Metropoulos Forum

Can't seem to get rid of this buzzing or humming sound in my new build. It comes though as notes decay. I assume it's a ground loop because it is the same pitch as 60 cycle hum. I used an old power transformer out of an antique radio and had to use two 100ohm resistors to center tap the filament leads, but the HT secondary was center tapped. It also has a 5 volt winding, but I just taped it off on both ends and set it aside. Does the PT primary need to be center tapped when using a grounded power cord?

sweetwilliam wrote:Does the PT primary need to be center tapped when using a grounded power cord?

You're speaking about the SECONDARY, not the primary, right?

Of course you must ground the HT center tap, otherwise the electrons can't flow. You even can place the standby switch between the center tap and ground - in this case a SPDT would be enough.

Larry

I think he does mean the primary.

SW you can pull each preamp tube V1-V2 then even V3 and see which set of gain stages is the worst offender or even see if it is the preamop which it probably is.

No, the secondary center tap is grounded. I was just wondering if the primary should have a center tap, but I guess not after looking at some schematics.

What could cause such a loud ground loop, though? Would it have anything to do with the unused 5 volt secondary? Should I try flip-flopping leads around?

I've already tried that. If I pull V1 it makes the biggest difference, but it's still there. I also notice if I ground my signal path it goes away completely (of course, so does everything else).

This goes on whether a guitar is plugged in or not.

It could be your preamp grounds then. Is there any noise when you pulkl v2? Probably not right.

Right. No buzz when V2 is pulled. Grounding bus runs along the back of the pots and is grounded at the input. I used Fender-style 1/4" jacks.

I don't know your layout in the amp, but anyway...

... ground your HT center tap directly to the first filter cap ground and DON'T ground the filament's resistors to or next to this point!

To ground this resistors even better to or next to V1's ground then anywhere in the area of the chassis, where the PT is mounted.

Larry

Layout is pretty standard superlead, just simpified with dual volume pot and one tone control.

The HT is grounded to one of the PT mounting lugs, the filament resistors are soldered from V5 pins 2 & 7 to pin 8 and then to ground.

HT grounding is ok - dirty ground to dirty ground

But the filament's resistors better should run directly to ground - and still better at another place in the amp - at the begin, or still better at the end (V1) of the filament's supply.

Larry

What wattage rating should those 100 ohm resistors be? The ones I have are metal film, 1% tolerance, 1/4 watt.

sweetwilliam wrote:What wattage rating should those 100 ohm resistors be? The ones I have are metal film, 1% tolerance, 1/4 watt.

Nearly no current flows through these resistors, therefore 1/4 watt is enough. But if anywhen one of your powertubes is shorting, then one or both resistors anyway may fuck up.

Larry

OK, moved those resistors and it's pretty dang quiet with no guitar plugged in. Just a little hiss and hum with the volume dimed, but that's to be expected. I actually still get more buzz than I would like with a guitar plugged in. Maybe I should disconnect the ground bus from the input, as the input grounds itself.

Verify, that your wire to the presence pot doesn't run exactly below the HT diodes or the bias diode on the board. That's also a buzz-source.

Larry

No. All good there.