Bell amps ground hum

[Randall]

I have restored a pair of Bell amplifiers for a vinyl junkie to run as Left/Right in a home stereo configuration. The restorations went well, and each amp sounds good. Problem is when the inputs are coupled via a splitter for scope testing or simply a turntable left and right out, there is an audible hum. I measure about 0.5vac between the grounds at the RCA jacks. Can I just lift one of the ground connections here?

[Leo_Gnardo]

Are the Bells' grounds/chassis connected in some other way? Grounded AC cables for instance?

[TimmyP1955]

Schematic? Is signal ground connected to the chassis by a link, a resistor, or does it float?

Two wire or three wire AC cable?

Make sure that there's no caps from either side of the AC line to the chassis.

[Randall]

Bell amps ground hum

I have restored two Bell Sounds 2122A amps for use as a home stereo system. When used separately, they sound pretty nice, however, when connected to left/right of a turntable, or an mp3 player, there is a slight 60Hz hum. What could be causing this, and what can I do about it?

Also, when plugging just one side of the turntable outputs into a single amp, there is a very pronounced hum. It goes away and back to the slight hum when the other side is plugged into the other amp. It doesn't matter which side into which amp. I have also found that if I short the unplugged side tip to shield the noise stops. When not shorted, touching the sheild affects the noise, touching the tip does not. This makes no sense to me.

[Leo_Gnardo]

I have restored two Bell Sounds 2122A amps for use as a home stereo system. When used separately, they sound pretty nice, however, when connected to left/right of a turntable, or an mp3 player, there is a slight 60Hz hum. What could be causing this, and what can I do about it?

Also, when plugging just one side of the turntable outputs into a single amp, there is a very pronounced hum. It goes away and back to the slight hum when the other side is plugged into the other amp. It doesn't matter which side into which amp. I have also found that if I short the unplugged side tip to shield the noise stops. When not shorted, touching the sheild affects the noise, touching the tip does not. This makes no sense to me.

I had to check & see if my calendar was stuck on 2/2....

In any case there may be 2 things going on here.

1 is, most hi fi turntable to preamp connections include a separate grounding lead that connects the turntable metal chassis to the preamp/receiver. I don't see one in your description, you may have to add one to this system. On preamps/receivers there was often a lug with a knurled nut where that wire attached. You could simply run yours to one of the screws holding the cover on one of the Bells.

B: ground loop? are both Bells grounded, say thru their AC cables? If there's a second ground connection you may be setting up a ground loop. We see this all the time with guitarists who use 2 (or more) amps. The quick n dirty method is use a ground lift on one of the amp's power cable. It may help to have a very sturdy ground connection between the Bells if you do this, not just one accidentally made thru the turntable cartridge & associated wiring.

[Randall]

This has the ground wire, and it works fine when hooked to a SS receiver and it's ground lug. I suspect you are correct about lifting one of the amps, but don't ya know I don't have one on hand. The part about one solo amp howling when only one signal out cable plugged in makes me wonder if the Grado cartridge is wired correctly. I wouldn't know where to start to find that info.

[Leo_Gnardo]

Grado's website? A good ol' "Made in Brooklyn, USA" company. At worst you could email 'em or call. You may need a magnifying glass to read any letters or numbers pressed into the cartridge. Typically there are 2 coils, not connected electrically to each other, and "floating" iow not grounded. Maybe one is open, or there's some discontinuity in the wires to it? You should be able to test the connections by using an ohmmeter on each of the signal plugs. Each coil should measure a couple hundred ohms I expect.

I'm sure you'll pick up a ground lift at the grocery or hardware next time you're out for a drive.

[Randall]

Forgive me, It seems I posted this in two places. The other is in Music Electronics. Both amps have grounded AC cords now, so I gather one may have to be lifted. There is a seperate ground wire with a spade lug along side the L/R rca plugs. Doesn't matter where I connect it. No caps from AC to chassis. Signal ground is strapped to chassis with rivets.

[Randall]

Well, as it turns out, the Grado had two wires reversed. Correcting that set things right. Also, the ground wire was not fastened securely to the turntable connection point, the screw was backed out about 1/8". Trying a ground lift on one amp made the hum worse, so I still have that to track down. May just be that there is a bit of hum inherent in these 70 year old "hi fi" amps?

[Leo_Gnardo]

Well, as it turns out, the Grado had two wires reversed. Correcting that set things right. Also, the ground wire was not fastened securely to the turntable connection point, the screw was backed out about 1/8". Trying a ground lift on one amp made the hum worse, so I still have that to track down. May just be that there is a bit of hum inherent in these 70 year old "hi fi" amps?

Sounds like a major improvement already. The remaining hum, does it rise & fall with the position of the volume control? Something to remember, the coils in the phono cartridge are like guitar pickup coils, they will pick up an alternating magnetic field from any nearby transformer or motor and turn it into hum. Also, all the hum beating techniques you can use in a guitar amp also hold true, from wire dress, plenty of filtering, landing the lowest voltage filter near the first stage preamp's cathode to ground connection, even going to DC filament for the preamp tubes. Also I guess people did accept some hum level as normal way back when. But it's evident from top notch studio recordings made in the 40's & 50's that some engineers & studios were able to beat the noise floor all the way down. I'm sure some hi fi nuts also did similar back in the day.

[TimmyP1955]

If the turntable has a grounded AC cable, see if its signal ground is connected to its chassis. If so, open the link and see if that solves it.

Try lifting the shield on one of the RCA plugs from the generator or turntable.

Make sure that there is only one connection from signal ground to the chassis.

You could replace the signal ground-to-chassis link with a 10 Ohm or larger resistor. This may solve the problem. Of course it could also make the amp unstable.

[Randall]

Now that the hum issue is largely fixed (not sure I want to go to great lengths to re-engineer these since I already have more time than I care to admit in these) I will ask for help with another issue. When turned up past 3/4 one of the amps goes into what I can best describe as a slow tremolo oscillation. I have double checked my work against the other working amp. I can see the plate voltages fluctuate with the volume change on both plates of PI V3, and on pin 2 of V2, and a slight bit on pin 5. I do not see it on 47K R32 or 100K R15. Anyone have any ideas?

Edit:: forgot to attach schematic

Bell 2122A.pdf

[Leo_Gnardo]

I've had those sort of projects, the sort of gift that keeps on giving, right? The sort of wobble you describe sounds like motorboating. The classic reason is insufficient filtering. Did you replace the filter caps in those Bells? You can try clipping in an extra filter cap, try it on different stages, see if it stops the wobbulation. Old caps are of course suspect, but on occasion even a new one may fail for no particular reason.

[Randall]

Yes, I have completely rebuilt both amps for a friend, so all new filter caps. And both the same. I will say the plate voltages are high, even when powered on a variac aimed at 6.3 vac for the filaments and 117 vac for the line input. Where it shows for 85 - 95 vdc on the plates, I am getting more like 120- 130 vdc. Not sure why.

[Leo_Gnardo]

Wow, running 30-40% high? At the preamp that's not gonna hurt anything but it's still a little alarming. How's the B+ ? What kind of 5Y3 are you using? If it's Sovtek or other Russian, they have a reputation for developing more hi voltage than we expect. As for the motorboating, it could be correlated to a failed cap. As you say there's a hum problem too. That's why clipping in a known good cap can reveal quickly whether that's the case. If the wobble & hum go away with a test cap clipped in you know the one you clipped it to is suspect - no need to de solder each one & test them.