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I have a hum in only the V5 tube preamp section of my Sherwood. However, oddly enough it only seems to be new tubes and NOT with vintgage tubes.
I notice that any tube that has plates perpendicular - meaning you can not see through the tube while holding the pin gap facing you - it does NOT hum.
All the tubes that DO hum have their plate parallel to the pin gap plate. Meaning you can look down through the tube while holding the pin gap facing you.
This happens with my vintage Zenith, Bell&Howell and RCA - no hum. With the TungSol and Genalex - hum.
Is this really possible? Because it seems to be the ONLY thing I can find. I've shielded, tightened pins, cleans, worshiped Vaal, everything!
thanks
Charles
Follow Ups:
I found an old RCA 12AX7A in my old Heathkit OL-1 scope.
It has the plates aligned parallel so it should be humming. Plugged it in and NO hum - dead quite with no shields in place.
So the mystery continues! I'll check voltages tomorrow.
charles
In looking at the Sam photofact I see that v5 uses one side for left channel A F amp and the other side for right channel A F amp. Is the hum equal in both channels? I'm grasping at straws here.
PaulB
Both channels hum equally.
I happen to still be using the tube set that came with the amp when I bought it. They're all nos consisting of 3 Amperex BB in V5, V4, and V3 with telefunkins in V2 and V1 (the phono section). Some time ago,just for fun I put some reissue Genelex GL'S in V3 and V4. I got HUM in the left channel only. I traced it to the V3 spot. But when I moved things around the HUM went away and I haven't been able to dupicate it again. Weird. Maybe I hadn't pushed the tube in all the way or maybe a dirty socket.I'll clean them anyway. Don.t know.So I just checked and The GL and BB's have the plate orientation the same ( in relation to the pins). This puts the plates at about a 45 degree angle to the face of my amp. It is kind of strange that I did get some hum. I'll start playing with it and see if I come up with something concrete. But you may have to have Mike take a look if you want to get to the bottom of it . Very strange indeed. PaulB
Edits: 11/20/15
Hey Charles. Looking at my S5000 I see that V5 is the pre amp tube that's right next to the power tranny. Although I've never had this issue, I wonder if there still isn't some kind of shielding problem.
PaulB
This seems to be a common problem with certain new production tubes.
Shielding will not fix it. Running heaters on DC or floating a DC voltage on the heater circuit usually fixes it, although there may not be room in some units for the additional circuitry.
Or you can try various tubes until you find some really quiet ones that don't have that problem. I have had good luck with the 12AX7LPS.
In some cases I agree - but I routinely check these tubes on my AT-1000 which shows H-K leakage. IIRC I have never seen over 1 ua from a Genalex, I can't recall the Tung-Sols' readings though, sorry.
Edits: 11/27/15
I have NO doubt about that!
However, I've tried the Belton shields, doubling up on tube sleeve shields, attaching a second ground from the shield to chassis.
And all this with the full steel case shield screwed down in place covering the entire preamp section.
The only variable, obviously besides the actual tube, is the tube plate orientation.
Check out your tubes and try that experiment. Remove ALL the shields and see if it hums as described with what ever position the plates are in.
I removed all my shields. No hum. Moved all the pre tubes around checking plate orientation. No hum. I do have the s5000 with the slanted pre tubes and the stock shields but I don't think that should matter. Tis a puzzelment.
PaulB
post the plate orientation
You know I have to check about that two channel hum thing????
charles
Interesting. It's probably why there's the Sovtek 12ax7LPS with "Spiral Filiment" to reduce hum. Maybe the older tubes also commonly had the same feature.
This is a tube shield, specifically built for your problem.
Yes, the construction is likely contributing.
And it made NOOOOOOOOOOOOOOOOOO difference!
And I'm using a Genalex 12ax7LPS with "Spiral Filiment" - NOOOOO difference.
cheap crappy zenith with no shield - quiet as a church mouse.
The crappy Zenith was made for Zenith. You're likely using a vintage tube by RCA, Sylvania, Tung-Sol, etc. that is better than your new production tube(s).
"I can't compete with the dead". (Buck W. 2010)
"You're likely using a vintage tube by RCA, Sylvania, Tung-Sol, etc. that is better than your new production tube(s)."
That's pure speculation at best. The old stock tube may be different, but better? What criteria is better in such a way to not hum?
As Chip posted in this thread, it could be the old tubes are tired while the new tubes are fresh ("better"?) - the exact opposite of what you said. The reissue Genalex has a spiral wound hum-rejecting heater winding, the reissue Tung-Sol does not. So the Genalex is equivalent in construction to the old tubes' heaters - but is humming just like the Tung-Sol without the spiral wound heater.
The tubes can't hum on their own - the hum is either induced by an external hum field that exists either above OR below the chassis deck - or there is hum coming in on one of the connections to the rest of the circuitry. We simply have not been able to track down the source so corrective action could be taken.
It has nothing to do (with the possible exception of what Chip posted) with "betterness".
I try to avoid using better in here. That was an error. Most of what people say in here are only opinions. Saying better, best, greatest....which we hear all to often in this forum, is just a descriptor in an opinion.
"I can't compete with the dead". (Buck W. 2010)
and a new tube has higher output/sensitivity. I do not think you are dealing with magic or evil spirits.
To really understand, you will have to look at the voltages on the tube: Heater Voltage, Plate Voltage, Plate Current. My guess is that one or more of those is making your new tubes unhappy.
Hmm. So much for my spiral filament theory. Was the Sherwood rebuilt to stock specs? I think that V5 is for the tone controls. I'm really curious to what it can be.I experienced something kinda of like this that ended up being a bad solder joint but it did it with any tube.
Edits: 11/18/15
I would like to check Chips theory out. But I really don't want to invest in a NOS/NIB tube just to try it out.
The Sherwood was restored to what I believe was stock or near stock by Mike Samra. So I trust his expertise in this matter.
The ONLY thing I could try is testing the voltages in V,4,5. Because there is NO hum using new tubes in positions V3 and V4. Only with a new tube in V5.
If I test the voltages what pin/pins would be the culprit voltage?
I'm sure that Mike does fantastic work but i'd double check under the socket to be sure something didn't fall through the cracks. Did you try any other new tubes besides the New Sensor made ones? Maybe a JJ or Chinese 12ax7 would do better.
Just the Genelex LPS and the TungSOl.
Oddly enough the Genelex with the spiral filament hums louder/harder (hard to actually describe being isn't a unique hum) than the TungSol.
One more thing to look for, and it's a super long shot, is that the heater wires are tightly twisted going to V5. I hope that you figure this out. I know how frustrating it can be. I have a Citation V with some hum that I can never figure out and I'm too lazy to send it to someone that can.
Does the hum change at all when you move your hand near the tube - or when you install or remove the shield?Does the hum change when you adjust the tone controls?
Do the new tubes hum in any other socket?
Note: edited to add content
Edits: 11/19/15
No change with hands near the tubes.No change with or without the shields or steel case shield.
The new tubes do NOT hum in any other socket. ONLY the V5 socket. The new tubes work very well in sockets V3 and V4.
If the tube is quite it is quiet WITH or WITHOUT the shield.
So for some reason the shielding has nothing to do with it????
Edits: 11/19/15
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