regularly repeating buzz - Reduction preamp

[jciii]

Hope this topic is in the right place...

Just built a reduction preamp kit. Great instructions, well thought out - great kit. But, as I am a noob at electronics, I have very little idea of where to start diagnosing a problem sound that I have.

First the sound itself: it is a buzz that goes on-off-on-off-on-off in a very regularly repeating way with maybe around a half-second period. There is hum below this, but first things first.

All resistance values are very close to values in the manual. All voltages are nearly spot on.

The reduction is grounded to the chassis on my receiver, and the TT is grounded to the reduction. TT power cord goes to receiver and the receiver and reduction are both plugged into a good power center - so not a ground issue I think.

All of my solder joints look good... any ideas where I should start to look? Many thanks in advance from this tube noob!

[Doc B.]

That kind of sound is usually the preamp picking up noise from an outside source. One of the most likely sources is a cell phone. Could also be a refrigerator, computer, or other electronic device.

[fullheadofnothing]

First thing I'd do is remove the ground wire going between the Reduction and receiver. That ground lug is there for when you are plugging your table into the receiver. Best case scenario is it's doing nothing, worst case is it's creating noise. Overall you want to try avoiding loops; plugging the table into the receiver could also be causing problems.

Have you tried shorting the input of the Reduction to determine your noise source?
http://www.bottlehead.com/smf/index.php/topic,586.0.html

[jciii]

Thanks for the replies!

Doc - if it is something contaminating the A/C power in the circuit, like a fridge, or a nearby device (EM field?), wouldn't this show up in my receiver-amplified sources? Granted my receiver is solid state (Marantz 2250), but I do not get any kind of similar noise when I play my TT through my receiver for example. Or is tube gear particularly sensitive in this regard?

Fullhead - I will reroute my grounds as you suggest and see what that does. No, I have not tried building the shorting rca's - that may be a weekend project.

Another thing that I tried was reversing the tubes to see if there was perhaps a bad tube. Initially, the sound seemed to be coming mostly from the left channel. When I swapped the tubes around, I got the noise from both channels. Weird. 

[Doc B.]

There wasn't the incredible amount of EMI that we experience these days back when tubes were the main method of amplification and hence they haven't been designed to be as resistant to the emissions from things like cell phones and computers. Luckily a little distance can make a huge difference in how much noise is picked up. Sometimes just moving the offending gear across the room can solve the problem. If the noise is coming over the power line getting the preamp on a different circuit than the noise generator can help.

[jciii]

Okay, I organized wiring, moved a load of stuff around, and the intermittent buzzing is much more subdued now. It's still there, but only when i turn the volume on the 2250 up past 50%, so that sound is probably some kind of EMF.

Now that that one has subsided a bit, I'll ask about the background sound now. This one more subtle - it's an intermittent, static-y, or kinda 'crunchy' sound, with a low hum as well. The sound completely switches sides when I move the tubes to the opposite sides. Could this one be a bad tube?

Thanks for the suggestions so far - they've been really helpful!

Joe

[Paul Joppa]

Yes, both can be the tubes, Give them some time - 50 to 100 hours is the usual estimate - to let the cathode form fully and the getters do their clean-up of residual gas atoms. If that doesn't solve it (it does most of the time) then contact the replacement parts email.

[jciii]

Thanks Paul.  I'll power it up for 8 hours a day for a week and see what differences I can hear in a weeks time. As this is my first piece of tube gear, I have no other tubes to try to immediately rule this in or out, so bummer there. Bidding on a pair of Amperex BB's now, so if I get those for a decent price, I'll swap 'em in and let y'all know what I hear.

Thanks again everyone!

Joe

[jciii]

10+ hours: the 'crunchy' noise is less extreme, but still present.

Just ordered a pair of Bugle Boy tubes of fleabay - will swap em in when they show up and see if there is a differnece in the noise.

[jciii]

Okay, so an update.

The tubes now have 30+ hours on them.

I now have a noise that is kind of like a helicopter coming from the channel with the tube that I suspect is bad (and also to a much less extent from the other channel too). The sound is intermittent and fast, like a helicopter, but is rather a soft static-y hum.

To confirm it wasn't my construction or soldering, I unpowered the unit, waited for the caps to discharge, and checked all of my solder joints once again. There were a couple with a bit of excess solder that I removed, but everything else looks fine, no crossed leads, everything neat and tidy. After checking everything once again, I did another voltage check and the results are below.

1    0
2    68
3    0
4    148
5    0
6    0
7    81
8    0
9    0
10  0
11  0
12  0
13  75
14  75
15  81
16  0
17  0
18  94*
19  88
20  87
21  0
22  0
23  0
24  0
25  0
26  94*
27  0
28  0
29  148
30  0
31  82
32  0
33  6
34  0
35  0
36  0
37  6
38  11
39  4
40  0
41  4
42  11
43  0
44  177
45  0
46  0
47  0
48  0
49  0
50  162
51  162
52  177
53  177
54  0
55  0
56  148
57  148
58  0
59  162
60  162
61  0
62  0

Terminals 18 and 26 are are above the published range of 70-90 - not by much, but I was wondering if this could be related to which tube was in the socket, so I reversed them. This resulted in the following:
2    increased from 68v to 93v
14  slight increase from 75 to 77v
18  decrease from 94 to 68
19  decrease from 88 to 62v
26  decrease from 94 to 68

Plugged the preamp back in, wired it up, and there was no discernible change.

So, it seems that one of the tubes produces dramatically higher voltages than the other across certain terminals. Is this enough for me to have these tubes replaced? Also, could this be the cause of my soft, static-y 'helicopter' noise?

Thanks again everyone!

Joe
 

[Paul Birkeland]

I'd put 100+ hours on the tubes, then remeasure the voltage that sits at 94.  Based on experience, it should pop down a bit with some use, and I believe your noises will subside.

-PB

[jciii]

Okay, Paul. I'll check back in around 100 hours. Thanks!

Joe

[Paul Joppa]

Agreed - the high plate voltage is characteristic of an incompletely-formed cathode. "Forming" in this case means getting the surface (on the molecular/atomic level) to have the right composition, and is accomplished by running current through the cathode. It should have been done at the factory of course, but it seems a certain percentage of most modern production tubes fall short, and need some burn-in to get going properly.

Of course if the problem is a badly-applied cathode coating, it could still have some problems, such as noises. That's why we suggest running it in for 100 hours or so, and sending for a replacement if that does not resolve the problem. Really, both halves of that protocol are equally important - this is just the most efficient approach.

[jciii]

About 50-60 hours on the tubes. One is still pretty noisy, but the other one sounds pretty good. I now have a comparison point - a pair of NOS Amperex BB 6DJ8. All I can say is WOW! Thanks for all of the help everyone - will check back in when the tubes have 100+ hours, but that will probably take a bit of time now that I have the BB's 

The veneer just showed up today for my base too!

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.ebayimg.com%2Ft%2FSouth-American-rosewood-sap-veneer-9-5-x68-%2F00%2Fs%2FODY2WDE2MDA%3D%2Fz%2FbLoAAOxy4t1SeDDc%2F%24%28KGrHqN%2C%21q8FJ15d%21S3uBSeDDc7UBQ%7E%7E60_12.JPG&hash=bd6caddc39c392a57d8e918a30c579f8c591c623)
Merry christmas, everyone!

[Paul Birkeland]

That is some stellar veneer!