ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Hum trouble with 6G15 Reverb kit

8/1/2004 12:28 AM
Howard	Hum trouble with 6G15 Reverb kit Hi, I'm not sure this is the place to ask this but I'm trying to get this annoying 60Hz hum out of my friends DIY 6G15. He's thrown his hands up and I'm trying to figure out what's going on. It was a kit from somebody, but I'm not sure who. It differs from the vintage unit in a couple of ways. 1. No center tap for the filament secondary, (but there is one for HV secondary). I'm using a 2 wire filament circuit and I did the virtual CT thing by using 2-100 ohm resistors (actually 200 ohm since that's all I could find) from ground to each side of the filament circuit. 2. Full wave bridge using a pair of 1n007's. 3. Modern 3 prong plug with fuse and switch on hot leg. Ground lug is firmly screwed in to chassis and green ground wire is soldered to lug. 4. Neutral, HV CT, one of the black PT primaries and a wire to the coppor plate is soldered to another lug that's screwed to the chassis. 5. Everything else is grounded to the copper plate. 6. I'm running coax to pin 8 of the 12at7 (First stage of wet channel) with coupling cap to dry circuit connected directly to the same lug. I have not checked caps yet, nor have I pulled the board to check the underside. But I guess that's next unless someone can suggest something better. The only thing that I can say about the hum is that 1. It's there even when the reverb is powered down (and plugged into wall). 2. Goes away immediatly when the plug is pulled. 3. It gets better with an isolation transformer 4. It gets worse when the mix control is more dry. Full reverb it gets quieter, but is still there. Other controls have no effecton hum. It sounds great in every other way. How can I get rid of this #%$^%^$ hum? Thanks for reading this far. Howard Chandler

8/1/2004 12:28 AM

Howard

Hum trouble with 6G15 Reverb kit

Hi,

I'm not sure this is the place to ask this but I'm trying to get this annoying 60Hz hum out of my friends DIY 6G15. He's thrown his hands up and I'm trying to figure out what's going on. It was a kit from somebody, but I'm not sure who. It differs from the vintage unit in a couple of ways.

1. No center tap for the filament secondary, (but there is one for HV secondary). I'm using a 2 wire filament circuit and I did the virtual CT thing by using 2-100 ohm resistors (actually 200 ohm since that's all I could find) from ground to each side of the filament circuit.

2. Full wave bridge using a pair of 1n007's.
3. Modern 3 prong plug with fuse and switch on hot leg. Ground lug is firmly screwed in to chassis and green ground wire is soldered to lug.

4. Neutral, HV CT, one of the black PT primaries and a wire to the coppor plate is soldered to another lug that's screwed to the chassis.

5. Everything else is grounded to the copper plate.

6. I'm running coax to pin 8 of the 12at7 (First stage of wet channel) with coupling cap to dry circuit connected directly to the same lug.

I have not checked caps yet, nor have I pulled the board to check the underside. But I guess that's next unless someone can suggest something better.

The only thing that I can say about the hum is that

1. It's there even when the reverb is powered down (and plugged into wall).
2. Goes away immediatly when the plug is pulled.
3. It gets better with an isolation transformer
4. It gets worse when the mix control is more dry. Full reverb it gets quieter, but is still there. Other controls have no effecton hum.

It sounds great in every other way. How can I get rid of this #%$^%^$ hum?

Thanks for reading this far.

Howard Chandler

8/1/2004 1:33 AM
Wild Bill	This problem has been around a while, Howard. I went through it too and got some help here. What's probably happening is that the use of a 3-wire cord with both the amp and the reverb unit sets up a ground loop from each power cord ground and the grounded shield of the patch cord running between them. I can't remember if the fix was to use isolated jacks and not to ground anything inside the reverb unit to chassis except the green cord wire or to star everything to one common point only. Maybe someone will chime in and repeat the advice. Me, I was feeling lazy so I just disconnected the green ground wire inside the unit. The protection of the ground on the amp follows the patch and guitar cords to the guitar anyway. .02 ---Wild Bill

8/1/2004 1:33 AM

Wild Bill

This problem has been around a while, Howard. I went through it too and got some help here.

What's probably happening is that the use of a 3-wire cord with both the amp and the reverb unit sets up a ground loop from each power cord ground and the grounded shield of the patch cord running between them.

I can't remember if the fix was to use isolated jacks and not to ground anything inside the reverb unit to chassis except the green cord wire or to star everything to one common point only. Maybe someone will chime in and repeat the advice.

Me, I was feeling lazy so I just disconnected the green ground wire inside the unit. The protection of the ground on the amp follows the patch and guitar cords to the guitar anyway.

.02

---Wild Bill

8/1/2004 3:56 PM
rooster	Between the green wire from your power cord and your chassis ground, insert the following: 2 diodes back to back (anode of one connected to cathode of the other, so you end up with a unit that breaks over at .7V in each direction); strap a 22 ohm resistor across that, and a .01uF cap across that. Mine doesn't hum, but if I run it into a high gain amp, it makes a gawdawful racket when the Vibrato is on. But when would you run vibrato with high gain? rooster.

8/1/2004 3:56 PM

rooster

Between the green wire from your power cord and your chassis ground, insert the following: 2 diodes back to back (anode of one connected to cathode of the other, so you end up with a unit that breaks over at .7V in each direction); strap a 22 ohm resistor across that, and a .01uF cap across that.

Mine doesn't hum, but if I run it into a high gain amp, it makes a gawdawful racket when the Vibrato is on. But when would you run vibrato with high gain?

rooster.

8/1/2004 6:52 PM
MBSetzer	DANGER! Will Robinson! You differ too much from the vintage unit. Difference No. 4 is the potentially deadly one. Never, ever, connect either the hot or the neutral AC supply wires to the chassis. There are usually at least 15amps of current at 120V available on those wires before a circuit breaker in the building will be tripped. That's why you just use the incoming hot & neutral to connect only to the primary of the PT, that's a coil all it's own which is isolated from everything else in the amp. The amp then gets its power from the PT secondary, which is also isolated from everything else except when you connect the B- of your HV DC power supply to the chassis and/or earth ground for reference. You're expected to have a relatively small fuse on one of the incoming power leads, like 1/2 amp or less for a reverb unit, along with a switch, and that's conventional wiring. The neutral connects to earth somewhere in the building, not inside equipment. The green dedicated earth lead is the only one from the power cord which is supposed to be connected to the chassis. After you correct this safety defect, it may also reduce the hum. If still not low enough in hum, you may consider using an isolated phone jack for the output of the reverb unit. This is the way Fender has it on my SF Tube Reverb, which is not exactly a 6G15, but you could compare those schematics for differences to see if it can be helpful. Mike

8/1/2004 6:52 PM

MBSetzer

DANGER! Will Robinson!

You differ too much from the vintage unit.

Difference No. 4 is the potentially deadly one.

Never, ever, connect either the hot or the neutral AC supply wires to the chassis.

There are usually at least 15amps of current at 120V available on those wires before a circuit breaker in the building will be tripped.

That's why you just use the incoming hot & neutral to connect only to the primary of the PT, that's a coil all it's own which is isolated from everything else in the amp. The amp then gets its power from the PT secondary, which is also isolated from everything else except when you connect the B- of your HV DC power supply to the chassis and/or earth ground for reference.

You're expected to have a relatively small fuse on one of the incoming power leads, like 1/2 amp or less for a reverb unit, along with a switch, and that's conventional wiring.

The neutral connects to earth somewhere in the building, not inside equipment.

The green dedicated earth lead is the only one from the power cord which is supposed to be connected to the chassis.

After you correct this safety defect, it may also reduce the hum.
If still not low enough in hum, you may consider using an isolated phone jack for the output of the reverb unit.
This is the way Fender has it on my SF Tube Reverb, which is not exactly a 6G15, but you could compare those schematics for differences to see if it can be helpful.

Mike

8/1/2004 7:08 PM
DD	Never, ever, connect either the hot or the neutral AC supply wires to the chassis. Good point! I did this once a long time ago when I knew a lot less than I know now. On summer afternoon I was working in my basement on my amp while standing on a damp concrete floor in my bare feet. I then touched the amp chassis that was wired the way you described ... Every cell in my body felt that jolt. I'm lucky to still be alive.

8/1/2004 7:08 PM

Never, ever, connect either the hot or the neutral AC supply wires to the chassis.

Good point! I did this once a long time ago when I knew a lot less than I know now. On summer afternoon I was working in my basement on my amp while standing on a damp concrete floor in my bare feet. I then touched the amp chassis that was wired the way you described ...

Every cell in my body felt that jolt. I'm lucky to still be alive.

8/1/2004 10:19 PM
Howard	Thanks for catching that. An easy fix, but it still hums. Next thing to try is pulling the output jack away from the chassis. I'm a little nervous about the suggestion that involved partially lifting the ground wire from the chassis. Another potential ground loop seems to be the non isolated RCA jacks for the tank. I might try and isolate them too and ground them to the brass plate. Right now the reverb xformer secondary is grounded through the chassis by being soldered ring of the RCA jack. Thanks

8/1/2004 10:19 PM

Howard

Thanks for catching that. An easy fix, but it still hums. Next thing to try is pulling the output jack away from the chassis. I'm a little nervous about the suggestion that involved partially lifting the ground wire from the chassis.

Another potential ground loop seems to be the non isolated RCA jacks for the tank. I might try and isolate them too and ground them to the brass plate. Right now the reverb xformer secondary is grounded through the chassis by being soldered ring of the RCA jack.

Thanks

8/2/2004 1:43 AM
Wild Bill	Howard, you might try cutting the shield on one of the tank patch cables. Shielded wires should and only need to be grounded at one end. If you check the ground connections at the chassis of the tank to its connectors you can disable the ground connection on one cable so that the ground currents flow from one jack, to the tank and along the return cable but stopped before it hits the return jack. Does this make sense? BTW, I gather your unit doesn't have a power transformer to provide isolation. The warnings about shocks with a line connection to chassis are quite valid. In the golden days of tube stuff we could easily tell if a radio had a power transformer or was a simple line rectified circuit. Sooner or later a knob broke or was lost. If we turned the bare shaft while standing on something grounded we quickly knew if there it was a power transformer supply or not! ---Wild Bill

8/2/2004 1:43 AM

Wild Bill

Howard, you might try cutting the shield on one of the tank patch cables.

Shielded wires should and only need to be grounded at one end. If you check the ground connections at the chassis of the tank to its connectors you can disable the ground connection on one cable so that the ground currents flow from one jack, to the tank and along the return cable but stopped before it hits the return jack.

Does this make sense?

BTW, I gather your unit doesn't have a power transformer to provide isolation. The warnings about shocks with a line connection to chassis are quite valid.

In the golden days of tube stuff we could easily tell if a radio had a power transformer or was a simple line rectified circuit. Sooner or later a knob broke or was lost. If we turned the bare shaft while standing on something grounded we quickly knew if there it was a power transformer supply or not!

---Wild Bill

Page 1 of 2

Next>

Last Page>>