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Thread: Bell amps ground hum

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    Supporting Member Randall's Avatar
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    Bell amps ground hum

    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?
    It's weird, because it WAS working fine.....

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    Old Timer Leo_Gnardo's Avatar
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    Are the Bells' grounds/chassis connected in some other way? Grounded AC cables for instance?
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    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.

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    Supporting Member Randall's Avatar
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    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.
    It's weird, because it WAS working fine.....

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Randall View Post
    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.

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    Supporting Member Randall's Avatar
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    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.
    It's weird, because it WAS working fine.....

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    Old Timer Leo_Gnardo's Avatar
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    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.
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    Supporting Member Randall's Avatar
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    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.
    It's weird, because it WAS working fine.....

  9. #9
    Supporting Member Randall's Avatar
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    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?
    It's weird, because it WAS working fine.....

  10. #10
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Randall View Post
    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.

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    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.

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    Supporting Member Randall's Avatar
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    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
    Last edited by Randall; 04-01-2017 at 03:38 AM.
    It's weird, because it WAS working fine.....

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    Old Timer Leo_Gnardo's Avatar
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    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.

  14. #14
    Supporting Member Randall's Avatar
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    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.
    It's weird, because it WAS working fine.....

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    Old Timer Leo_Gnardo's Avatar
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    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.

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    Supporting Member Randall's Avatar
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    With the variac dialed back to put 6.3vac on the filaments, the B+ is 331v. All tubes are American made, I swapped them from amp to amp with no difference. I will try clipping in a test cap next.
    It's weird, because it WAS working fine.....

  17. #17
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Randall View Post
    With the variac dialed back to put 6.3vac on the filaments, the B+ is 331v. All tubes are American made, I swapped them from amp to amp with no difference. I will try clipping in a test cap next.
    B+ sounds comfortable for 6V6's. You could always select higher value dropping resistors in the hi voltage chain to bring the voltage to spec'd values in the preamp, no harm in that.

  18. #18
    Supporting Member Randall's Avatar
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    Clipping a test cap across each cap makes no difference.
    It's weird, because it WAS working fine.....

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    Old Timer Leo_Gnardo's Avatar
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    Drat, I'm running out of tricks. Is there any reason Bells would need to be turned up to 7 - 8 out of 10 where this motorboat effect starts up? If you're well into clipping before that point, you could pad the volume controls to stay out of the trouble zone.

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    Supporting Member Jazz P Bass's Avatar
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    Have you replaced all of the power supply caps?

    The ones further down the preamp chain are considered decoupling caps.

    If not that, I vote a bad interstage coupling cap.

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    Supporting Member Randall's Avatar
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    Interestingly, the Bells as they are now are fed directly from the mag cartridge, so there is no clipping going on on the inputs. I believe there should be an RIAA pre-amp between the turntable and the 12AX7s, but I'm not 100% on this. If so, I think this should be enough extra juice to where the volumes could be run at lower levels. At least I hope so. I don't want to spend the rest of my days on these, helping out a buddy. Still would like to know why it is oscillating, tho. I too, am running out of rope here.
    It's weird, because it WAS working fine.....

  22. #22
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Randall View Post
    Interestingly, the Bells as they are now are fed directly from the mag cartridge, so there is no clipping going on on the inputs. I believe there should be an RIAA pre-amp between the turntable and the 12AX7s, but I'm not 100% on this. If so, I think this should be enough extra juice to where the volumes could be run at lower levels. At least I hope so. I don't want to spend the rest of my days on these, helping out a buddy. Still would like to know why it is oscillating, tho. I too, am running out of rope here.
    I can see what looks like a passive RIAA sort-of EQ following the 1st stage 12AX7 triode. You've replaced the cathode bypass caps in the preamp too? 2nd & 3rd stage triodes share a cathode RC pair. If that cap is defective you'd have unexpected feedback thru the common cathode resistor. If that turns out OK, I can only recommend 2 things in general, less low frequency response: cut the values of cathode bypass caps, reduce value of interstage caps. And/or limit the volume control with a pad resistor to stay out of the trouble zone.

    If there was no RIAA or similar eq built in, your magnetic cartridge would sound super tinny, all screeching highs & no bass. Have you played a record thru this rig, did it sound that bad? If it sounded reasonably good, you may well not need an external RIAA pre. And besides it looks like there's plenty of gain in the Bell to me.

    Hold on cap'n, I sees sumpin' weird: take a look at the 3rd stage triode's grid, I don't see any grid leak resistor! Neither do I see one on the 4th triode, marked 2nd AF AMP - PHASE INV. Hm, if that's really the case, you could expect instability. As for the 3rd stage, if you put a shorting plug into the crystal cart or radio inputs, the resistors leading to those jacks would serve as grid leaks. As for the 4th triode, there just isn't one, how they get away with that? Slap a 1M resistor across that grid (marked NO 5 OR = orange?) to ground, see wha happens. Also wouldn't hurt to put a shorting plug into unused crystal & radio inputs, then triode 3 will have a resistive path from ground for its grid.
    Last edited by Leo_Gnardo; 04-02-2017 at 03:22 AM.

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    Supporting Member Randall's Avatar
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    It does not sound super tinny with a nice turntable. And it does the oscillation even with the 12AX7 missing. So it's not that stage. What would make the plate voltages swing that hard?
    It's weird, because it WAS working fine.....

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Randall View Post
    It does not sound super tinny with a nice turntable. And it does the oscillation even with the 12AX7 missing. So it's not that stage. What would make the plate voltages swing that hard?
    It has more to do with the cartridge than the table, but your report is useful: if it sounds good, it IS good. And likely does not need an additional RIAA pre. It's easy enough to find a schemo for a passive RIAA and compare that to the filter that follows your 12AX7 triode, I think you'll find them similar.

    Check paragraph 4: in the next stages there's a couple of grids with no leak resistor. You could verify that with an ohmmeter quick enough, but what I see in the schematic is alarming. Also, the shared cathode resistor/cap in the 2nd & 3rd stages, reminds me a bit of similar shared Rk/Ck in Fenders. If cascaded stages have this setup, and the cap goes bad or gets disconnected, you get some strange behavior - could be the same here. Please check that bypass cap & its associated 2200 ohm R, you can clip a cap on there as you did for the power supply ones. And let's find out whether there's some hidden grid leak R's that I'm not seeing on the schematic. One quick check, you could remove the 2nd tube 6SL7 and see if the wobble stops.

    What a long fezzle. I just had a BA115 Ampeg in here, same thing, the repair party never ends. "How much can go wrong?" Oh crap, there's another one demanding my attention.

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    Supporting Member Randall's Avatar
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    Ding Ding Ding, we have a winner! Leo, I grounded the radio input and it stopped oscillating. Man, what a slog that was! Now I have to figure out why now this amp is way louder than the other amp, and why some of the hum returned. Everything I measured was within bounds, and almost every component has been replaced.
    It's weird, because it WAS working fine.....

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Randall View Post
    Ding Ding Ding, we have a winner! Leo, I grounded the radio input and it stopped oscillating. Man, what a slog that was! Now I have to figure out why now this amp is way louder than the other amp, and why some of the hum returned. Everything I measured was within bounds, and almost every component has been replaced.
    Yay! Did you ground the radio input on the other amp too? Now it's down to "what's wrong with this picture?" One must be different from t'other in some way. That dangling grid on the next stage still gives me the heebie jeebies.

    You would think - a pro manufacturer like Bell would have seen to this, even if they put in a very high value resistor for the "gone to hell" situation of having no resistive ground path to control the grids. I guess they assumed the radio input would be plugged into the output of a - radio - or other device that had a resistive path between ground and signal line. Like maybe a volume pot at the output. Crystal cartridge input? Couldn't expect much of a resistive path there, crystals generally have super high resistances more or less infinite.

    I see our fellow MEFster eneumann has as his byline Hofstadter's law. It definitely applies in this case: It always takes longer than you expect, even when you take into account Hofstadter's Law.

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    Supporting Member Chuck H's Avatar
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    Quote Originally Posted by Randall View Post
    Ding Ding Ding, we have a winner! Leo, I grounded the radio input and it stopped oscillating. Man, what a slog that was! Now I have to figure out why now this amp is way louder than the other amp, and why some of the hum returned. Everything I measured was within bounds, and almost every component has been replaced.
    The subject amp may be louder than the other AND have more hum simply because of higher gain. There may have been some leakage of signal at any shared components (especially cathode circuits). Did you replace ALL the electrolytic in the amp, or just the power suppply caps? Sort of skimmed, but if it's fixed bias you'll definitely want to address the bias filters and for my money, in any amp that age, any other electrolytic for sure.

    You may still have a hum issue that wasn't apparent before you "fixed" the signal bleed issue on the test amp. That still may be improved by replacing the cathode bypass caps.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

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    Supporting Member Jazz P Bass's Avatar
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    My vote is on 'coupling' caps.

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    Pretty much all components have been replaced on both amps, and they were close in output with a sine wave on a scope before I started unsoldering and measuring. It seems to me the instability was around V2. I checked everything that was suggested and all seems well. Lifting the .05 C8 made the wobble stop, and I read no leakage there. The cathode bypass R and C look good on V2 and V3. I did swap some tubes between amps, maybe there is a gain issue with one of them?
    It's weird, because it WAS working fine.....

  30. #30
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    Try swapping the whole lot of tubes from one amp to another. Then you will know if it's a tube or not.
    Also, if you have an idea of what size input signal the amps expect, try to guess whether one amp is weak, or is one amp too 'hot' ?
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