S.E.X buzzing

[jrebman]

Have you thoroughly checked the whole power supply terminal strip and components -- making sure all diodes are in the correct orientation and are not damaged, filter cap for dc filament supply ok?, and especially the soldering around the joints where the thick wires all come together from the big diodes -- those can suck up a lot of heat and can be difficult to solder well?  Also, too much heat here without heatsinks on the component leads can damage the diodes and cap.

HTH,

Jim

 

[Doc B]

Can you measure the hum level? It's very difficult to help someone determine whether residual hum is normal or not without knowing what the level is.

 

[mcandmar]

I'm not entirely sure how to do this correctly so i measured both ways.

Measuring between the chassis plate and L/R at the headphone jack i see .49v/.48v AC on the multimeter. Between the ground point on the jack and L/R i see .002v AC, i think that's as low as my meter goes so i'll need a better one.

I got the scope out (old analog) and with the earth lead clipped onto the chassis plate i see an AC waveform of ~.34v @20ms (50hz). Between the ground point on the jack and L/R i'm way down at 500mv/div and see an odd looking seesawing ripple ~200mv peak to peak in 10ms divisions (100hz).

Hope i did that right, and thanks again for the help..

Mark

 

[Paul Birkeland]

Done a bit more testing this evening and discovered the hum starts immediately when i power on,

Sure sounds a lot like a bad solder joint in the power supply.

 

[Doc B]

Are all the filter capacitors oriented properly?

 

[mcandmar]

Ok so i assume those readings aren't right, and point to an issue in the main power supply section.  Everything looks ok and has been reflowed but to be 100% i'll use a solder sucker this time and disassemble/rebuild that entire section.

Thanks again,

Mark

 

[mcandmar]

Stripped and rebuilt the HV power supply section (terminal 1-5), and the two power feed posts 10 & 20 which seemed to improved things a little.  Good news is the power on noise is much better, after the initial bwong noise of the transformer energizing it all goes quite before the tubes start working so we are making progress.

I think i am now down to tube noise with a light hum in one channel, and a slightly louder buzzing sound in the the other, but the noises switch channels when i swap the tubes around.  The question is if the light hum is from the tube or the amp so i will source a replacement set of tubes and work from there.

Cheers,

Mark

 

[Mach2]

I have a similar buzzing problem after finishing my S.E.X build yesterday. My readings are quite similar to the other users' here,  3.83V @ C5, and -2.38V @ C4, and I have hum noises from both channels. I can live with the (barely audible) noise from one channel, however the noise from the other channel is very audible and annoying, especially with a low impedance phones such as my ATH-AD2000.

After tearing off half of my beard and going through and checking that all the soldering joints all over again, I swapped the tubes and the hum switches channels, so I guess the tubes are the problem here. I'll let my S.E.X play for a while and see if burn in will solve the humming problem. I hope those 6DN7 tubes are easy to find if I happened to have a bad tube.

PH

 

[Grainger49]

PH,

If this kit was a recent purchase Bottlehead will send you a new tube if the noise doesn't settle down after 50 to 100 hours of burn in.  You can just leave it on.

 

[physicsmajor]

I have a similar buzzing problem after finishing my S.E.X build yesterday. My readings are quite similar to the other users' here,  3.83V @ C5, and -2.38V @ C4, and I have hum noises from both channels. I can live with the (barely audible) noise from one channel, however the noise from the other channel is very audible and annoying, especially with a low impedance phones such as my ATH-AD2000.

After tearing off half of my beard and going through and checking that all the soldering joints all over again, I swapped the tubes and the hum switches channels, so I guess the tubes are the problem here. I'll let my S.E.X play for a while and see if burn in will solve the humming problem. I hope those 6DN7 tubes are easy to find if I happened to have a bad tube.

PH

If you're using low impedance phones, did you wire it up using the 4 ohm taps? If not, that's probably the easiest way to reduce your noise floor. At least on the lower noise channel, where the tube probably isn't the issue.

 

[Mach2]

PH,

If this kit was a recent purchase Bottlehead will send you a new tube if the noise doesn't settle down after 50 to 100 hours of burn in.  You can just leave it on.

Thanks! I'll give the tubes 50 hours before asking Bottlehead for a replacement. I'm living on the other side of the world so getting a replacement is rather tricky...

If you're using low impedance phones, did you wire it up using the 4 ohm taps? If not, that's probably the easiest way to reduce your noise floor. At least on the lower noise channel, where the tube probably isn't the issue.

I wired the amp using the 32 ohm taps, but I installed the impedance switch today and still observe the hum after switching to 4 ohm setting. I think it's definitely a tube problem here.

On the plus side, the amp sounds absolutely delicious even with the hums! I have yet to install the C4S kit so it can only go up from here.

 

[thomas27]

Just following up on this. I plugged the same 24ohm phonon headphones in my SEX amp yesterday. After 2/3 extra hundred hours of use, suprise: the background noise is almost completely gone! (the amp is still 100% stock and running on the 8ohm tap)

 

[Mach2]

An update on my S.E.X. After a few dozen of hours my Raytheon tubes seem to settle a little bit, the buzz (caused by one of the tube) seems to be a little less audible, but it's still there if I'm using my low impedance AD2000.

I finally got a set of replacement tubes from the Queen (thanks, Eileen!). Instead of a replacement Raytheon tube I got a new pair of RCA tubes. I plugged the new set of tubes in and yes, one of the tubes gives me the nasty buzz again, while the other one is completely quiet. After a while I got fed up and mixed the good Raytheon and the good RCA and voila, no more buzz! Now I only need to explain to whoever trying my SEX why I got two different tubes on the same amp...

-PH-

 

[mcandmar]

I've not had the best of luck either with these tubes, of the first three sets i bought one of each was unusable, and they were to replace the set that came with it.  Gets annoying spending days at a time burning in tubes to find they wont get any quieter.  I'm starting to wonder if were scraping the barrel of worldwide supply, or if i'm just unlucky.

 

[mauroj]

So, to get back to the original information, with these new tubes that are now quiet, are you still getting the weird voltages like 3.9 V and -2.5 V? I am having this buzzing issue and these voltages exactly. I just received 2 new pairs of tubes but I have not tried them yet. Please advise. Thanks in advance.

 

[Paul Birkeland]

I've not had the best of luck either with these tubes, of the first three sets i bought one of each was unusable, and they were to replace the set that came with it.  Gets annoying spending days at a time burning in tubes to find they wont get any quieter.  I'm starting to wonder if were scraping the barrel of worldwide supply, or if i'm just unlucky.

Did this actually end up being an issue after you installed the resistors at the headphone jack?

 

[mcandmar]

Oh yeah they went beyond the normal background hum, anything from a constant bzzzzt noise to intermittent russling, and one of them kept making loud popping noises that would make you jump out of your skin. They were just bad tubes.

Karma balanced out in the end as i picked up four International Servicemaster coin base tubes from Australia for $3 each which turned out to be re branded Sylvania black plates and completely silent. That totally made my day

 

[mcandmar]

So, to get back to the original information, with these new tubes that are now quiet, are you still getting the weird voltages like 3.9 V and -2.5 V? I am having this buzzing issue and these voltages exactly. I just received 2 new pairs of tubes but I have not tried them yet. Please advise. Thanks in advance.

Just measured again, +3.61dc and -2.49dc, with 2.99vac/3.00vac at the transformer.  Pretty sure they would read evenly with no load on them.

 

[Paul Birkeland]

This has nothing to do with noises in the amp.

(if you don't believe me, unground the center tap and ground one leg of the 6.3V supply)

 

[mcandmar]

We believe you Paul, it just throws people off as the voltage check list in the manual says it should be +3.15 and - 3.15, hence people query it.