I bought a recapped Sherwood s5000 which hums, most horrendously on the phono stage. Hum is low and acceptable on the monitor input where I plug my ipod in. Problem is I mostly listen to vinyl. My tube-tech friend found that the 4 output tubes weren't rated for the high voltage that this thing runs on so I bought some nos military spec 7189 output tubes. There is Sylvania 5U4GB rectifier tube. In the preamp section there are 2 nos telefunken 12ax7's, a Sylvania 12ax7, a JJ 12ax7, and an unknown 12ax7 (installed in that order from left to right). There are nos Amperex Bugle Boy 6U8A's where the 7199's go. All tubes test good. My friend found that the power supply capacitors and bias diodes are all updated. Another friend installed a ground 3-prong cord (grounded to chasis). So that's my adventure thus far and it still hums! Now what?
My Sherwood s5000 hums. Help!
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The original power cord was 2 prong. It could be the 3 prong cord, although, i find it difficult to believe that the person who installed the new power cord did not do a sound check before and after to make sure it did not mess up your amp. Tracing down the source of a hum can be most challenging. Since it is in the phono stage which is a high gain stage you should at least try different 12ax7. Getting a "low noise" pair might be good or at least a new pair in case it is the tubes. Try subbing you 12ax7 in the phono stage to see if it makes a difference. Also you really need to make sure that it is not your turntable and grounds, interconnects, etc that is the problem. You dont want to be chasing a problem in the amp when it is actually the turntable hook up that is the problem. good luck
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I'd also go with lifting the ground on the cord to see if that helps first. One of those "cheater" adapters will do fine for testing. Ground loops on old gear can be a royal PITA. I have an S5500-II myself, as well as an S-7000. They are both very quiet amplifiers when things are proper with them. My 5500 is missing the bottom plate too, and its still quiet in every mode with original filter caps.
Is your turntable grounded to the chassis of the amp? I don't think these have a dedicated ground screw, but any chassis screw will do the trick. Does the hum on other modes stop if you unhook the turntable? Are the input connections clean and tight? I know both of my Sherwoods have been a little problematic in that area.
Is your turntable grounded to the chassis of the amp? I don't think these have a dedicated ground screw, but any chassis screw will do the trick. Does the hum on other modes stop if you unhook the turntable? Are the input connections clean and tight? I know both of my Sherwoods have been a little problematic in that area.
well in my reading of your issue i c that u stated that 6u8a are inplace of the 7199s now this is not a direct replacement you have to change the wiring on the tube sockets there are many instructional sites on the net that describes the process as when i bought a stromburg carlson asr 110 it had 7199s in it. one was faulty instead of paying out the rear for nos 7199s i chose the much cheaper 6u8as but had to rewire the sockets if u cannot find the info on it i can walk u through it.
Thanks all! I have tried a two prong cheater adapter to no avail. Yes, the turntable is grounded to the chassis (you're right, there is no official phono ground on the stereo). One bit of info that may be helpful is that the hum is not present when the turntable is not hooked up. The same table on my other amps does not hum. As for the 6u8as, there are socket adapters. Also, I found the following passage in a post elsewhere (I think I will ask my tech friend to look into this):
"The bias supply turns out to be a finicky thing on this amp. If you read the schematic, either Sherwood OR Sams, you see that there are three capacitor sections in the supply. In the original amp, these are all in a twistlock can and share a common positive, rather than a common negative like just about every other twistlock can. If you look at the pictures I posted earlier, you see that there are several things tied to the "ground" on this can, but that is incredibly misleading. You see, while this cap has a common positive, it is NOT on the can body!! There is a terminal that has the common positive, and it is NOT connected to the can. Whoever drew the schematics made the same assumption I did, that the common positive is connected to ground. It is not. It is connected only to the center-tap of bias winding, the DC filament return, and the ground bus that for the entire preamp section!! If you follow the schematic or your eyes when you rebuild the bias section, and you connect the common positive to ground, you will be instantly greeted by a NASTY hum as soon as you connect a magnetic phono cartridge! JRC, since your bias supply was rebuilt, and kinda sloppily, you may wish to redo that section and be sure that the common positive is not tied to the chassis!
I got everything dead quiet in this amp until I tried the phono section which was working beautifully before my rebuild. It now had a horrible hum, and so I knew it must have been something I had done. I poured over the amp with a fine-tooth comb, uncovering 3 mistakes from the factory in the process including two joints that were never soldered, one was the center-tap of the AC primary at its chassis junction!! The other was a tone control resistor that was connected but never soldered to the pot. Obviously, this amp had some pretty green hands making it. After hours of frustration, I started measuring things. I compared the bias circuit carefully to my restored version. After finally getting my meter out and measuring the original cap (still in place but disconnected) I discovered the truth! The schematics are wrong and that circuit connects to the center tap of the bias secondary ONLY, and is floated from the chassis. One wire snipped out and voila! A dead quiet phono pre!"
So maybe I just need to snip a wire!!!!
"The bias supply turns out to be a finicky thing on this amp. If you read the schematic, either Sherwood OR Sams, you see that there are three capacitor sections in the supply. In the original amp, these are all in a twistlock can and share a common positive, rather than a common negative like just about every other twistlock can. If you look at the pictures I posted earlier, you see that there are several things tied to the "ground" on this can, but that is incredibly misleading. You see, while this cap has a common positive, it is NOT on the can body!! There is a terminal that has the common positive, and it is NOT connected to the can. Whoever drew the schematics made the same assumption I did, that the common positive is connected to ground. It is not. It is connected only to the center-tap of bias winding, the DC filament return, and the ground bus that for the entire preamp section!! If you follow the schematic or your eyes when you rebuild the bias section, and you connect the common positive to ground, you will be instantly greeted by a NASTY hum as soon as you connect a magnetic phono cartridge! JRC, since your bias supply was rebuilt, and kinda sloppily, you may wish to redo that section and be sure that the common positive is not tied to the chassis!
I got everything dead quiet in this amp until I tried the phono section which was working beautifully before my rebuild. It now had a horrible hum, and so I knew it must have been something I had done. I poured over the amp with a fine-tooth comb, uncovering 3 mistakes from the factory in the process including two joints that were never soldered, one was the center-tap of the AC primary at its chassis junction!! The other was a tone control resistor that was connected but never soldered to the pot. Obviously, this amp had some pretty green hands making it. After hours of frustration, I started measuring things. I compared the bias circuit carefully to my restored version. After finally getting my meter out and measuring the original cap (still in place but disconnected) I discovered the truth! The schematics are wrong and that circuit connects to the center tap of the bias secondary ONLY, and is floated from the chassis. One wire snipped out and voila! A dead quiet phono pre!"
So maybe I just need to snip a wire!!!!
Interesting about the cap, though if that were the cause of the problem I would expect it to hum whether or not the turntable is hooked up. Unless I just don't understand what precisely the circuit is supposed to look like and what effect grounding the common positive would have on things.
Yesterday I watched as my my tube tech friend did a test by isolating the bias supply from the chassis to see what would happen - this was done to follow the suspected problem pointed out in the quoted passage above. I don't recall why exactly (something about no negative voltage appearing where it should) but he determined that it needed to be grounded to the chassis. And with that my friend decided that he can't help me any further (he's more of a guitar amp guy than a hi-fi specialist). So that leaves me stranded. Anyone in the Seattle area that can help me out? I live in Olympia.
I was born there, but haven't been back since. Been on the east coast since before my first birthday.
Bias caps being can common vs common to a terminal shouldn't really matter, though its possible something just isn't connected correctly. Dunno, its just been my experience that when the bias supply is messed up, its messed up all the time and not just when a turntable is connected. I had a lot of issues with my Sherwood's input jacks being generally crappy. I wonder if that isn't the whole of your issue.
Happen to have another turntable? I wonder if maybe the problem isn't specific to that turntable, maybe you've got a wiring problem there. I've had them that got funny about grounds depending on particular cartridges and mounting arrangements. The ones with all metal arms can get cranky if its got a metal eared cartridge that also happens to have a ground strap. My old Shure and Stanton cartridges need to have that strap removed or it hums.
Bias caps being can common vs common to a terminal shouldn't really matter, though its possible something just isn't connected correctly. Dunno, its just been my experience that when the bias supply is messed up, its messed up all the time and not just when a turntable is connected. I had a lot of issues with my Sherwood's input jacks being generally crappy. I wonder if that isn't the whole of your issue.
Happen to have another turntable? I wonder if maybe the problem isn't specific to that turntable, maybe you've got a wiring problem there. I've had them that got funny about grounds depending on particular cartridges and mounting arrangements. The ones with all metal arms can get cranky if its got a metal eared cartridge that also happens to have a ground strap. My old Shure and Stanton cartridges need to have that strap removed or it hums.
I need to to correct some info that I stated in my first post: the stereo has a hum/buzz on the am/fm, mx/tv, aux, inputs as well. Note that I say hum/buzz - not just hum as is the case on the phono input. There is also a tape selection on the same selector knob where there is no hum or buzz but there is a lot of hiss. On the Monitor input there is awesome clean silence! That's where I plug my devices into for now. I have tried other turntable and have the same problem.
The tape input runs through the same preamp circuit as the phono, it just has some slightly different eq parts. The hiss is probably noisy tubes. The fact that it is quiet tells me that the amplifier itself is OK. The monitor input bypasses the function selector but all of the other circuits are the same. I'm thinking you've got something funny with that selector switch since it seems that all of the electronics are actually working correctly.
That seems like some solid logic. Now I just need to find someone who can look into that. As far as tubes, they all test good. I guess that wouldn't rule out the possibility of hiss? One thing I'd like to do is get together a second compliment of 12Ax7's to see how much it might change things for the better or worse. I have two nice Telefunkens in the first two sockets. I believe that these are phono stage sockets. Thanks for your thoughts!
Tubes can test fine and still be noisy. Could also be noisy resistors. Its sort of hard to chase down without some poking and probing, but you could always swap tubes around to see if it changes. The quietest ones need to be in the phono stage, they can get less quiet as you get towards the back.