S.E.X buzzing

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Offline Mach2

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Reply #30 on: November 25, 2013, 09:31:05 PM

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.

PH


Offline thomas27

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Reply #31 on: December 08, 2013, 07:03:25 AM
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)



Offline Mach2

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Reply #32 on: January 07, 2014, 05:49:40 AM
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-
« Last Edit: January 07, 2014, 04:49:27 PM by Mach2 »

PH


Offline mcandmar

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Reply #33 on: January 07, 2014, 10:41:46 AM
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.

M.McCandless


Offline J. Mauro

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Reply #34 on: January 13, 2014, 07:34:45 AM
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.

Jeremy Mauro

So long, and thanks for all the fish.


Offline Paul Birkeland

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Reply #35 on: January 13, 2014, 07:39:51 AM
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?

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline mcandmar

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Reply #36 on: January 13, 2014, 01:04:04 PM
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 :)

M.McCandless


Offline mcandmar

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Reply #37 on: January 13, 2014, 01:49:15 PM
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.

M.McCandless


Offline Paul Birkeland

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Reply #38 on: January 13, 2014, 01:57:20 PM
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)

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline mcandmar

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Reply #39 on: January 13, 2014, 02:01:16 PM
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.

M.McCandless


Offline Paul Joppa

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Reply #40 on: January 13, 2014, 04:10:21 PM
In the SEX 2.1 (with PT-7 power transformer) the heater power is only connected to ground for two short periods during the 60Hz power line cycle - just when the diodes are conducting. Most of the time it floats. For that reason, if there is a little leakage between heater and cathode, the voltage will drift away from symmetrical. You can put a couple 100-ohm resistors in series across the DC power and ground the center point if you want to see matched voltages; you are not however going to hear a difference unless the leakage is so bad the tube should be replaced.

The center tap is grounded so that the winding itself is always grounded. This allows it to act as an electrostatic shield for other windings.

Paul Joppa


Offline mcandmar

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Reply #41 on: January 18, 2014, 07:06:35 AM
Paul,

I just tried this out of interest and it does make a difference, enough that i am going to have to wire it in permanently.

I initially tried it with just the scope connected and could see the sharp peaks were knocked off the noise with the resistor center point grounded, i then plugged in a pair of headphones (also before the 120r resistors at the headphone socket) and i could clearly hear a reduction in the higher pitched bzzzt noise any time i connected the ground. Voltages evened out a bit from +3.6/-2.4, to +2.9v/-3.1v, though i find it curious they swung the other way to be slightly more negative.

Before i warm up the soldering iron is it worth experimenting with small value caps across the heater circuit, or directly on the transformer secondary to help further reduce any switching noise?

M.McCandless


Offline Paul Joppa

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Reply #42 on: January 18, 2014, 05:07:06 PM
Interesting - thanks so much for posting!

My best guess at this point is that it's common-mode noise. Capacitors aren't going to help unless they have lower impedance at 60Hz than the two 100 ohm resistors, which are effectively in parallel. 50 ohms at 60Hz is 53uF, that's getting pretty silly.

You can replace the resistors with a pot, probably anything from 50 to 500 ohms will do. You'll want a 3-watt or greater rating. And make sure the tubes are the same make - some have series heaters, so the best balance will be off center for those. (Also make sure they are the same phase - pin 7 to pin 7, 8 to 8.) Adjust it for the lowest hum.

The Schottky diodes don't have reverse recovery spikes, but they do still have switching noise. So it's possible that a common-mode choke will have an effect. If you put one between the transformer and the rectifiers, it will reduce coupling to the other windings (i.e. the HV winding). Putting one between the DC filter and the tubes will have more effect on what noise gets to the heaters. I have, at this point, no idea which would be more effective, or whether either would be audible at all. But it's a standard approach, so there's a good chance.

Before getting too carried away, do consider getting a second set of tubes. I imagine that different tubes, especially different makes, might have different sensitivity to heater noise coupling.

Paul Joppa


Offline mcandmar

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Reply #43 on: January 19, 2014, 10:31:05 AM
My best guess at this point is that it's common-mode noise. Capacitors aren't going to help unless they have lower impedance at 60Hz than the two 100 ohm resistors, which are effectively in parallel. 50 ohms at 60Hz is 53uF, that's getting pretty silly.

I was thinking smaller in the 10-100nf range directly on the secondary winding as i've seen it on various schematics with the explanation of preventing switching noise from propagating back through the transformer into adjacent windings.

You can replace the resistors with a pot, probably anything from 50 to 500 ohms will do. You'll want a 3-watt or greater rating.

I will look into that, from the bit of research i did i found this schematic you posted a few years back, http://usr.audioasylum.com/images/0/5625/FC-1_application_300B.GIF , i'm curious as to the purpose of the RC filter, whats the advantage to that vs directly connecting the center of the pot to ground?

And make sure the tubes are the same make - some have series heaters, so the best balance will be off center for those. (Also make sure they are the same phase - pin 7 to pin 7, 8 to 8.) Adjust it for the lowest hum.

Tubes are always identical pairs, i'm a little OCD about that.  I only have one series wired tube which is one of the Zeniths supplied with the kit, i never used that set as they were very noisy, but i'm now wondering if that is because one is series wired and the other is parallel wired.  I cant remember which of the two had the problem so i will need to retest them.

The Schottky diodes don't have reverse recovery spikes, but they do still have switching noise. So it's possible that a common-mode choke will have an effect. If you put one between the transformer and the rectifiers, it will reduce coupling to the other windings (i.e. the HV winding). Putting one between the DC filter and the tubes will have more effect on what noise gets to the heaters. I have, at this point, no idea which would be more effective, or whether either would be audible at all. But it's a standard approach, so there's a good chance.

I'm curious so i'll keep an eye on ebay for something common-mode and capable of handling a few amps to experiment with.  Unless something salvaged from a computer ATX power supply would be suitable, i have a box of those toroidal style chokes here that are rated for a few amps at 240v.

Before getting too carried away, do consider getting a second set of tubes. I imagine that different tubes, especially different makes, might have different sensitivity to heater noise coupling.

I have a bit of a collection at this stage of various different brands, interestingly one of my tubes started making an intermittent russling sound last night, i'm not sure if its just coincidence or if i finished them off while testing yesterday as i had the amp powered on/off a few times during the process.  I went back to an older set that i had it my bad pile marked as "slight hum" and so far they are silent, i need to go back and re-evaluate my collection of bad sets to see if they will work fine now.

M.McCandless


Offline J. Mauro

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Reply #44 on: January 20, 2014, 06:40:41 AM
Guys, I am still having this problem. It is a loud buzzing sound more in the left channel than the right but the volume doesn't change with the volume control. I have tried 3 different sets of tubes and my voltages are -2.5, 3.7, -2.5, 3.7 at C1,2,4,5 and A7,8, B7, 8. All other voltages and impedances are perfect. I have gone over every terminal and resoldered any that had even a hint of error. I don't know what the problem could be. Please help. Thanks in advance.

Jeremy Mauro

So long, and thanks for all the fish.