[becomethemould]

hi there, i recently received a NOS sylvania 5751 tube for my mg head otl. however when i plugged it in, i noticed a very noticable hum/ background static noise. the previous owner of the tube didn't experience any such noise. the noise isn't a big issue, when the music comes through i can't hear the hum

just interested in knowing why it happens.

oh, the previous owner suggested that i do the modification to the power supply of the mg head. i have yet to do it.

thanks!
peter

 

[Hirsch]

Some tubes are inherently noisy. Not all tubes age well, and noisy tubes are a risk, particularly when you're buying from a private seller instead of a reputable dealer. It's not an indicator of the brand or tube type, usually, but simply a bad tube. If all 5751's produced the same noise in your amp, it would indicate that you shouldn't use 5751's, probably due to an impedance mismatch that might not be present with other tubes. The 5751 is not an exact substitute for a 12AX7.

 

[braillediver]

I've had similar problems with the Sylvania 5751 Gold Brands in the MG Head. 60 cycle hum and a scratchy noise when adjusting the volume pot. More than half of the 12 I've tried so far had this problem. I was inclined to believe it was caused by the AC Filament heaters but never changed the MG Head to DC heaters to find out. I just returned the tubes.

People have told me it means something is wrong with the amp but the problem only occurs with that particular tube, I've tried several dozen different types of tubes. Maybe it's too sensitive, to tight of tolerances? If you could solve the problem they're seems to be quite a few Sylvanias around that exhibit this anomaly.


Good Luck

Mitch

 

[Hirsch]

Quote:

Originally posted by braillediver I've had similar problems with the Sylvania 5751 Gold Brands in the MG Head. 60 cycle hum and a scratchy noise when adjusting the volume pot.

IME that indicates that the tube is not meeting an amps needs. It's possible that the lower gain on the 5751 is right at the limits of tolerance of the amp's circuit. Some better tubes might work, others would not. This suggests to me that the 5751 isn't great with the amp, unless Sylvania is the only brand this occurs with.

 

[ddriveman]

I've tried many brands of 5751 (Sylvania, RCA, Raytheon, GE) on the MG head with no issues. I do get some scratchy sound when turning volume know on some but not limited to 5751. Even some 12AX7 have them.
So far, I have had 4 Sylvania gold brand 5751 and 2 black plate 3 mica Sylvania with no hum. Guess I am lucky ?
I have also tried 12AY7 and 12AU7 whihc have even lower gain thatn 5751 with no issues. Only tube I had a hum problem was a CIFTE 12AY7 but its due to the tube itself because the hum tracked with the tube.

 

[fredpb]

My humble opinion....

Bad tube.

Some tubes are just prone to problems.

When I worked on tube gear for many years, the worst offender in equipment was a JAN6005. This sucker either was designed poorly or just did not like being operated upsided down, which is how one piece of equipment used it.

It would work fine for a while. Many did. But many many times the tube elements would short out and fry the cathode bias resistor.... crispy enough to burn the resistor in half. Yeah, tried higher wattage resistors.

 

[becomethemould]

the tube i bought, Sylvania 5751 3 Mica Black Plate D-getter NOS in original box date packed 8-58, was bought from j_herrod who bought it from braillediver. im pretty sure both of them didn't experience any noise from the tube. unless you are a dodgy seller braillediver

it's not the 5751 type tube causing the hum as i have a GE 5751 in the amp before and it made no noise. the stock tube, a sovtek or EI 12ax7 made noise when adjusting the volume.

i'm a little stuck

... the mg head i have apparently has some cap upgrades and an upgraded power supply.

 

[ddriveman]

Quote:

Originally posted by becomethemould ...it's not the 5751 type tube causing the hum as i have a GE 5751 in the amp before and it made no noise. the stock tube, a sovtek or EI 12ax7 made noise when adjusting the volume...

Dear becomethemould,

I'm afraid its boils down to your 5751 having GONE bad. Hum usually comes to the tube when some of its components get loose...perhaps during shipping. Only other thing I can suggest is (DON'T LAUGH) to "stroke the tube a few times with your fingers". It helped with one of my tubes...I think because it helped to dampen the resonance and put it into a stable condition.

 

[Number9]

Does the tube have a metal base?

When I owned the Wheatfield HA-2, I noticed with some tubes with metal bases, I would get some hum when the base touched the chassis. Just moving it by a millimeter so they did not touch fixed the problem.

Another time, I corrected hum by reconnecting my power cord to the wall socket. That's right. The plug was just a tiny bit loose, and plugging it in more firmly removed the hum. Took me a long time to "debug" this one.

Tubes can be tempermenatal.