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Power on, standy off: hummmmmmm ???

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7/25/2000 10:25 PM

Power on, standy off: hummmmmmm ???

Hi everybody!

My second DIY amp hums as soon as I switch on the power; the standby, however, is off!! When I switch on the standby, the hum remains and cannot be dialled down by volume control.

It's single-ended EL 84 amp with a Vox top boost preamp (a la Matchless parallel input triode). The heaters have a virtual ground by means of two 150 ohms resistors, there is enough filtering and the grounding is ok (the same grouding works great in my plexi clone).

So what?

Carlos

7/25/2000 10:35 PM
Ken Gilbert	carlos, this is a classic sign of magnetic coupling between your power transformer and your output transformer. are they mounted on different planes (i.e. facing in different directions)? how close to one another are they? does the hum start IMMEDIATELY when you apply power to the mains tranny, or does it fade in with the heaters as they warm up? kg

7/25/2000 10:44 PM
Carlos	It starts right away. The transformers, yes, theey are close to each other, but mounted in a 90 degree angle. They are about one and half inch close. regards Carlos

7/27/2000 8:51 AM
Carlos	The transformer are now 7 inches apart. But there still is this hum. No magnetic coupling then. So what? Regards Carlos

7/27/2000 1:52 PM

Carlos,

First, I would remove your power tubes just to completely eliminate about 75 percent of the amp's circuitry (I know it shouldn't matter with the amp in standby, but this is a new build and anything is possible). Then, I would 'float' your speaker jack (isolate it from the chassis), reconnect the speaker, and try the amp again (power on, standby off). If you have a three-prong cord, have you tried lifting the ground? I would NOT touch the chassis during this test; it's possible you have a power transformer fault that is putting a ton of AC onto chassis ground. I would then measure the AC voltage from the chassis to the outlet ground and see what you get. I would also make sure that the standby switch is working correctly, and that you have no high voltage with the switch in the standby position.

Is the hum 50/60 Hz, or 110/120 Hz? I'm almost certain it's 50/60, but I had to ask.

Ray Ivers

7/27/2000 8:20 PM

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

Thanks Ray!

It's the common nasty European 50Hz hum.
I pulled the EL84 (it's a single-ended amp) and the hum remained.
I already use a Rean nylon jack for speaker out. The hum remains whether the speaker ground is connected to chassis or not.
Standby switch is ok.
I even turned the speaker in the combo 90 degrees to make sure that the speaker terminals are not close to OT or PT... (induction?)
The OT is brandnew and has been tested, the PT is from an old Telefunken radio (1957) that I took apart.

I haven't lifted the ground yet, though I might give it a try!

Regards

Carlos

7/28/2000 3:45 AM
Bruce	I'd like to know how you can hear any hum through the speaker with no power tube in the amp! The only thing that could cause that is the speaker wires or OT picking up raw AC and delivering it right to the speaker. Bruce

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