic from the PT/chassis.
(2)If speakers pull out all the tubes. If it still hums, it's either gotta be magnetic induction to the OT, and therefore correctable only by changes to placement and/or shielding, or an AC leakage to the OT by a quirk of incorrect wiring.
(3) remove the tubes, and temporarily unsolder the CT line and one filament wire (on each filament winding) from the PT. If it still hums, it's magnetic induction/pickup, and you gotta do placement/wire routing changes. If it doesn't still hum, the OT is being fed AC through the power wiring somehow; fix the wiring. You can tell which wiring is causing it by which winding makes it start when you reconnect it. IF it's the B+ wire that restarts it, you have to check for leaky caps, leaky inductor, or wiring short on the B+ line.
(4) if chassis, pull all the tubes; nothing can be amplifying, so any hum that remains has to be mechanical. There are two possibilities: "normal" magnetostriction and bum wiring making the PT shake harder than normal.
(5) To separate out bum wiring, check the no-load power transformer current drain, either by measuring the AC line current or doing the AC-bulb-in-the-AC-line trick. With no tubes in, (and rectifier disconnected if solid state) the power used should be very small. A 25W bulb should be dim, not bright.

Note that you could have a very leaky filter cap or inductor with a short to chassis causing high current drain on the B+, and therefore high current and magnetostriction on the PT, leading to mechanical hum acoustically amplified by the chassis, so you do have to separate out excess loading on the

7/29/2000 6:18 PM
Carlos	Magnetic coupling!! Thanks folks! It was magnetic coupling. I disconnected the secondary side of the PT and moved it about 2.5 inches away from the OT and the hum was gone. I moved it back to original position and the hum was back again. Since the PT is already at the edge of the chassis, I've got to move the OT. Now I understand why Marshall tops are so wide ... to get maximum distance between OT and PT. But how do they reduce magnetic coupling in small tube heads? Thanks Carlos

7/29/2000 6:18 PM

Carlos

Magnetic coupling!!

Thanks folks!

It was magnetic coupling. I disconnected the secondary side of the PT and moved it about 2.5 inches away from the OT and the hum was gone. I moved it back to original position and the hum was back again.
Since the PT is already at the edge of the chassis, I've got to move the OT.

Now I understand why Marshall tops are so wide ... to get maximum distance between OT and PT.

But how do they reduce magnetic coupling in small tube heads?

Thanks

Carlos

7/29/2000 6:52 PM

Ray Ivers

Carlos,

From your previous post:

quote:
"The transformer are now 7 inches apart. But there still is this hum. No magnetic coupling then.
So what?"

Did you move them back together again?

Ray

7/29/2000 11:28 PM
Carlos	I moved them further apart. I didn't realize how big the magnetic field of a PT could be. Thanks Carlos

7/29/2000 11:42 PM
Ken Gilbert	carlos, does the power tranny have a belly band on it? this is visible as a copper band wrapping around the core on the outside. if not, you can remove the end bells (you've got those, right?), find a thin, wide, long piece of copper, wrap it around the OUTSIDE of the core, and solder it to itself as a loop. this will create a shorted turn which will pass currents induced by stray magnetic fields. it is VERY important that this band be on the OUTSIDE of the core (because a true shorted turn on the inside woud fry your tranny in fractions of a second). if the tranny already has a belly band, this is how it works... kg

7/29/2000 11:42 PM

Ken Gilbert

carlos,

does the power tranny have a belly band on it? this is visible as a copper band wrapping around the core on the outside. if not, you can remove the end bells (you've got those, right?), find a thin, wide, long piece of copper, wrap it around the OUTSIDE of the core, and solder it to itself as a loop.

this will create a shorted turn which will pass currents induced by stray magnetic fields. it is VERY important that this band be on the OUTSIDE of the core (because a true shorted turn on the inside woud fry your tranny in fractions of a second).

if the tranny already has a belly band, this is how it works...

7/29/2000 11:43 PM
Richie{~}==:::	Another odity is if you use a rectifier tube. Sometimes these will pick up vibrations from the transformer.. and make a buzzing noise..or rattling noise. Just something to add. Richie{~}==:::

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